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Departamento Ecología, Genética y Evolución

Guía de Trabajos Prácticos - Parte II

Primer Cuatrimestre 2016

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ECOLOGÍA GENERAL – Primer Cuatrimestre 2016 http://www.ege.fcen.uba.ar/materias/general/general.htm

DOCENTES

Nombre E-mail Prof. Cecere, Carla [email protected] Prof "Pedro Flombaum [email protected] JTP Gustavo Thompson [email protected] JTP Diana Rubel [email protected] JTP Sebastián Torrela [email protected] Ayudante Primera Verónica Loetti [email protected] Ayudante Primera Rosario Lovera [email protected] Primera Carolina Guerra [email protected] Segunda Pedro Montini [email protected] Segunda Samanta Thais Efron [email protected] Ayudante Segunda laura Calfayán [email protected]

Día Horario Teóricas

Martes Viernes

18 a 21 14 a 17

Trabajos Prácticos Miércoles y viernes 9 a 13 (G. Thompson) Martes y jueves 13:30 a 17:30 (D. Rubel) Miércoles y viernes 17:30 a 21:30 (S. Torrela)

Lista de Alumnos EGE: es una lista de distribución de información para alumnos del Departamento de Ecología, Genética y Evolución, acerca de becas, cursos y otras cuestiones de interés. No es una lista de discusión. Puede Ud. suscribirse a la lista en:

http://www.ege.fcen.uba.ar/mailman/listinfo/alumnos

o accediendo desde el link en la página del Departamento.

También en esta página encontrará los correos electrónicos de los representantes

estudiantiles del EGE, para realizar cualquier consulta.

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REGIMEN DE APROBACIÓN DEL CURSO Los requisitos para aprobar los trabajos prácticos son: (1) Aprobar 2 exámenes parciales con un mínimo de 60 puntos. Se podrán recuperar los 2 exámenes parciales. La fecha de recuperación será a posteriori del segundo examen parcial. A los fines de calcular la nota final de la materia se considerarán las notas de todos los exámenes. (2) Aprobar el 80% de los informes de los trabajos prácticos. El informe deberá entregarse la semana siguiente a la finalización del trabajo práctico. En caso de no ser aceptado será devuelto para su corrección y nueva entrega. En caso de no ser aceptado luego de la segunda corrección se considerará desaprobado. (3) Asistir al 80% de los trabajos prácticos. Se tomará asistencia al comienzo del trabajo práctico. Los alumnos que lleguen 10 minutos después de iniciado el mismo tendrán media inasistencia y los que lleguen luego de 20 minutos tendrán ausente. Los requisitos para aprobar la materia por promoción sin dar examen final son: (1) Aprobar los 2 exámenes parciales (sin la opción del recuperatorio) con un mínimo de 70 puntos cada uno y tener un promedio mínimo en los parciales de 80 puntos. (2) Aprobar todos los informes de los trabajos prácticos. (3) Tener los finales aprobados de las materias correlativas. La nota final para aquellos alumnos que hayan promovido será elaborada en base al promedio de los 2 exámenes parciales, los informes de laboratorio y el desempeño en Trabajos Prácticos. Aquellos alumnos que hayan aprobado los trabajos prácticos pero que no hayan promovido deberán dar un examen final escrito, cuya nota de aprobación es de 60/100

PROGRAMA ANALÍTICO INTRODUCCIÓN A LA ECOLOGÍA ¿Qué es ecología? Niveles de organización. Método científico en ecología. Nociones generales de biología evolutiva. Métodos de muestreo y diseño de experimentos en ecología. Escalas espaciales y temporales. Problemas ecológicos actuales. FACTORES QUE LIMITAN LA DISTRIBUCIÓN DE LOS ORGANISMOS Recursos y condiciones. Temperatura. Salinidad. Radiación. CO2. H2O. Nutrientes. Espacio. Ectotermos y endotermos. Nicho ecológico. Aclimatación, migración, almacenamiento y letargo. Principales recursos para plantas y animales. Generalistas, especialistas, oportunistas y selectivos. Biomas. POBLACIONES Concepto de población. Atributos poblacionales. Composición de la población. Abundancia y rango de distribución, tamaño corporal y latitud. Densidad absoluta y relativa e índices de densidad. Censos. Curvas poblacionales. Métodos basados en marcado y recaptura y en la reducción del tamaño poblacional. Disposición espacial: al azar, regular y contagiosa. Distribución de Poisson y Binomial negativa. Demografía. Estadística vital. Tablas de vida y de fecundidad. Curvas de supervivencia. Tasas de reproducción, tiempo generacional y tasas de incremento. Distribución de edades. Valor reproductivo. Poblaciones con generaciones discretas y con solapamiento. Historias de vida. Plasticidad fenotípica.

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Esfuerzo reproductivo. Edad de la primera reproducción. Iteroparidad y semelparidad. Tamaño y número de crías. Senescencia. Dinámica poblacional. Densodependencia y densoindependencia. Competencia intraespecífica. Curvas exponencial y logística: teoría y ejemplos de poblaciones naturales y de laboratorio. Modelos que incorporan un retraso temporal. Regulación poblacional. Demografía humana. Relaciones interespecíficas. Distintos tipos. Competencia interespecífica. Modelo de Lotka y Volterra. Concepto de nicho y principio de exclusión competitiva. Efectos de los predadores sobre la población de presas. Ciclos predador-presa: hipótesis sobre sus causas. Modelo de Lotka-Volterra y derivados. Parasitismo: Micro y macroparásitos. Infección y enfermedad. Transmisión y distribución. Efecto del parasitismo sobre el hospedador individual y su población. Herbivoría. Relaciones positivas entre especies: comensalismo, simbiosis. Coevolución. ESTRUCTURA Y DESARROLLO DE LA COMUNIDAD Características de la comunidad. Clasificación y ordenación de las comunidades. Descripción de la composición de la comunidad. Indices de diversidad. Análisis de gradientes. Comunidad clímax. Mecanismos del proceso de sucesión. Organización de la comunidad. Influencia de la competencia y predación en la estructura de la comunidad. Cadenas alimenticias y niveles tróficos. Especies principales y especies dominantes. Control "top-down" y "bottom-up" de las tramas tróficas. Gremios. Estabilidad de la comunidad. Dinámica temporal de las comunidades: concepto de sucesión. Sucesión primaria y secundaria. Tipos de sucesión. Determinantes de la biodiversidad. Efectos del clima, heterogeneidad espacial y temporal, perturbaciones, productividad. FLUJO DE ENERGÍA Y MATERIA A TRAVÉS DEL ECOSISTEMA Flujo de energía y materia a través del ecosistema. Redes y cadenas tróficas. Productividad primaria. Productividad secundaria. Eficiencias de transferencia de energía entre niveles tróficos. ¿Qué limita el número de niveles tróficos? Factores que limitan la productividad primaria en ecosistemas terrestres y acuáticos. Factores que limitan la productividad secundaria en ecosistemas terrestres y acuáticos. Ciclos biogeoquímicos. Alteraciones de los principales ciclos biogeoquímicos. ECOLOGÍA DE PAISAJES Y REGIONES Desarrollo histórico. Conceptos de paisaje, región y ecosistema local. Modelo de parche-corredor-matriz. Mosaicos y gradientes. Patrones espaciales. Teoría jerárquica. APLICACIONES DE LA ECOLOGÍA DE POBLACIONES Manejo y explotación de recursos naturales. Rendimientos máximo sostenible. Modelos de explotación. Rendimiento económico óptimo. Declinación de la abundancia de ballenas y otros ”stocks” pesqueros. Control de plagas y malezas: control biológico, cultural, genético y químico. Manejo integrado de plagas. Pesticidas: efectos adversos y positivos sobre la plaga y otros organismos. Nivel de daño económico y de umbral de acciones. BIODIVERSIDAD Y CONSERVACIÓN Concepto de biodiversidad. Valor intrínseco y utilitario de la biodiversidad. ¿Cuántas especies existen? Patrones geográficos de distribución de especies. Relaciones especies-area. Biogeografía de islas y modelo del equilibrio. Biodiversidad y estabilidad de los ecosistemas. Tasas de extinción históricas y recientes. Principales causas de extinciones recientes. Poblaciones viables mínimas. Conservación de especies amenazadas. Fragmentación del hábitat y efecto de borde. Diseño de reservas. CONTAMINACIÓN EN ECOSISTEMAS ACUÁTICOS Y TERRESTRES Tipos principales de contaminantes en el ambiente: orígenes y fuentes de emisión, ingreso y dinámica en el ambiente. Niveles ecológicos de acción. Bioconcentración y biomagnificación. Evaluación y diagnóstico de la contaminación: parámetros físicos y químicos de referencia. Bioindicadores.

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Respuesta de la biota al estrés ambiental. Indices ecológicos para cuantificar el deterioro ambiental. Bioensayos. Bibliografía Begon M, Harper JL y Townsend CR (1996) Ecology: individuals, populations and communities.

Blackwell Sci., Oxford (Versión en español de la 2da. edición inglesa: (1990), Ed. Omega, Barcelona).

Caughley G (1977) Analysis of vertebrate populations. Wiley, New York. Dobson AP (1996) Conservation and biodiversity. Scientific American Library, New York. Forman RTT (1995) Land mosaics. The ecology of landscapes and regions. Cambridge Univ. Press,

Cambridge. Krebs CJ (1989) Ecological methodology. Harper Collins, New York. Krebs CJ (1994) Ecology: the experimental analysis of distribution and abundance. Harper Collins,

New York (Versión en español de la 3ra. edición inglesa: (1985), Ed. Pirámide, Madrid). Rabinovich JR (1980) Introducción a la ecología de las poblaciones animales. CECSA, Caracas. Ricklefs RE (1997) The economy of nature. W. Freeman & Co., New York (Versión en español:

Invitación a la ecología. Ed. Médica Panamericana, Buenos Aires). Smith, R. & Smith, T (2001) Ecología. 4ta. edición. Addison – Wesley. Madrid. Stiling PD (1996) Ecology: theory and applications. Prentice Hall, New Jersey. Townsend CR, Harper JL y Begon M (2000) Essentials of ecology. Blackwell Sci., Oxford.*

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ECOLOGIA GENERAL 2016 (1° Cuatrimestre). Cronograma ECOLOGIA GENERAL 2016 (1° Cuatrimestre). CRONOGRAMA Trabajos Prácticos

DÍA FECHA TRABAJOS PRÁCTICOS

Ma 15-mar

Vier 18-mar

Ma 22/23mar

(LABO) -Inscripción/presentación -Actividad noticias. -Diseño experimental. Hipótesis y predicciones -Explicación TP Recursos y Condiciones e inicio de trabajo experimental con Lemnas

Ju/Vier 24/25-mar FERIADO

Ma 29/30mar

(LABO + AULA) -Recuento de Lemnas (Semana 1). -Abundancia I: problemas, actividades de discusión -Introducción las áreas de estudio (Magdalena) -Muestreo: conceptos generales, técnicas (bosque/sotobosque) -Explicación/planificación de la salida campo Magdalena

Vier 1-abr Salida a la Reserva de Magdalena

Ma 5/6-abr

(LABO + AULA) -Recuento de Lemnas (Semana 2). -TP Abundancia II: técnicas II (animales) -Explicación/planificación de la salida campo RECN -Informe científico, actividades y Guía TP.

Vier 7/8-abr Salida de campo a la RECN

Ma 12/13-abr (LABO + COMPU) -Recuento de Lemnas (Semana 3) -Análisis datos de la salida de campo: armado de base de datos (MAGDALENA)

Vier 14/15-abr (COMPU) -Análisis datos de la salida de campo 2: vegetación, matecitos

Ma 19/20-abr

(LABO+COMPU) -Recuento de Lemnas (Semana 4). -Crecimiento poblacional: actividad sobre ideas previas, modelos, simulación, problemas

Vier 22-abr Salida de campo San Vicente (a confirmar)

Ma 26/27-abr (COMPU) -Crecimiento poblacional: fin problemas -Tablas de vida I (explicación-ejercicios).

Vier 28/29- abr (COMPU) -Tablas de vida II (ratones). -Cierre Lemnas.

ECOLOGIA GENERAL 2016 (1° Cuatrimestre). Cronograma ECOLOGIA GENERAL 2016 (1° Cuatrimestre). CRONOGRAMA Trabajos Prácticos

DIA FECHA TRABAJO PRACTICO

Ma 17/18may (LABO/AULA) REPASO

Vier 20may (COMPU) (AULA) PARCIAL

Ma 24/25may POR EL FERIADO del MIERCOLES 25 de mayo

Vier 26/27may

(LABO/AULA) -Atributos de las comunidades (Definiciones, Área Mínima, Diversidad e Índice Similitud).

Ma 31/1jun

(LABO/AULA) -Atributos de las comunidades (cierre ejercicios Guia TP - Costanera Sur), - Invasiones Biológicas I: Seminario

Vier 2/3-jun (COMPU) -Invasiones Biológicas II: parte I: TP ardillas

Ma 7/8-jun (COMPU) -Invasiones Biológicas II: parte II: finalización + discusión de video -Análisis resultados avistaje de aves RECN

Vier

9/10jun

(COMPU) -Ecología del Paisaje: Escalas + Gradiente urbano rural (GoogleEarth)

Ma 14/15-jun (COMPU) -Cierre Eco de paisaje -Cambio Climático (actividad ciclo carbono)

Vier 16/17-jun ( AULA) -Cierre Cambio Climático (video y discusión

Ma 21/22-jun (AULA) -Actividad sobre producciones escritas de la salida Magdalena (informe). -Actividad Evaluación (Preguntas de Magdalena)

Vier 23/24-jun (AULA) REPASO Ma 28-jun SEGUNDO PARCIAL Vier 1-jul Entrega notas 2º parcial Lu 4-jul Recuperatorio 1º parcial

juev 7 julio Recuperatorio 2º parcial

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Seguridad en laboratorios de docencia (

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ÍNDICE GENERAL DE TRABAJOS PRÀCTICOS – Parte 2

7. ATRIBUTOS DE LAS COMUNIDADES (1) Introducción (2) Guía de trabajo

8. SEMINARIO INVASIONES BIÓLOGICAS

9. INVASIONES BIOLÓGICAS

(1) Introducción (2) Actividades

10. ECOLOGIA DEL PAISAJE (1) Introducción (2) Actividades

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Trabajo práctico 7 ATRIBUTOS DE LAS COMUNIDADES INTRODUCCIÓN En los libros de texto de ecología se pueden encontrar varias definiciones de “comunidad”, que cubren un rango considerable de significados. Algunos consideran a la comunidad como “un ensamble de poblaciones de plantas, animales, bacterias y hongos que viven en un ambiente y que interactúan unos con otros, formando juntos un sistema viviente distintivo con su propia composición, estructura, relaciones ambientales, desarrollo y función” (Whittaker 1975). En el otro extremo, se la ha considerado como “cualquier conjunto de organismos que viven cerca unos de otros y acerca de los cuales es interesante hablar” (MacArthur 1971). Todas las definiciones, no obstante, concuerdan en que las comunidades son conjuntos de individuos de distintas especies que aparecen juntos en tiempo y espacio, y la mayoría destaca la importancia de las interacciones entre esas poblaciones. Varios autores, por otra parte, señalan la existencia de propiedades emergentes de las comunidades, atributos de estructura (e.g., la composición de especies) o de funcionamiento (e.g., el flujo de energía) que son característicos de este nivel de organización.

En alguna medida, las diferentes definiciones de comunidad son consecuencia de los distintos objetivos de los investigadores que las propusieron. Los ecólogos de plantas, que tratan con ensambles espacialmente fijos, a menudo enfatizan la descripción de tales asociaciones y sus cambios en el tiempo; los ecólogos de animales, confrontados con organismos móviles y activos, le dan más importancia a las interacciones y a las relaciones funcionales entre las especies. Algunos definen a la comunidad en términos de unidades de hábitat (e.g., las comunidades del intermareal), otros por categorías de formas de vida (e.g., comunidades herbáceas) o por taxonomía (e.g., comunidades de aves). Lo común a todas estas formas de definir a una comunidad es su valor operativo: todas se centran en una parte del conjunto total de especies que coexisten, pues es prácticamente imposible trabajar con el concepto original de comunidad (i.e., el conjunto de todos los individuos de todas las especies que viven en un determinado lugar). Es muy claro que la noción de comunidad, aún cuando se utilicen solo formas “operativas”, ha contribuido notablemente al desarrollo de nuestro entendimiento de la naturaleza (Wiens 1989).

Los atributos comunitarios más comúnmente utilizados por los ecólogos son los siguientes: • Diversidad específica: es una función de la riqueza específica (número de especies presentes) y

de la equitatividad (grado de uniformidad de las abundancias relativas de las especies). La variación conjunta de ambos componentes determina los cambios en la diversidad.

• Dominancia: no todas las especies tienen la misma influencia sobre la comunidad; las dominantes

pueden ejercer un mayor control sobre la estructura comunitaria. La dominancia puede estar dada por su abundancia, tamaño o actividad.

• Abundancia relativa: las abundancias relativas entre las distintas especies también permiten

describir a las comunidades. • Estructura trófica: las relaciones alimentarias entre las especies de la comunidad determinan el

flujo de materia y energía. • Interacciones entre especies: una de las ideas implícitas en el concepto de comunidad es que

existen determinadas asociaciones de especies (i.e., que éstas aparecen juntas más a menudo que lo que uno esperaría por azar). Estas asociaciones pueden deberse a las interacciones entre ellas, como en el caso de las mutualistas, o ser consecuencia de afinidades de su biología (e.g., requerimientos de hábitat similares).

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De estos atributos comunitarios, uno de los más estudiados históricamente por los ecólogos ha sido la diversidad. El concepto de diversidad ha provisto un marco teórico importante para el desarrollo de muchas especulaciones acerca de la estructura y el funcionamiento de las comunidades (Magurran 1988). Al mismo tiempo, el interés por este tema se ha incrementado debido a la creciente necesidad de comprender los factores que gobiernan los patrones globales de biodiversidad (Sala et al. 2000). OBJETIVOS El objetivo general del trabajo práctico es familiarizarse con el cálculo de algunos atributos comunitarios, con algunos aspectos del muestreo de comunidades y con la determinación de la diversidad y sus componentes.

Los objetivos específicos son: (1) Determinar el área mínima necesaria para estimar la riqueza específica de una comunidad. (2) Calcular la riqueza específica, la diversidad y la equitatividad de una comunidad. (3) Construir una curva rango-abundancia para una comunidad. (4) Evaluar el grado de similitud entre distintas comunidades. (5) Estimar similitudes y diferencias en los patrones de diversidad de varias comunidades y explorar las posibles causas. DESARROLLO Se utilizará un “mapa” de una comunidad imaginaria (ensamblar las cuatro hojas grilladas para obtener el “mapa”) . En el mapa se encuentran indicados (con distintas letras) los individuos de las distintas especies. El grillado facilita el muestreo, al mismo tiempo que cada cuadrado constituye una unidad muestreal mínima. 1.- Determinación del área mínima y de la riqueza específica de la comunidad Para evaluar el área mínima de muestreo necesaria para estimar la riqueza de la comunidad hay que realizar muestreos sucesivos de la riqueza en unidades de tamaño cada vez mayor (véase figura 1). En cada paso se duplica la superficie de muestreo, extendiendo la unidad anterior (Matteucci y Colma 1982). Las unidades muestrales sucesivas son: un cuadrado, dos cuadrados, cuatro cuadrados (2x2), ocho cuadrados, 16 cuadrados (4x4), 32 cuadrados, 64 cuadrados (8x8), etc. Comenzar el muestreo en alguna de las esquinas del borde izquierdo del mapa. Graficar el número de especies en función del tamaño del cuadrado. ¿Cuál es el tamaño que usted emplearía para determinar la riqueza de la comunidad y por qué? Examine cómo se modifica su resultado si hubiera comenzado el muestreo en alguna de las esquinas del borde derecho del mapa. ¿A qué lo atribuye? ¿Qué implicancias tiene esto para el muestreo de comunidades?

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Figura 1: Tamaños sucesivos de muestreador para la evaluación del área mínima.

57 58 59 60 61 62 63 64

49 50 51 52 53 54 55 56

41 42 43 44 45 46 47 48

33 34 35 36 37 38 39 40

25 26 27 28 29 30 31 32

17 18 19 20 21 22 23 24

9 10 11 12 13 14 15 16

1 2 3 4 5 6 7 8

2.- Estimación de la diversidad y de la equitatividad Para la diversidad, se utilizarán el índice de Shannon-Wiener (H) y el de Simpson (D). Utilizando un muestreador de 8 x 8, tomar 10 muestras al azar (elegir coordenadas x e y al azar, y colocar el extremo superior izquierdo del cuadrado muestral sobre la celda correspondiente; descartar el punto si no cabe el muestreador entero). En cada uno de los 10 casos, contar el número de individuos de cada una de las especies presentes, completando la tabla 1. Tabla 1. Número de individuos de cada especie por muestra (M), y suma de abundancias para cada especie.

spp. A B C D E F G H I J K L M N

M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 Suma

Sobre la base de la suma correspondientes a las 10 muestras, calcular los siguientes índices, utilizando la tabla 2:

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(a) Índice de Shannon-Wiener. Basado en la teoría de la información, predice cuál es la probabilidad de que un individuo en una muestra sea de la misma especie que el de la muestra anterior. S H = - ∑ (pi)(ln pi) i=1 donde H = contenido de información de la muestra (diversidad); S = riqueza específica; pi = proporción de individuos de la especie i respecto al total de individuos. Varía entre un valor mínimo de 0 y un máximo que depende de la riqueza específica (véase más abajo). (b) Índice de Simpson. Basado en la teoría de probabilidades. ¿Cuál es la probabilidad de que dos individuos tomados al azar pertenezcan a una misma especie? Si pi es la proporción de individuos de la especie i respecto al total de individuos, entonces en una muestra de dos individuos, la probabilidad de que sean los dos de la misma especie es pi * pi , o sea pi

2. Si se suman las probabilidades para todas las especies presentes, se obtiene el índice de Simpson: S D = 1 - ∑ (pi)2

i=1

Este índice otorga mayor peso a las especies abundantes que a las raras. Varía entre un valor mínimo de 0 (cuando todos los individuos pertenecen a la misma especie) y un máximo de (1 - 1/S) cuando los individuos se reparten equitativamente entre especies. (c) Equitatividad. El valor máximo de diversidad varía con el número de especies presentes. Usando el índice de Shannon-Wiener, para un S dado, el H será máximo cuando los individuos se distribuyan equitativamente entre las especies (i.e., cuando todos los pi sean iguales entre sí e iguales a 1/S). Reemplazando en la fórmula de H: S Hmáx = - ∑ (1/S)(ln 1/S) = - S(1/S)(ln(1/S)) = ln S i=1 Equitatividad = H/Hmáx = H/ln S Tabla 2. Valores utilizados para el cálculo de la diversidad y la equitatividad.

spp. Suma pi ln pi (pi)(ln pi) (pi)2

A B C D E F G H I J K L M N ∑

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3.- Construcción de la curva de rango-abundancia Utilizando los datos de abundancia relativa (pi) de cada una de las especies (tabla 2), se debe ordenarlas en orden decreciente de abundancia, asignándole rango 1 a la más abundante, 2 a la segunda, y así sucesivamente. Volcar los datos de abundancia relativa en función del rango en la figura 2. Figura 2. Curva de rango-abundancia. Abundancia relativa (pi)

Rango 4.- Estimación de la similitud entre comunidades Existen distintos índices que permiten comparar comunidades de a pares (Matteucci y Colma 1982, Crisci y López Armengol 1983). Estos índices pueden ser cualitativos o cuantitativos. Los primeros se basan solo en la presencia o ausencia de las distintas especies en las dos comunidades que se comparan, mientras que los cuantitativos utilizan información de la abundancia relativa de las especies. (a) Índice de Jaccard. Este índice cualitativo tiene en cuenta la relación entre el número de especies comunes a las dos comunidades que se comparan y el total de las especies en ambas comunidades. En la tabla 3 se muestra un esquema de los valores utilizados. Los valores oscilan entre 0 y 1. El índice es: Similitud = a / (a + b + c) (b) Índice de Sorensen. Este índice cualitativo concede mayor significación a las presencias conjuntas. Los valores oscilan entre 0 y 1. El índice es: Similitud = (2a) / (2a + b + c) Tabla 3. Esquema de los valores utilizados para calcular la similitud cualitativa entre dos comunidades A y B. En la tabla, a es el número de especies comunes a A y B, b es el número de especies exclusivas de B, c es el número de especies exclusivas de A, y d es el número de especies ausentes en ambas muestras simultáneamente.

Comunidad A Presente Ausente

Presente a b Comunidad B Ausente c d

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(c) Índice de Czekanowski. Este índice cuantitativo está basado en la menor abundancia de cada especie en las comunidades que se comparan. Los valores oscilan entre 0 y 1. El índice es: S Similitud = ∑ mín (pi1, pi2) i=1 donde pi1 = proporción de individuos de la especie i respecto al total de individuos en la comunidad 1; pi2 = proporción de individuos de la especie i respecto al total de individuos en la comunidad 2. Tabla 4. Abundancia de las especies de aves acuáticas en la Reserva Costanera Sur (C.S.) durante primavera, verano, otoño e invierno, y de las especies de aves acuáticas en humedales cercanos a Chascomús, Chasicó (al oeste de Bahía Blanca) y Mar Chiquita (en la costa, al norte de Mar del Plata). Los valores corresponden al número de individuos observados en censos estandarizados.

spp. C.S. primav

C.S. verano

C.S. otoño

C.S. inv

Chascomús Chasicó Mar Chiquita

Podiceps rolland 140 128 322 45 77 Podiceps major 69 63 28 23 Phalacrocorax olivaceus 29 19 43 12 36 Egretta thula 25 3 1 7 12 5 Bubulcus ibis 27 1 2 7 7 18 12 Plegadis chihi 19 1 6 25 12 Coscoroba coscoroba 25 14 21 Cynus melancoryphus 16 32 5 7 Anas georgica 10 2 11 23 12 Anas flavirostris 12 3 32 11 13 Anas platalea 9 2 12 2 17 Anas cyanoptera 8 2 8 1 Anas versicolor 9 3 25 15 33 Dendrocygna bicolor 15 5 13 32 Dendrocygna viduata 2 1 5 2 14 Netta peposaca 14 8 31 46 Heteronetta atricapilla 32 26 16 24 Oxyura vittata 23 24 Rallus sanguinolentus 1 5 1 1 6 2 Fulica rufifrons 25 26 17 44 124 87 Fulica leucoptera 125 145 98 65 155 107 Fulica armillata 168 58 529 426 198 98 Gallinula chloropus 18 10 6 35 Jacana jacana 7 17 1 12 23 Himantopus melanurus 6 20 1 9 15 45 Vanellus chilensis 5 12 2 4 2 8 Larus maculipennis 1 1 14 21 67 Larus dominicanus 1 1 48 Sterna trudeaui 3 1 19 Podiceps occipitalis 3 Anas sibilatrix 1 Anas bahamensis 3 Aramus guarauna 1 Charadrius falklandicus 25 Zonibyx modestus 12 Gelochelidon nilotica 1 Tringa flavipes 7 38 Charadrius collaris 5 25

Ecología General

21

En la tabla 4 se presentan datos correspondientes a las abundancias de especies de aves acuáticas en la Reserva Costanera Sur (Buenos Aires) en distintas épocas del ciclo anual y en otros humedales de la provincia de Buenos Aires. Usando dichos datos, calcule la similitud (con los tres índices descriptos arriba) entre la comunidad de aves acuáticas de Costanera Sur y las otras tres comunidades (para Costanera Sur utilice solamente los valores invernales, pues los datos de las comunidades de la provincia de Buenos Aires fueron tomados en dicha estación). Comparar los resultados obtenidos con los distintos índices. ¿Cuáles podrían ser las causas del grado de similitud observado entre las comunidades. 5.- Patrones de diversidad Los datos de Costanera Sur de la tabla 4 corresponden a cuatro períodos del ciclo anual, y fueron tomados en un año en el cual se produjo una inusual sequía. Los niveles de agua de las lagunas de la reserva (que dependen del régimen de precipitaciones y de las temperaturas) son máximos durante el invierno, y van disminuyendo hasta su mínimo durante el verano y el otoño subsiguiente. Durante el verano estudiado casi la totalidad de las lagunas estaban secas, dejando solo grandes extensiones barrosas. Para el otoño, aunque el nivel del agua era bajo, ya había una superficie anegada considerable. Calcular la diversidad (usando el índice de Shannon-Wiener), la riqueza de especies y la equitatividad correspondiente a los cuatro muestreos. Examine comparativamente los valores obtenidos. ¿De qué manera afectan la riqueza y la equitatividad a las estimaciones de diversidad? Construya la curva de rango-abundancia de cada muestreo. ¿Qué conclusiones puede alcanzar comparando las cuatro curvas? ¿Cómo se relaciona la forma de las curvas con los valores de diversidad y sus componentes? ¿Cuáles podrían ser las causas de los patrones observados? REFERENCIAS Crisci JV y López Armengol MF (1983) Introducción a la teoría y práctica de la taxonomía numérica.

Monografía 26, Serie de Biología. OEA, Washington DC. MacArthur RH (1971) Patterns of terrestrial bird communities. Pp. 189-221 en: Avian biology. Farner

DS y King JR (eds). Academic Press, New York. Magurran AE (1988) Ecological diversity and its measurement. Princeton University Press, Princeton. Matteucci SD y Colma A (1982) Metodología para el estudio de la vegetación. Monografía 22, Serie

de Biología. OEA, Washington DC. Sala OE, Chapin III FS, Armesto JJ, Berlow E, Bloomfield J, Dirzo R, Huber-Sanwald E, Huenneke LF,

Jackson RB, Kinzig A, Leemans R, Lodge DM, Mooney HA, Oesterheld M, LeRoy Poff N, Sykes MT, Walker BT, Walker M & Wall DH (2000) Global biodiversity scenarios for the year 2100. Science 287:1770-1774.

Whittaker RH (1975) Communities and ecosystems. Second edition. Macmillan, New York. Wiens JA (1989) The ecology of bird communities. Volume 1. Foundations and patterns. Cambridge

University Press, Cambridge.

Biological invasions are having a major impact on the Earth’s ecosystems [1], giving urgency to a betterunderstanding of the factors that affect them. Somerecent reviews have considered invasions from avariety of viewpoints, including the characteristics of invaders [2], the characteristics of invadedcommunities [3], resources [4,5] and natural enemies[6]. As these issues are not independent, it is essentialto find a means of considering them jointly. Towardsthis goal, a theoretical framework for invasion ecologybased on community ecology theory is proposed here.We show how this framework applies to the analysis

of the factors promoting invasion, and use it toexamine correlations between invasion resistanceand species diversity.

Invasion involves two essential stages: transportof organisms to a new location [7,8]; andestablishment and population increase in the invadedlocality [9]. A third stage, applicable to the mostworrisome invasions, is regional spread from initialsuccessful populations [10]. We focus on the secondstage, where community ecology theory has most tooffer. There is much evidence that the chance ofestablishment increases markedly with the rate ofarrival of an alien species at a potential invasion site[2]. However, for establishment and growth, a speciesmust be able to increase in abundance at the invadedlocality. This depends on the opportunities that theparticular invaded community provides for theinvader in question.

Niches and niche opportunities

Three main factors contribute to an invader’s growthrate: resources [4,5,11,12], natural enemies [7,13,14]and the physical environment [15,16],all of whichvary in time and space. How a species responds tothese factors, including their spatial and temporalvariation, determines its ability to invade. Once aninvader has achieved an appreciable density, it willhave effects on the invaded locality – for example, by consuming resources and maintaining naturalenemies. Such responses and effects are the twodefining aspects of an organism’s niche, according to a recent definition (Box 1). The response aspect of the niche is fundamental to an alien species’ability to invade, and the effect aspect is fundamental to the impact that the invader has in the invadedcommunity (Box 2). Both effects and responses ofresident species in a community determine whetherthat community provides opportunities for invasion –that is, whether it provides niche opportunities(Box 1). In simple circumstances, niche opportunitiescan reduce to either resource opportunities or naturalenemy escape opportunities.

TRENDS in Ecology & Evolution Vol.17 No.4 April 2002

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170 Opinion

Community ecology

theory as a

framework for

biological invasions

Katriona Shea and Peter Chesson

Community ecology theory can be used to understand biological invasions by

applying recent niche concepts to alien species and the communities that they

invade. These ideas lead to the concept of ‘niche opportunity’, which defines

conditions that promote invasions in terms of resources, natural enemies, the

physical environment, interactions between these factors, and the manner in

which they vary in time and space. Niche opportunities vary naturally between

communities but might be greatly increased by disruption of communities,

especially if the original community members are less well adapted to the

new conditions. Recent niche theory clarifies the prediction that low niche

opportunities (invasion resistance) result from high species diversity.

Conflicting empirical patterns of invasion resistance are potentially explained

by covarying external factors. These various ideas derived from community

ecology provide a predictive framework for invasion ecology.

Katriona Shea*

Dept of Biology,208 Mueller Laboratory,The Pennsylvania StateUniversity, UniversityPark, PA 16802, USA.*e-mail: [email protected]

Peter Chesson

Section of Evolution andEcology, University ofCalifornia, Davis,CA 95616, USA.

45 Fowler, S.V. et al. (1996) Comparing thepopulation dynamics of broom, Cytisusscoparius, as a native plant in the UnitedKingdom and France, and as an invasive alienweed in Australia and New Zealand. InProceedings of the Ninth InternationalSymposium of Biological Control of Weeds(Moran, V.C. and Hoffmann, J.H., eds),pp. 19–26, University of Cape Town

46 DeLoach, C.J. (1995) Progress and problems inintroductory biological control of native weeds inthe United States. In Proceedings of the EighthInternational Symposium on Biological Control ofWeeds (Delfosse, E.S. and Scott, R.R., eds),pp. 111–112, CSIRO Publishing

47 Hawkins, B.A. et al. (1999) Is the biologicalcontrol of insects a natural phenomenon? Oikos86, 493–506

48 Hosking, J.R. (1995) The impact of seed- and pod-feeding insects on Cytisus scoparius, InProceedings of the Eighth InternationalSymposium on Biological Control of Weeds(Delfosse, E.S. and Scott, R.R., eds), pp. 45–51,CSIRO Publishing

49 Straw, N.A. and Sheppard, A.W. (1995) The role ofplant dispersion pattern in the success and failureof biological control. In Proceedings of the EighthInternational Symposium on Biological Control ofWeeds (Delfosse, E.S. and Scott, R.R., eds),pp. 161–168, CSIRO Publishing

50 Southwood, T.R.E. et al. (1982) The richness,abundance and biomass of the arthropodcommunities on trees. J. Anim. Ecol. 51, 635–649

51 MacFarlane, R.P. and van den Ende, H.J. (1995)Vine-feeding insects of old man’s beard (Clematisvitalba), in New Zealand. In Proceedings of the

Eighth International Symposium on BiologicalControl of Weeds (Delfosse, E.S. and Scott, R.R.,eds), pp. 57–58, CSIRO Publishing

52 Burki, C. and Nentwig, W. (1997) Comparison ofherbivore insect communities of Heracleumsphondylium and H. mantegazziaum inSwitzerland (Spermatophyta: Apiaceae).Entomol. Gen. 22, 147–155

53 Syrett, P. and Smith, L.A. (1998) The insect faunaof four weedy Hieracium (Asteraceae) species inNew Zealand. New Zealand J. Zool. 25, 73–83

54 Hight, S.D. (1990) Available feeding niches inpopulations of Lythrum salicaria L. (purpleloosestrife) in the northeastern United States. InProceedings of the Seventh InternationalSymposium on Biological Control of Weeds(Delfosse, E.S., ed.), pp. 269–278, IstitutoSperimentale per la Patologia Vegetale


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171Opinion

Resource opportunitiesResource opportunities arise when the resources thata species needs are high in availability. This is notsimply because resources are supplied at a high rate,but also because the effects of resident species havenot reduced resource densities [17–20] or interferedwith access to resources too greatly [21].

In cases where a potential invader and a residentspecies are limited by a single resource, Tilman’sR* rule (where R is resource availability; Box 2)predicts that invasion will occur if the resident’s R*is greater than the invader’s R*. For example,invasion would result if the invader had a higher

resource acquisition rate than that of the residentspecies at the same resource densities (e.g. by havinga superior foraging technique; Box 2 [11]). Invasionwould also occur if the invader had a lowermaintenance requirement than that of the resident(Box 2). Simple resource limitation might beapproximated in the case of space or food competitionof a species invading the habitat of a very similarspecies [11,22]. Invader success, however, is alsopredicted for the case of limitation by multipleresources [23], as long as the invader always has a higher response (per-capita growth) than theresident. Most importantly, these conclusions are

Ecological niches are defined by the relationships betweenorganisms and the physical and biological environment,taking into account both time and space. A particularcombination of physical factors (e.g. temperature andmoisture) and biological factors (e.g. food resources andnatural enemies) at a particular point in time and spacedefines a point in niche space. A modern definition of aspecies’ ecological niche is the response that the specieshas to each point in niche space and the effect that thespecies has at each point [a]. Responses are defined interms of demographic variables, such as survival andindividual growth; but of most importance is the overalloutcome of these responses, the per-capita rate of increase.Effects include consumption of resources, interference withaccess to resources by other organisms, support of naturalenemies and occupancy of space.

Organisms respond to resource availability [b], which is the density of unused resource in the environment – soilwater content, for example. Resource supply [b] is the netrate at which resources enter the system, discounting useby the organisms present (for example, rainfall minusevaporation and runoff but not transpiration). Resourceavailability is the net resource result of the effects of all theorganisms in a system and the supply of the resource.

A resource opportunity is defined as a high availabilityof resources on which a potential invader depends.Resource opportunity includes the effects of mutualists,such as pollinators, because they provide services [c,d] that could also be considered resources. Similarly, a natural enemy escape opportunity is defined as a low levelor low efficiency of natural enemies to which invadersmight be susceptible.

A niche opportunity is the potential provided by a givencommunity for alien organisms to have a positive rate ofincrease from low density. This might occur because of aresource opportunity, an escape opportunity or because ofsome favorable combination of resources, natural enemiesand physical environmental conditions, including theirfluctuations in time and space (Fig. I). Low levels of nicheopportunities lead to invasion resistance of a community –that is, few alien species are able to successfully invade the community.

Maturity is the opportunity a system has had toaccumulate species, and for adaptation to the system tohave taken place. It depends on the time that the systemhas had the current climate, including its short-termfluctuations and recurring disturbance events. Maturitydepends also on the size of the species pool that hashistorically served as a source of species to the system [e].

References

a Chesson, P. (2000) Mechanisms of maintenance of speciesdiversity. Annu. Rev. Ecol. Syst. 31, 343–366

b Tilman, D. (1982) Resource Competition and CommunityStructure, Princeton University Press

c Richardson, D.M. et al. (2000) Plant invasion – the role ofmutualisms. Biol. Rev. 75, 63–93

d Mack, R. et al. (2000) Biotic invasions: causes, epidemiology,global consequences, and control. Ecol. Appl. 10, 689–710

e Huston, M. (1994) Biological Diversity, Cambridge University Press

TRENDS in Ecology & Evolution

Enemy escapeopportunities

CompetitorsResources

Specialist Generalist

Environment

Resourceopportunities

Invader

Fig. I. Main components of niche opportunity. Blue arrows arepositive effects on the invader; red arrows are negative effects.Arrows from the outer boxes to the inner box represent directeffects of the community on the invader. Arrows betweencommunity components represent indirect effects on the invader,and they are colored according to their indirect effect on the invader, which is opposite to their direct effect on the communitycomponent. The environment directly affects all components andmodifies their interactions with other components. Not shown arethe effects of specialist natural enemies of community members.These specialists will limit community members and so havepositive indirect effects on the invader. Sometimes generalistsmight not attack the invader, in which case their effects would be likethose of specialists on community members. (Weevil photographcourtesy of CSIRO, Australia.)

Box 1. Concepts and definitions

robust to environmental fluctuations [24] and are notrestricted to strict equilibrium scenarios, even thoughthe R* rule was first derived in that context (Box 2).Such uniform superiority of an invader would make it an invader of large effect, because it would depressor displace all resident species relying on the sameresources, with the details of the invasion dependingon the effect component of the invader’s niche (Box 2).

More generally, an invader would not beuniformly superior to any resident species, butinstead might have a superior response to aparticular resource, certain abundances of resources,or resources found in certain places or times [25]. Anysituations in which residents do not keep resourcesat uniformly low levels are a potential resourceopportunity [4,5] (Box 3).

In some situations, the effect aspect of a species’niche can also have a role in its invasion. Where aspecies has spatially localized effects, as in the case of plants, it will potentially have a strong effect on asmall spatial scale, benefiting itself more than otherspecies. For example, allelopathic effects of aninvader might reduce densities of other species,increasing resource availability to the invader [26].

Also, some species generate disturbance or alterdisturbance regimes, freeing resources and therebyfacilitating their own and other invasions [4,27–29].

Natural enemy escape opportunitiesEscape opportunities arise when natural enemies,such as diseases, predators and parasites, are in lowabundance or are less effective against new species[13,14]. Community ecology theory claims strongsymmetries between the effects of natural enemiesand the effects of resources on community dynamics[30–32]. Thus, parallel to the R* rule for resources,there is a P* rule for natural enemies (where Pindicates predator, pathogen or parasite density).Responses to natural enemies include elevatedmortality rates and reduced feeding rates, leading tolower per-capita growth rates. Effects of residents onnatural enemy densities are parallel to the effects ofresident species on resource availability. Althoughspecies can vary greatly in resource dependence,natural enemies vary greatly in their specificity [33].An invader might not be affected by specialistnatural enemies preexisting in the invadedcommunity and might gain a considerable advantagebecause it leaves its own specialist natural enemiesbehind or loses them early in the invasion processwhile at too low a density to maintain them [13,14].This potential forms the basis of biological control of invaders [7,8] (but see Keane and Crawley in thisissue [6]). Generalist natural enemies of the invadedcommunity, however, will have effects that vary withtheir ability to attack the invader [6]. A naive invadermight not be well defended against these enemies, in which case they reduce the escape opportunity of that invader [34]. However, generalists of theinvaded community might not be equipped to attackthe invader, in which case they increase the invader’sescape opportunity [6].

Interactions between the physical environment,resources and natural enemiesIn some studies, there has been a strong emphasis on the physical environment as a constraint oninvasions [15,16]. However, many species have broadenvironmental tolerances, and the interaction ofenvironmental factors with resources and naturalenemies has a potentially important role. Forexample, with plant species, higher temperaturescould mean higher evaporative water loss, lowerwater-use efficiency and, therefore, higher demandfor water as a resource [35]. Similarly, a harshphysical environment could lead to a highermaintenance requirement as a result of highermortality, higher biomass attrition rates (in plants) or higher metabolic costs (Box 2). A harsher physicalenvironment might therefore require higher resourceavailability to achieve the same capacity for increase.However, as both residents and invaders respond toenvironmental harshness, it is the difference in theresponse of the residents and invader that determines



172 Opinion

The per-capita growth rate of a population limited by a single resource is the neteffect of the gain from consumption of the resource, and the losses due tometabolism, tissue death (e.g. leaf fall and herbivory) and death of individuals. Thus:

(I)

where N is population density, R is resource availability, af (R) defines the gain (theresponse of the species to the resource), and m (the maintenance requirement) isthe total of all losses. The gain consists of the two parts: the constant a defining theoverall magnitude of the response to the resource and the function f (R) defininghow response to the resource changes with resource abundance. The dynamics ofthe resource are given by:

(II)

where S(R) is the supply rate of the resource, and g(R) the per-capita effect of thespecies on the resource. Multiplying g(R) by N gives the total effect of the speciesand determines the impact of the invader in the invaded community.

Normally, there would be a unique value R* of R at which equilibrium wouldoccur – that is, af (R*) = m. For an alien species to invade, it must have a smallervalue of R* than does a resident at equilibrium with the resource, because this isthe value of resource availability that the alien experiences when it arrives in thesystem. Only if the resident’s R* value exceeds that of the invader can the invaderhave a positive growth rate. This is Tilman’s R* rule [a]. An alien might have asmaller R* value by having a smaller maintenance requirement. Alternatively, itmight have a higher response than a resident [i.e. a larger value of af (R)]. Thismight be achieved by taking up the resource at a faster rate or by being moreefficient at converting that uptake into gain. Different responses for differentspecies might be modeled by having a vary with the species. In this case, R* wouldbe an increasing function of m/a, and an invader would be successful if it had asmaller value of m/a than that of a resident.

Although this R* theory was originally developed as an equilibrium theory [a], inthe form where species can differ in a or m, the theory is robust to fluctuations in Rand m, with the result being that an alien having a mean of m/a smaller than that of aresident can invade [b]. See Box 3 for situations in which R* theory does not hold.

References

a Tilman, D. (1982) Resource Competition and Community Structure, PrincetonUniversity Press

b Chesson, P. and Huntly, N. (1997) The roles of harsh and fluctuating conditions in thedynamics of ecological communities. Am. Nat. 150, 519–553

( ) ( )NRgRSdtdR

−=

( ) mRafdtdN

N−=

1

Box 2. Resource competition ideas

whether invasion is promoted or inhibited byharshness [24]. An invader will be at an advantage if its maintenance requirement does not increase as much as that of a resident with environmentalharshness, or if it has a stronger response toincreased resources than the residents (Box 2).

Environmental fluctuations in time and space also have major effects. The key issues again aredifferential effects of the fluctuations on invaders andresidents, with the added parameter that differentspecies might be favored at different times and indifferent places [4,5,25] (Box 3).

Environment–resource interactions naturallyhave parallels in environment–natural enemyinteractions, but one should also consider naturalenemy–resource interactions. Indeed, important

invader advantage might accrue as a result of theseinteractions, because residents and invaders could be differentially susceptible to specialist naturalenemies of the residents. In particular, high densitiesof specialist natural enemies should lead to lowerdensities of resident species, increasing resourceavailability [24,30,36]. Although generalist naturalenemies might have similar effects on resources asthe specialists, the increased resource availability for an invader could be countered by a decrease inescape opportunity [24,30]. The net outcome of all these different interactions determines themagnitude and nature of the niche opportunitiesprovided by the community.

How niche opportunities arise

Natural enemy escape opportunitiesMost communities provide escape opportunitiesbecause they do not have the specialist naturalenemies of invaders from geographically distantlocations [7]. Although invasions are sometimescompared with range expansions [5,37], escapeopportunities imply an important distinction,because specialist natural enemies must be lost atmuch lower rates during range expansions than theyare during invasions of relatively small numbersfrom great distances.

Invaders that offset losses to natural enemies in their native range by having high fecundity orindividual growth (grazed plants and clonal animals)[38] could gain a strong advantage in a systemwithout their specialist natural enemies. Potentiallydiminishing this advantage are generalist naturalistenemies to which the invader might be particularlyvulnerable in the invaded location because it has no evolutionary history with them and might not be adequately protected against them [34]. Thespectacular success of biological control for someinvaders, however, implies that loss of naturalenemies is sometimes an important escapeopportunity [7,39].

Escape opportunities can allow a species to win in resource competition in a similar way to a lowmaintenance requirement (Box 2), potentiallyallowing an invader to reach high densities and havea large effect on resources. Apparent competition can have a similar outcome. An invader with fewspecialist natural enemies could rise to a high density,maintaining generalist natural enemies that severelyimpact the native community.

Resource opportunitiesDisturbance is commonly assumed to releaseresources and provide opportunities for invaders [5],an idea that has been generalized to consider anyform of temporal variation in resource availability[4,5]. As emphasized by spatio-temporal resourcecompetition theory (Box 3) an invader still must have some advantage over residents. However, thatadvantage might occur at particular times or in



173Opinion

Community ecology theory predicts that spatial and temporal environmentalvariation has an important role in species coexistence [a]. For invaders, this canmean spatial and temporal niche opportunities, provided that invaders andresidents differ in their responses to varying factors. Most species have periods ofrelative activity and relative inactivity during a year [b]. Niche opportunities ariseduring times when resident species are relatively inactive and are not placing highdemands on resources. Using the notation given in Box 2, the per-capita growthrate of a species might be represented as:

[I]

with a now a function of time to represent temporally varying growth activity: it is theresponse of the organism to time. The theory of the storage effect [b] shows that aspecies can invade even if it has an average value of a(t) less than that of residentspecies, provided that the invader’s a(t) is sufficiently large compared with residents’a(t)s some of the time; for example, the activities of the invader and residents mightfluctuate out of phase. Fluctuations in a(t) can be deterministic (e.g. seasonal) orstochastic (e.g. in response to yearly weather variation) [b,c]; the key feature is that theinvader must show a different temporal pattern of response than that of the residents.

An invader might also gain an advantage by responding differently to changesin resource levels than do residents – that is, by having a function f (R) that differsnonlinearly from that of residents [a,d]. For example, the invader might have astronger response than residents at both high and low resource availabilities, butnot at intermediate resource availabilities. Resource fluctuations between high andlow values would then be a resource opportunity for the invader, a mechanismreferred to as ‘relative nonlinearity of competition’ [a].

Spatial variation can also provide niche opportunities, and this can occur throughspatial versions of the storage effect and relative nonlinearity of competition [e].Much emphasized in the literature, however, are competition–colonizationtradeoffs [f,g], which can work on a variety of scales. At the level of a localcommunity, the mechanism can be driven by disturbance. In sessile communities,death of an individual is a substitute for disturbance. Most importantly, thismechanism means that an inferior competitor for resources could invade if it hadsuperior colonization or local resource exploitation ability [g].

References

a Chesson, P. (2000) Mechanisms of maintenance of species diversity. Annu. Rev. Ecol.Syst. 31, 343–366

b Chesson, P. et al. (2001) Environmental niches and ecosystem functioning. In FunctionalConsequences of Biodiversity (Kinzig, A. et al., eds), pp. 213–245, Princeton University Press

c Lehman, C.L. and Tilman, D. (2000) Biodiversity, stability, and productivity incompetitive communities. Am. Nat. 156, 513–552

d Armstrong, R.A. and McGehee, R. (1980) Competitive exclusion. Am. Nat. 115, 151–170e Chesson, P. (2000) General theory of competitive coexistence in spatially-varying

environments. Theor. Popul. Biol. 58, 211–237f Tilman, D. (1994) Competition and biodiversity in spatially structured habitats. Ecology

75, 2–16g Bolker, B. and Pacala, S. (1999) Spatial moment equations for plant competition:

understanding spatial strategies and the advantages of short dispersal. Am. Nat.153, 575–602

( ) ( ) mRftadtdN

N−=

1

Box 3. Resource opportunities in fluctuating environments

particular places, or it might be in a life-history trait,such as colonizing ability (Box 3). Sher and Hyatt [4]emphasize that an advantage to invaders commonlyarises through disruption of the historical pattern ofresource supply and consumption.

There are many ways in which human activitiesdisrupt historical patterns of resource fluctuations,including alteration of patterns of fire [40], harvestingof biomass, nutrient enrichment [12], alteration ofpatterns of spatial heterogeneity, and climate change[1,41,42]. Resident species might not be adapted tothe changed environmental conditions, lesseningtheir ability to reduce resource availability uniformlyin time and space, and thus providing resourceopportunities for invaders. Some invaders, such asspecies that inhabit human disturbed environmentsin their native range, might have critical adaptationsto human disturbed environments that residentspecies lack [43,44], giving them the advantages theyneed for successful invasion.

Community maturityDo natural communities vary in the nicheopportunities that they provide, independently of thedisruptions discussed above? Particular systems andgeographical regions have relatively high rates ofinvasions. Among these are freshwater systems,oceanic islands, regions with a Mediterranean

climate, and North America in comparison withEurasia [29]. Greater disruption by humans andgreater rates of commerce between geographicallysimilar regions might contribute to some of theseelevated rates, but particular features of thecommunities themselves could also contribute [45].Species in different systems might vary incompetitive ability [11,26] or their degree ofspecialization [29] – that is, the breadth of conditionsunder which individual members have positiveresponses to their environment. In the presence oftradeoffs that benefit specialization [46], severalspecialized species would reduce resources moreeffectively than one generalist species covering thesame range of circumstances.

The maturity concept (Box 1) might explain suchcommunity differences: communities that have had less time to assemble, and less time for theirconstituent species to adapt to the local conditions,are likely to have fewer species with broader niches.Their species might also have lower competitiveabilities than those in communities that have had a longer time under their present environmentalregime. These communities tend to be less invasionresistant (Box 1). Similar effects on invasionresistance might result from the size of the speciespool from which a community has assembled.Maturity undoubtedly also affects invasion resistancethrough escape opportunities, but clear predictions inthis area are not so apparent.

Variation in niche opportunities with resident diversity

Maturity is one theoretical approach to explaininginvasion resistance. Ideally, invasion resistance couldbe predicted from directly observable communityproperties. In a classic work, Elton [47] proposed that communities with high species diversity shouldbe invasion resistant. Indeed, models and someexperimental studies suggest that high speciesdiversity does lead to invasion resistance – that is,there is a negative relationship between invasionsuccess and species diversity [48–51]. However, large-scale observational patterns mostly show that morediverse systems tend to have higher numbers of exoticspecies [3,51,52]. There have been various attemptsto understand these conflicting patterns.

Scale and the role of covarying factorsSpecies diversity varies widely with physicalextrinsic factors, such as latitude, climate (givenlatitude), soils and the supply rates of physicalresources to a system. This observation can helpexplain the discrepancy between different studies. Ifextrinsic factors favorable to high species diversityalso lower invasion resistance, the positiverelationship between species diversity and invasionsuccess seen on broad spatial scales is explained [50].A negative pattern of invasion success as a functionof diversity, for fixed extrinsic conditions, isconsistent with this proposal, as illustrated in Fig. 1:



174 Opinion


Number of native species

Num

ber

of e

xotic

spe

cies

Fig. 1. Reconciliation of relationships between invasion success andspecies richness on different spatial scales. In this illustration, extrinsicconditions are assumed to be the same within each cluster of pointsbut to differ between clusters. Within any cluster, higher numbers ofnative species lead to poorer niche opportunities for invaders,generating the negative relationship between the numbers of alienspecies and native species often observed in models, experiments and at small spatial scales. However, extrinsic factors can varyconsiderably on broad spatial scales. If extrinsic factors that favor highnumbers of native species also directly increase niche opportunity forinvaders, changes in these extrinsic factors will lead to clusters ofpoints whose mean numbers of alien and native species are positivelyrelated, as depicted. Thus, there is an overall positive relationshipbetween alien and native species when the data are combined on abroad spatial scale.

the broad-scale positive relationship is the outcomeof combining data from a series of negativerelationships, where each negative relationshipcomes from different extrinsic conditions.

This reconciliation of conflicting patterns isconsistent with the outcome of models [51] in whichextrinsic factors are not generally varied. Speciesdiversities differ because of different sizes of thespecies pool colonizing the local system, or differentamounts of time for species to accumulate in the localsystem, or simply due to chance: randomness in theprocess of community assembly leaves some systemswith fewer species than others. In all these ways of varying species diversity, there is a consistenttendency in models for invasion success to decreasewith species diversity. Generally, equivalent resultshave been obtained by experimental studies thatcarefully control extrinsic factors, or randomize them,and define species diversity as the number of speciessupplied to the system [53]. By contrast, studies thatdefine species diversity as the number of speciessuccessfully established might confound uncontrolledvariation in extrinsic factors, including propagulepressure. According to Fig. 1, a positive relationshipbetween invasion success and diversity could result.

The role of positive interactionsAn alternative explanation of positive relationshipsbetween species diversity and invasion success, whichhas some support from experiments in agriculturalsystems, is that high species diversity creates nicheopportunities – for example, by mutualisms, both

direct and indirect, that facilitate the entry of otherspecies [54]. The modeling and experimental studiesthat found negative relationships might not haveprovided the conditions for sufficiently beneficialmutualisms to occur, possibly explaining whyinvasion success did not increase with diversity.

The role of niche differentiationTheoretical explanations of why higher speciesdiversity might confer invasion resistance, whenextrinsic factors are controlled, depend on how theniches of the various residents and invaders relate toone another. According to the empty niche hypothesis[55], lower species diversity might simply mean theexistence of circumstances where resources are notbeing exploited efficiently because species withsuitable niches are lacking. Niche opportunitiestherefore exist for species able to benefit fromresources in those particular circumstances [7]. Inthis case, it should not be just the diversity of residentspecies that matters but how their niches differfunctionally [56], including their spatial and temporalpatterns of effects on resources [48].

At the opposite extreme is the samplinghypothesis, where species are not differentiatedfunctionally but vary in their ability to reduceresources [57], or (presumably) to maintain naturalenemies. According to this hypothesis, higherdiversity does not broaden the circumstances underwhich resources are exploited efficiently but insteadincreases the probability that a high-rankingcompetitor is present. Invasion success depends not on filling a vacant niche but on being a betterexploiter of resources or a better avoider of naturalenemies than resident species. However, acompetitor’s rank might vary in space and time,allowing a high diversity of species to be maintainedin the system [25] (Box 3). Thus, invader successdepends on finding a time or place where it issuperior to resident species, and the distinctionbetween the empty niche hypothesis and thesampling hypothesis is lost [57,58].

Conclusion

Over much of its history, invasion ecology hasdeveloped on a relatively separate path from otherareas of ecology, potentially to the detriment of thediscipline [59]. The framework presented here,however, shows how core issues of invasion ecologycan be discussed as topics in community ecology.Recent advances in community ecology theory havemade this possible by providing detailed predictionsabout topics of particular importance in invasionecology, such as resource and natural enemyinteractions [30,32], and disturbance and moregeneral kinds of variation in space and time[24,25,60]. The link between community andinvasion ecology is a natural one, because theessential criterion for a species to persist in acommunity is its ability to increase from low density,



175Opinion

The niche opportunity framework raises many questions and provides manyavenues for new research. The following are some of the most immediate.•• Interactions between resource and escape opportunity: much of the recent

invasion literature emphasizes resources. However, an invader released fromnatural enemies would have a low maintenance requirement and therefore a lowR* value. It would thus be a strong competitor. Hence, the cause of the invasionmight appear to be a resource opportunity when, in fact, it is an escape opportunity.Studies of the interaction between natural enemies and competition in the nativerange of the invader, and in the invaded community, would resolve this issue.

•• Community maturity: the theoretical concept of community maturity has beenexplored implicitly through models of community assembly [a]. However, thereis a need for systematic studies of invasion resistance distinguishing the effectsof time, species pool, the number of established species and establishedfunctional diversity. Important challenges are to understand the accumulationsof natural enemies over time, their degree of specialization and their interactionswith resources.

•• Covarying extrinsic factors in field studies: covarying extrinsic factors easilyconfound the relationship between invasion resistance and diversity. There is aneed for field techniques to control for extrinsic factors. Methods of data analysisaccounting for covarying extrinsic factors also need to be developed.

•• Improved understanding of coexistence mechanisms in nature: a betterunderstanding of how species coexist in natural systems would give a betterappreciation of the kinds of niche opportunity that a system might provide. Forexample, studies of changes in dominance patterns over time and in space couldtest whether species coexist by having different responses to spatially andtemporally varying environmental factors. This information would also revealthe kinds of changes to the system that might favor invasion.

Reference

a Morton, R.D. and Law, R. (1997) Regional species pools and the assembly of localecological communities. J. Theor. Biol. 187, 321–331

Box 4. Future directions

Acknowledgements

K.S. is a collaborator via a fellowship under theOECD Co-operativeResearch Programme:Biological ResourceManagement forSustainable AgriculturalSystems and alsoacknowledges theAustralian CRC for WeedManagement Systems,NMFS (grant toM. Mangel, UC Santa Cruz)and NIH/NSF Ecology ofInfectious Diseases grant(1R01ES11067-01) forsupport. P.C. wassupported by NSF grantDEB 9981926. We alsothank Ottar Bjørnstad,Ryan Keane, MarkLonsdale, Marc Mangel,Peter Moyle, Bill Murdoch,Marcel Rejmánek,Jay Stachowicz,Don Strong, Mark Torchin,Michael Turelli and MarkWilliamson for comments.

References

1 Vitousek, P.M. et al. (1997) Introduced species: a significant component of human-caused global change. New Zealand J. Ecol.21, 1–16

2 Kolar, C.S. and Lodge, D.M. (2001) Progress ininvasion biology: predicting invaders. TrendsEcol. Evol. 16, 199–204

3 Lonsdale, W.M. (1999) Global patterns of plantinvasions and the concept of invasibility. Ecology80, 1522–1536

4 Sher, A.A. and Hyatt, L.A. (1999) The disturbedresource-flux invasion matrix: a new frameworkfor patterns of plant invasion. Biol. Inv.1, 107–114

5 Davis, M.A. et al. (2000) Fluctuating resources inplant communities: a general theory ofinvasibility. J. Ecol. 88, 528–534

6 Keane, R.M. and Crawley, M.J. (2002) Exoticplant invasions and the enemy releasehypothesis. Trends Ecol. Evol. 17, 164–170

7 Mack, R. et al. (2000) Biotic invasions: causes,epidemiology, global consequences, and control.Ecol. Appl. 10, 689–710

8 Williamson, M. (1996) Biological Invasions,Chapman & Hall

9 Veltman, C.J. et al. (1996) Correlates ofintroduction success in exotic New Zealand birds.Am. Nat. 147, 542–557

10 Shigesada, N. and Kawasaki, K. (1997)Biological Invasions: Theory and Practice,Oxford University Press

11 Petren, K. and Case, T. (1996) An experimentaldemonstration of exploitation competition in anongoing invasion. Ecology 77, 118–132

12 Jefferies, R.L. (2000) Allochthonous inputs:integrating population changes and food webdynamics. Trends Ecol. Evol. 15, 19–22

13 Settle, W.H. and Wilson, L.T. (1990) Invasion bythe variegated leafhopper and biotic interactions:parasitism, competition and apparentcompetition. Ecology 71, 1461–1470

14 Torchin, M.E. et al. (1996) Infestation of anintroduced host, the European green crab,Carcinus maenas, by a symbiotic nemertean eggpredator, Carcinonemertes epalti. J. Parasitol.82, 449–453

15 Moyle, P.B. and Light, T. (1996) Fish invasions inCalifornia: Do Abiotic Factors DetermineSuccess? Ecology 77, 1666–1670

16 Sutherst, R.W. et al. (1999) CLIMEX: Predictingthe Effects of Climate on Plants and Animals,CSIRO Publishing

17 Tilman, D. (1982) Resource Competition andCommunity Structure, PrincetonUniversity Press

18 Davis, M.A. et al. (1998) Competition betweentree seedlings and herbaceous vegetation: supportfor a theory of resource supply and demand.J. Ecol. 86, 652–661

19 Murdoch, W.W. et al. (1996) Competitivedisplacement and biological control in parasitoids:A model. Am. Nat. 148, 807–826

20 Byers, J.E. (2000) Competition between twoestuarine snails: implications for invasions ofexotic species. Ecology 81, 1225–1239

21 Holway, D.A. and Suarez, A.V. (1999) Animalbehavior: an essential component of invasionbiology. Trends Ecol. Evol. 14, 328–330

22 Thompson, J.D. (1991) The biology of an invasiveplant. BioScience 41, 393–401

23 Leibold, M.A. (1998) Similarity and localcoexistence of species from regional biotas. Evol.Ecol. 12, 95–110

24 Chesson, P. and Huntly, N. (1997) The roles ofharsh and fluctuating conditions in the dynamicsof ecological communities. Am. Nat. 150, 519–553

25 Chesson, P. (2000) Mechanisms of maintenance ofspecies diversity. Annu. Rev. Ecol. Syst.31, 343–366

26 Callaway, R.M. and Aschehout, E.T. (2000)Invasive plants versus their new and oldneighbors: a mechanism for exotic invasion.Science 290, 521–523

27 Mack, M.C. and D’Antonio, C.M. (1998) Impacts ofbiological invasions on disturbance regimes.Trends Ecol. Evol. 13, 195–198

28 Simberloff, D. and VonHolle, B. (1999) Positiveinteractions of nonindigenous species. Biol. Inv.1, 21–32

29 Huston, M. (1994) Biological Diversity,Cambridge University Press

30 Holt, R.D. et al. (1994) Simple rules forinterspecific dominance in systems withexploitative and apparent competition. Am. Nat.144, 741–771

31 Holt, R.D. and Lawton, J.H. (1994) The ecologicalconsequences of shared natural enemies. Annu.Rev. Ecol. Syst. 25, 495–520

32 Grover, J.P. and Holt, R.D. (1998) Disentanglingresource and apparent competition: realisticmodels for plant–herbivore communities.J. Theor. Biol. 191, 353–376

33 Murdoch, W.W. et al. (1985) Biological control intheory and practice. Am. Nat. 125, 344–366

34 Mack, R.N. (1996) Biotic barriers to plantnaturalization. In Proceedings of the IXInternational Symposium on Biological Control ofWeeds (Moran, V.C. and Hoffman, J.H., eds),pp. 39–46, University of Cape Town

35 Larcher, W. (1983) Physiological Plant Ecology,Springer-Verlag

36 Grover, J.P. (1994) Assembly rules forcommunities of nutrient-limited plants andspecialist herbivores. Am. Nat. 143, 258–282

37 Thompson, K. et al. (1995) Native and alieninvasive plants: more of the same? Ecography18, 390–402

38 Mauricio, R. et al. (1997) Variation in the defense strategies of plants: are resistance andtolerance mutually exclusive? Ecology78, 1301–1311

39 Hawkins, B.A. and Cornell, H.V., ed. (1999)Theoretical Approaches to Biological Control,Cambridge University Press

40 D’Antonio, C.M. (2000) Fire, plant invasions andglobal changes. In Invasive Species in a ChangingWorld (Mooney, H.A. and Hobbs, R.J., eds),pp. 65–93, Island Press

41 Dukes, J.S. and Mooney, H.A. (1999) Does globalchange increase the success of biologicalinvaders? Trends Ecol. Evol. 14, 135–139

42 Mooney, H.A. and Hobbs, R.J., ed. (2000)Invasive Species in a Changing World,Island Press

43 Brown, J.H. (1989) Patterns, modes and extentsof invasions by vertebrates. In BiologicalInvasions: A Global Perspective (Drake, J.A. and Mooney, H.A., eds), pp. 85–110, John Wiley & Sons

44 Drake, J.A. and Mooney, H.A., eds (1989)Biological Invasions: A Global Perspective, John Wiley & Sons

45 Niemelä, P. and Mattson, W.J. (1996) Invasion ofNorth American forests by Europeanphytophagous insects. BioScience 46, 741–753

46 Rosenzweig, M.L. (1995) Species Diversity inSpace and Time, Cambridge University Press

47 Elton, C. (1958) The Ecology of Invasions byAnimals and Plants, Methuen

48 Stachowicz, J.J. et al. (1999) Species diversity andinvasion resistance in a marine ecosystem.Science 286, 1577–1579

49 Knops, J.M.H. et al. (1999) Effects of plant speciesrichness on invasions dynamics, diseaseoutbreaks, insect abundances, and diversity. Ecol.Lett. 2, 286–293

50 Naeem, S. et al. (2000) Plant diversity increases resistance to invasion in the absence of covarying extrinsic factors. Oikos91, 97–108

51 Levine, J.M. and D’Antonio, C.M. (1999) Eltonrevisited: a review of evidence linking diversityand invasibility. Oikos 87, 15–26

52 Stohlgren, T.J. et al. (1999) Exotic plant speciesinvade hot spots of native plant diversity. Ecol.Monogr. 69, 25–46

53 Levine, J.M. (2000) Species diversity andbiological invasions: relating local process tocommunity pattern. Science 288, 852–854

54 Palmer, M.W. and Maurer, T.A. (1997) Doesdiversity beget diversity? A case study of cropsand weeds. J. Veg. Sci. 8, 235–240

55 Simberloff, D. (1995) Why do introduced speciesappear to devastate islands more than mainlandareas? Pac. Sci. 49, 87–97

56 Dukes, J.S. (2001) Biodiversity and invasibilityin grassland microcosms. Oecologia126, 563–568

57 Crawley, M.J. et al. (1999) Invasion resistance inexperimental grassland communities: speciesrichness or species identity? Ecol. Lett.2, 140–148

58 Chesson, P. et al. (2001) Environmental niches and ecosystem functioning. In Functional Consequences of Biodiversity(Kinzig, A. et al., eds), pp. 213–245,Princeton University Press

59 Davis, M.A. et al. (2001) Charles S. Elton and the dissociation of invasion ecology from the rest of ecology. Div. Distrib. 7, 97–102

60 Bolker, B. and Pacala, S. (1999) Spatial moment equations for plant competition:understanding spatial strategies and theadvantages of short dispersal. Am. Nat.153, 575–602



176 Opinion

which is also the condition for an alien to be able to invade a community. This means that invasionecology has the potential to contribute greatly tocommunity ecology. Invasions provide case studies onparticular communities subject to perturbation by

invaders; they also provide the challenge ofexplaining invader success and invader impact.Indeed, there is every reason to expect a healthysynergy between invasion ecology and communityecology (Box 4).

TRENDS in Ecology & Evolution Vol.16 No.8 August 2001

http://tree.trends.com 0169–5347/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S0169-5347(01)02194-2

454 Review

Erika Zavaleta*

Harold A. Mooney

Dept of BiologicalSciences, StanfordUniversity, Stanford,CA 94305, USA.*e-mail:[email protected]

Richard J. Hobbs

School of EnvironmentalScience, MurdochUniversity, Murdoch,WA 6150, Australia.

Invasive alien species interact with other elements ofglobal change to cause considerable damage tomanaged and natural systems and to incur huge coststo society1. In response, several measures have beendeveloped and deployed to control, contain oreradicate a wide range of invasive species in affectedareas. Where possible, ERADICATION (see Glossary) isthe favored approach. Control, which reduces thepresence of the invader, and containment, whichlimits further spread, both require indefiniteinvestments of time, tools and money to keep aninvader at bay. Although eradication can require largeshort-term investments, successful removal can beachieved within months or years and gives the bestchance for native biodiversity to recover.

The results of eradication efforts so far areencouraging and have been detailed recently2. Manycase studies demonstrate success for a range of taxa,particularly on small islands and at local scales.Additional examples include the removal of the exoticlittle red fire ant Wasmannia auropunctata fromSanta Fe Island in the Galapagos3 (which resulted inthe increase in density of several native ant species),and the nearly complete removal from Laysan Island,Hawaii of the exotic annual grass Cenchrusechinatus, which once covered 30% of the vegetatedarea of the island (E.N. Flint, unpublished).Successful eradications often lead to dramaticrecovery of native species and ecosystems. Removal ofintroduced rabbits from Pacific islands off Mexico(C.J. Donlan, unpublished) and the USA have allowedrecovery of two rapidly declining endemic species ofnative succulents Dudleya linearis and D. traskiae4.Lowland vegetation on Santa Fe Island has recoveredsteadily following the removal of exotic goats Caprahircus nearly 30 years ago.

However, other cases suggest that more refined andintegrated approaches to invasive removal couldimprove results. Successes are still largely confined tosmall islands. The ecological context of eradication is

increasingly complex. Major damage caused by long-established invaders, systems that are affected bymultiple invaders, and systems that are affected byboth invaders and other global changes are nowcommon. In these settings, straightforward deploymentof standard eradication tools, such as poisons, trappingand mechanical harvesting, might not accomplish thedesired level of recovery of native ecosystems5.

We suggest that, although there is a crucial need forthe continued development and application of effectiveeradication methodologies, a parallel need exists toplace these methodologies in the context of the overallecosystem that is being managed. Ideally, there shouldbe both: (1) pre-eradication assessment, to tailorremoval to avoid unwanted ecological effects; and(2) post-removal assessment of eradication effects, onboth the target organism and the invaded ecosystem.

The requirements for successful removal of aninvader have been discussed recently2. We focus onthe possible impacts that result from the successfulremoval of invasive species, regardless of the methodsemployed to remove them. We reviewed recentliterature for examples where the successfuleradication of invasives had or was likely to haveimportant secondary impacts, a task that was madedifficult by the relatively few verified eradicationsuccesses that included the monitoring of post-removal system behavior.

Eradication: what can go wrong

Successful eradication efforts have generally benefitedbiological diversity. However, there is also evidence that,without sufficient planning, successful eradications canhave unwanted and unexpected impacts on nativespecies and ecosystems. These inadvertent impacts are ofmany types. Excessive poisoning of non-target organismsand transfer of poisons up food chains6 are problems thatcan result from the removal method used7,8. Someeradication efforts fail because they do not eliminate thetarget organism, because they either miss individuals ordo not include steps to reduce post-eradicationsusceptibility to reinvasion3. Eradication alone might notallow ecosystems to recover, because some invaderschange the condition of the habitat so as to render itunsuitable for native species. For instance, in sites fromthe Middle East to the western USA, high soil salinity iscaused by the invasive ice plant Mesembryanthemumcrystallinum, and tamarisk Tamarix spp., which makesit difficult for salt-sensitive native species to re-establish9.In these cases, eradication must be followed by additionalsite restoration.

Eradications of invasive species often have striking positive effects on native

biota. However, recent research has shown that species removal in isolation

can also result in unexpected changes to other ecosystem components. These

secondary effects will become more likely as numbers of interacting invaders

increase in ecosystems, and as exotics in late stages of invasion eliminate

native species and replace their functional roles. Food web and functional role

frameworks can be used to identify ecological conditions that forecast the

potential for unwanted secondary impacts. Integration of eradication into a

holistic process of assessment and restoration will help safeguard against

accidental, adverse effects on native ecosystems.

Viewing invasive species removal in a

whole-ecosystem context

Erika S. Zavaleta, Richard J. Hobbs and Harold A. Mooney

Successful eradications can also have undesiredeffects that result from the successful removal of theinvader. In several cases, removal of one exoticspecies has led to the establishment or increase of oneor more other invasive species. For example, severaleradications of exotic herbivores have been linked to

increases in exotic plant populations. Removal of oneinvader can lead to increased impacts of anotherinvader; for example, when removal of exotic preyleads to increased predation on native prey by exoticpredators10. Finally, removal of invasive plant speciescan reduce habitat or resources available for nativefauna if the removal is not accompanied by furtherrestoration measures (Box 1). These unexpectedoutcomes will become more probable both as thevariety of interacting invaders contained in anecosystem increases, and as exotics in late stages ofinvasion largely or wholly eliminate native speciesand replace their functional roles. Althoughresearchers have begun to explore the implications ofmultiple, interacting invaders, little attention hasbeen paid to the implications of these interactions foreradication efforts.

Secondary effects: a conceptual framework

A useful basis from which to tackle when and whysecondary effects of eradication occur is that systemscontaining invasives function according to the samebasic principles as do other systems. Invaded systemscan, therefore, be considered using the frameworksthat are usually used to analyze community andecosystem dynamics.

Trophic cascades in multiply invaded systemsA large literature has been devoted to how food-webinteractions limit populations of producers, consumersand predators11–13. Much work has been done on therelative roles of top-down regulation of food-webcomponents by higher-level consumers or predators,and of bottom-up regulation of populations by foodavailability or resource limitation. Evidence fromseveral ecosystem types shows that both top-down andbottom-up population regulation of producers andconsumers occur under some conditions14–16. Theexistence of these regulatory links can give rise toTROPHIC CASCADES16,17 (but see Ref. 13).

When combined with the use of simple terrestrialfood webs6 (Fig. 1), this framework helps to explain howmany animal eradications have allowed populationrecovery of native species. Removal of an exoticpredator can release native prey from strong top-downregulation, increasing prey abundance with potentialcascading impacts on other food-web components,including native predators (Fig. 1b). Similarly, exoticherbivores in the absence of predators can becomesufficiently abundant to exert top-down pressure onnative plants14. Removal of these herbivores can lead torapid recovery of native plant populations4.

Predator–prey interactionsHowever, the presence of multiple invaders atdifferent trophic levels complicates matters. Considerthe case where an exotic predator and an exotic preyspecies co-occur (Fig. 1c). Removal of the invasivepredator only could lead to MESOPREDATOR RELEASE

(release of the invasive prey from top-down



455Review

Exotic saltcedar Tamarix spp. shrubs have replaced much of the native riparianvegetation of the arid western USA, where they consume large quantities ofwater, narrow river channels, salinize soil and degrade wildlife habitata.

Saltcedar removal has been repeatedly delayed in parts of its rangebecause it provides significant nesting habitat for an endangered nativesongbirdb. The southwestern willow flycatcher Empidonax trailii extimus,currently reduced to fewer than 500 breeding pairs, nested historically inriparian cottonwood (Populus spp.)–willow (Salix spp.) stands in thesouthwestern USA (Refs c,d). Urbanization, agriculture, fire, waterdiversion and livestock grazing all contributed to the decline of its nativehabitatb. The replacement of much of the habitat that remained by saltcedarrequired the flycatcher to make use of the invader, which it seems to preferin some areas, despite its reduced breeding successe,f.Stepwise saltcedar removal could strongly benefit the flycatcher by givingnative trees the opportunity to re-establish and provide replacementhabitatg. However, some saltcedar-invaded areas might no longer be ableto support native vegetation, because lowered water tables and salinesoils, the results of saltcedar dominance, might complicate native re-establishmenth–j. Region-wide flood suppression hinders re-establishment of flood-associated native species such as cottonwoodsand increases the likelihood of saltcedar reinvasionj,k.

Managers are confident that, if accompanied by planning and carefulrestoration, saltcedar removal can benefit the endangered flycatcher as wellas other native speciesg. However, poorly planned removal without stepssuch as flooding and vegetation restoration, might fail, harming anendangered species in the process.

References

a Zavaleta, E.S. (2000) Valuing ecosystem services lost to Tamarix invasion in the UnitedStates. In Invasive Species in a Changing World (Mooney, H.A. and Hobbs, R.J., eds), pp. 261–300, Island Press

b USFWS (1997) Endangered and threatened wildlife and plants; final determination ofcritical habitat for the southwestern willow flycatcher. Fed. Reg. 62, 39129–39147

c Rosenberg, K.V. et al. (1991) Birds of the Lower Colorado River Valley, University of ArizonaPress

d Sogge, M.K. et al. (1997) A Southwestern Willow Flycatcher Natural History Summaryand Survey Protocol, National Park Service

e DeLoach, C.J. et al. (1999) In Ecological Interactions in the Biological Control of Saltcedar(Tamarix sp.) in the US: Toward a New Understanding, US Department of Agriculture

f McKernan, R.L. and Braden, G. (1999) Status, Distribution, and Habitat Affinities of theSouthwestern Willow Flycatcher Along the Colorado River; Year 3 – 1998, US Dept of theInterior–Bureau of Reclamation

g Dudley, T.L. et al. (2001) Saltcedar Invasion of Western Riparian Areas: Impacts andNew Prospects for Control, US Department of Agriculture

h Jackson, J. et al. (1990) Assessment of the Salinity Tolerance of Eight Sonoran DesertRiparian Trees and Shrubs, US Dept of the Interior–Bureau of Reclamation

i Shafroth, P.B. et al. (1995) Effects of salinity on establishment of Populus fremontii(cottonwood) and Tamarix ramosissima (saltcedar) in southwestern United States.Great Basin Nat. 55, 58–65

j Taylor, J.P. and McDaniel, K.C. (1998) Restoration of saltcedar infested flood plains onthe Bosque del Apache National Wildlife Refuge. Weed Technol. 12, 345–352

k Stromberg, J. (1998) Dynamics of Fremont cottonwood (Populus fremontii) and saltcedar(Tamarix chinensis) population along the San Pedro River, Arizona. J. Arid Environ. 40,133–155

Box 1. When a harmful exotic harbors an endangered native species

regulation) (Fig. 1d). If the exotic prey consume nativespecies, the removal of the exotic top predator couldlead to net negative impacts on native populations ofconservation value18. For example, exotic cats onStewart Island, New Zealand, prey upon the kakapoStrigops habroptilus, an endangered flightless parrot.However, the diet of the cats consists overwhelminglyof the three species of exotic rats on the island19. Cateradication would probably increase the impact ofrats on the kakapo as well as on other native biotaunless rats were simultaneously removed. Thepotential for mesopredator release following cateradication is widespread. Introduced rats Rattusspp., house mice Mus musculus, and/or rabbitsOryctolagus cuniculus co-occur with exotic cats on 22islands where the diets of cats have been studied. Innearly every case, cats exert important top-downcontrols on these other exotics by preying heavily onrabbits if they are present, and heavily on rats ifrabbits are not present20 (Table 1). Mice are also animportant part of the diet of feral cat on islands attemperate, but not tropical, latitudes20. The potentialfor these trophic effects is probably strongest onislands lacking native predators; however, it applies,in principle, to any system in which exotic predatorpopulations take advantage of abundant exotic prey.

The effects of mesopredator release can cascade toalter ecosystem-scale properties as well as alteringnative populations. Studies before cat eradication onsubantarctic Marion Island showed that the cats ate

many exotic house mice, which prey heavily upon aflightless endemic moth Pringleophaga marioni,which is important to nutrient cycling21–23. Removalof the cats only might have allowed increases inmouse populations, causing cascading declines inendemic moth abundance and, ultimately, changes insoil nutrient availability.

When exotic predators and prey co-occur,eradication of only the exotic prey can also causeproblems by forcing the predator to switch to nativeprey. In New Zealand, introduced rats R. rattus andpossums Trichosurus vulpecular are an importantpart of the diet of the stoat Mustela ermina, an exoticmustelid10. Efforts to remove all three species bypoisoning the prey species had an unexpected result:the stoat populations were not eliminated by either the prey eradication or the poison application and, in the absence of abundant exotic prey, the stoatsswitched their diets to native birds and bird eggs.

Without prey eradication, the co-occurrence ofexotic predators and exotic prey can impact heavily onnative prey populations by HYPERPREDATION. Theavailability of abundant exotic prey can inflate exoticpredator populations, which then increase theirconsumption of indigenous species24. Thisphenomenon was first elaborated to explain whynative Australian mammals suffered populationdeclines in areas invaded by cats only if exotic rabbitand mouse densities were also high25. The removal ofexotic prey to curb hyperpredation of native speciesby exotic predators has been suggested26. However,managers must consider carefully whether nativepopulations can withstand further, temporaryincreases in predation when the inflated predatorpopulation no longer has exotic prey to sustain it.

Herbivore–plant interactionsWhen exotic herbivores and plants co-occur (Fig. 1d),control or eradication of only the exotic plants could,in theory, lead to increased exotic herbivory on nativeplants. However, we do not know of a case in whichthis has occurred. This might reflect the paucity ofsuccessful plant eradications, the prioritization of



456 Review


Plant 1 Plant 2 Plant 3 Plant 4

Consumer 1 Consumer 2 Consumer 3

Predator 1 Predator 2 Predator 3




Plant 3 Plant 4






(a)

(b)

(c)

(d)

(e)

(f)







Plant 1 Plant 2

Fig. 1. Idealized food websindicating trophicinteractions betweenspecies. Closed boxesrepresent exotic speciesand open boxes representnative species. Arrowthickness indicates thestrength of trophicinteraction. Font sizerepresents populationsize. (a) shows acommunity containing asingle exotic predator. In(b), removal of thispredator increases nativeprey populations.(c) shows a communitycontaining both an exoticpredator and an exoticherbivore. In (d), removalof only the exotic predatorreleases the exoticherbivore population, withcascading impacts on twoplant species. (e) shows acommunity containingboth an exotic herbivoreand an exotic plantspecies. In (f), removal ofthe exotic herbivore onlyreleases the exotic plantpopulation.

Table 1. Importance of exotic rats in the diet of introduced cats on islandsa

Islands without Occurrence of Islands with Occurrence of

introduced rabbits rats in diet (%) introduced rabbitsb rats in diet (%)

Galapagos: Isabela 73 Gran Canaria 4Santa Cruz 88 Te Wharau, NZ 3Lord Howe 87 Kourarau, NZ TraceRaoul 86 Orongorongo, NZ 50Little Barrier 39 Mackenzie, NZ 2Stewart 93 Kerguelen 0Campbell 95 Macquarie 3aData from Ref. 20.bAbbreviation: NZ, New Zealand.

animal removals from multiply invaded ecosystems,or an absence of strict bottom-up regulation of exoticherbivores by plant biomass availability.

When exotic herbivores and plants co-occur,eradication of the herbivores only can lead to release ofexotic plants from top-down control (Fig. 1f). In nearlyall documented cases where exotic plants co-occur withexotic herbivores on islands, herbivore removal has hadmixed results for native vegetation (but see Refs 27,28).Feral herbivore removal from Santa Catalina Island,Channel Island National Park, led to an increase innative species richness, but also to large absolute andrelative increases in cover by exotic annuals29. Rabbiteradication on Round Island, Mauritius, led to strongrecovery of three endemic or locally restricted treespecies (Latania loddigesii, Pandanus vandermeerschiiand Hyophorbe lagenicaulis) and six reptile species [twoskinks (Leiolopisma telfaririi and Scelotes bojerii), threegeckos (Phelsuma guentheri, P. ornata and Nactusserpensinsula) and a snake (Casarea dussumerii)],including five endemics30. However, rabbit removal alsocaused the spectacular release of a previously sparseexotic grass Chloris barbata, rendering it a significantcomponent of the vegetation on the island30 (Box 2).Asiatic water buffalo Bubalus bubalis eradication fromKakadu National Park, Australia spurred large-scaleregeneration of the wetlands of the park31. However,alien plant species also proliferated, in particular,introduced para grass Brachiaria mutica, which nowcovers approximately 10% of the major floodplainhabitats in the park.

Although the removal of feral pigs Sus scrofa,sheep Ovis aries and goats has allowed some nativeplant species to recover slightly in Hawai’i32, manyHawai’ian lowland grasslands have responded to

ungulate removal with increases in the cover offlammable exotic grasses33. Accompanying increasesin fire frequency accelerate a positive feedback loopamong invasive grass establishment, fire, and loss ofnative woodlands and forest34.

The effects of exotic herbivore removal on nativevegetation, under certain circumstances, might alsohave indirect negative effects, because of the presenceof other exotic animals. Rabbit removal on MacquarieIsland in the Southern Ocean led to major increasesin cover by a native tussock grass Poa foliosa, which isthe preferred habitat of the introduced ship rat.Tussock expansion could bring the rats into contactwith burrow-nesting bird colonies on the island,which have escaped rat predation so far35.

Herbivore removal from islands has strong negativeeffects on vegetation in some cases. The removal ofsheep and cattle Bos taurus from Santa Cruz Island ledto an explosive expansion of exotic fennel Foeniculumvulgare, starthistle Centaurea solstitialis, and otherintroduced herbs, increases in relative cover of exotics,but the observable recovery of only one native species,Bishop pine Pinus muricata, after nine years ofmonitoring36–38. Moreover, the sudden expansion ofexotic forbs provided abundant food for feral Europeanbee Apis mellifera, colonies, and complicated eventualbee eradication from the island39. The greatest potentialfor negative impacts on native vegetation perhapsexists when herbivore eradication removes thedisturbance that is necessary to suppressestablishment of late successional (tree or shrub)exotics40. The removal of feral cattle from degradedgrasslands on San Cristobal Island in the Galapagosallowed previously suppressed exotic guava Psidiumguajava to grow rapidly into dense, extensive thickets41.



457Review

Before their extinction, two species ofgiant tortoise (Geocholone triserrata andG. inepta), endemic to the MascareneIslands, browsed the native vegetationand dispersed fruits of endemic trees suchas the Ile aux Aigrettes ebony Diospyrosegrettarum. Trade in tortoise meat,together with the introduction of rats andpigs in the 16th–18th centuries, extirpatedthe native browsers from the archipelago.

Introduced goats Capra hircus andrabbits Oryctolagus cuniculus replaced thetortoises as herbivores, suppressingnumerous introduced grazing-intolerantplant species until the late 20th century.However, the eradication of exoticherbivores from Round Island and Ile auxAigrettes in the 1970s and 1980s releasedpopulations of exotic weeds such asChloris barbata on Round Island and falseacacia Leucaena leucocephala on Ile aux

Aigrettes. Native tussock-forming grassesdeclined on Round Island, and increasinglytall exotic vegetation threatened low-growing endemics such as Aervacongesta, now found only on RoundIsland.

To restore and maintain nativevegetation, scientists at the MauritianWildlife Foundation are exploring theintroduction of a taxonomic and functional

substitute for the extinct tortoises, theAldabran tortoise G. gigantia (Fig. I). Fouradult Aldabran tortoises were releasedinto a fenced enclosure on Ile auxAigrettes in November 2000, and the firstpost-introduction vegetation survey tookplace in May 2001. Viable fruits of theendemic ebony have already been founddispersed in tortoise feces away fromparent trees. It is hoped that theintroduced tortoises will not only shift thecompetitive balance in favor of nativeplants, but also restore the broaderfunctional roles of their extinct congenersin the ecosystems of the Mascarenearchipelago.

Reference

a North, S.G. et al. (1994) Changes in thevegetation and reptile populations on RoundIsland, Mauritius, following eradication ofrabbits. Biol. Conserv. 67, 21–28

Box 2. Replacing extinct herbivores in the Mascarene Islands

I

In most settings, removing introduced herbivoresis an important and reasonable first step in ecosystemrestoration. However, in some cases (particularly onislands without native herbivores), herbivore removalmight actually cause harm if there are no concurrentefforts to control exotic vegetation (Fig. 2). Theclearest benefits from exotic herbivore removal arelikely to occur in settings that are still dominated bynative vegetation. In other settings, close monitoringafter herbivore removal, as well as pre-eradicationassessment, can help reduce unexpected negativeconsequences of the removal of invasives42.

Native species dependence in exotic-dominated habitatsIncreasingly, exotic species have been present inecosystems for long enough to dominate or replacenative species and habitats. In these cases, anecosystem or functional framework might be useful inwhich one asks whether removal of the invader willlargely or entirely remove from the system a functionnecessary to other biota in it. For example, aninvasive plant species might provide usable habitatfor native fauna in the absence of original vegetation.Rapid removal of the invader without restoring nativevegetation might not only increase the chances of anew invasion, but also leave native fauna withoutcover or food. Several examples of the potential forthis type of problem have been described43. However,examples of successful eradications that actually ledto such habitat loss have not been identified. Thismost probably reflects the lack of successfuleradications of plants, which usually provide habitat

for other biota. The case of Tamarix (Box 1) illustrateshow, under certain conditions, consideration of thiskind of undesirable impact can be important.

Conclusion

The type of species being removed, the degree to whichit has replaced native taxa, and the presence of othernon-native species can affect the eventual impacts ofremoval of an invasive species. Managers can take somesimple steps to reduce surprise outcomes. Pre-assessment, including qualitative evaluation of:(1) trophic interactions among exotics and betweennatives and exotics; and (2) potential functional roles ofexotics, is necessary for managers to anticipate the needfor special planning. Post-eradication monitoring is alsoextremely valuable, not least because it allowsmanagers to document the positive outcomes oferadication successes. It also provides the opportunityto learn from mistakes and gives managers the chanceto curtail negative effects before they become severe.More frequent ecological studies that take advantage oferadication programss as being large-scale ecosystemexperiments will speed the accumulation of knowledgeabout system responses to exotic species removals.

Specific guidance for tailoring eradication efforts tocomplex situations is emerging. In the case ofstoat–rat–opossum eradication in New Zealand10,follow-up study showed that the timing and method ofpoisoning used were important in determining stoatpopulation declines (as a result of secondary poisoning)as well as determining effects on native birds44. Amodel of interactions between exotic cats and rabbitsfound that simultaneous removal of both speciesmaximized the chances of success, but suggested thatthe next best alternative was to remove rabbits firstand cats later26. Data from several cases show thatattempts to restore a native species without removingall invaders that consume it are likely to fail45. Manyattempts to reintroduce native marsupials to areasfrom which they have been extirpated have failedbecause of the presence of uncontrolled exoticterrestrial predators such as cats and foxes Vulpesfulva. Success rates of reintroductions are an order ofmagnitude greater (82% versus 8%) on islands withoutexotic predators46. As they accumulate, these kinds ofanalyses – whether based on post-eradication data ormodeled on ecological principles – will enable thedesign of better eradication and restoration strategies.

Invasive species eradication is an increasinglyimportant component of the conservation andmanagement of natural ecosystems. However, innatural systems, a shift in emphasis from strict invasivesmanagement towards broader ecosystem restorationgoals is required. This will place more emphasis on thefull diagnosis of causal factors and the desired ecologicaloutcomes of eradications47. As knowledge abouteffective eradication methods accumulates, attentionshould turn to combining such methods with broaderecological principles to form cost-effective removalstrategies that accomplish overall restoration goals.



458 Review


(b)

(a)Fig. 2. An adverse effect of eradication. Thephotographs show acamp site on SariganIsland, Commonwealth ofthe Northern MarianaIslands, before (a) andafter (b) successfuleradication of feral goatsCapra hircus and pigs Susscrofa in 1998 explosivelyreleased a previouslyundetected exotic vineOperculina ventricosa.Arrows in (b) indicate thelocations of the twobuildings visible in (a).Reproduced, withpermission, from CurtKessler, ZoologyUnlimited.

Acknowledgements

We thank Curt Kessler,Josh Donlan, JohnMauremootoo, RobertBensted-Smith, BernieTershy, Rick Van Dam,Dick Veitch, and IngridParker and her lab groupfor their helpful input.



459Review

Eradication: removal of every individual andpropagule of an invasive species so that onlyreintroduction could allow its return.Hyperpredation: abnormally high predation ofindigenous prey species by a predator population thatis inflated by the availability of highly abundant exoticprey.

Mesopredator release: rise in a population of onespecies caused by the removal of a species that preysupon it. It can lead to a net increase in predation onnative populations of conservation concerna.Trophic cascade: when changes in one species affectthe abundances of other species across more than onelink in the food webb.

References

a Courchamp, F. et al. (1999) Cats protectingbirds: modelling the mesopredator releaseeffect. J. Anim. Ecol. 68, 282–292

b Pace, M.L. et al. (1999) Trophic cascadesrevealed in diverse ecosystems. Trends Ecol.Evol. 14, 483–488

Glossary

References

1 Mooney, H.A. and Hobbs, R.J., eds (2000) InvasiveSpecies in a Changing World, Island Press

2 Myers, J.H. et al. (2000) Eradication revisited:dealing with exotic species. Trends Ecol. Evol. 15,316–320

3 Abedrabbo, S. (1994) Control of the little fire ant,Wasmannia auropunctata, on Santa Fe Island inthe Galapagos Islands. In Exotic Ants: Biology,Impact, and Control of Introduced Species(Williams, D.F., ed.), pp. 219–239, Westview Press

4 Clark, R. and Halvorson, W.L. (1987) The recoveryof the Santa Barbara Island live-forever.Fremontia 14, 3–6

5 Donlan, C.J. et al. (2000) Island conservationaction in northwest Mexico. In Proceedings of theFifth California Islands Symposium (Browne,D.H. et al., eds), (CD-Rom), Santa BarbaraMuseum of Natural History

6 Innes, J. and Barker, G. (1999) Ecologicalconsequences of toxin use for mammalian pestcontrol in New Zealand: an overview. N. Z. J. Ecol.23, 111–127

7 Cory, J.S. and Myers, J.H. (2000) Direct andindirect ecological effects of biological control.Trends Ecol. Evol. 15, 137–139

8 Simberloff, D. and Stiling, P. (1996) How risky isbiological control? Ecology 77, 1965–1975

9 El-Ghareeb, R. (1991) Vegetation and soil changesinduced by Mesembryanthemum crystallinum L.in a Mediterranean desert ecosystem. J. AridEnviron. 20, 321–330

10 Murphy, E. and Bradfield, P. (1992) Change indiet of stoats following poisoning of rats in a NewZealand forest. N. Z. J. Ecol. 16, 137–140

11 Hairston, N.G. et al. (1969) Community structure,population control, and competition. Am. Nat. 94,421–425

12 Fretwell, S.D. (1987) Food chain dynamics: thecentral theory of ecology? Oikos 50, 291–301

13 Polis, G.A. and Strong, D.R. (1996) Food webcomplexity and community dynamics. Am. Nat.147, 813–846

14 Terborgh, J. et al. (1999) The role of top carnivoresin regulating terrestrial ecosystems. InContinental Conservation: Scientific Foundationsof Regional Reserve Networks (Soule, M.E. andTerborgh, J., eds), pp. 39–64, Island Press

15 Polis, G.A. (1999) Why are parts of the worldgreen? Multiple factors control productivity andthe distribution of biomass. Oikos 86, 3–15

16 Pace, M.L. et al. (1999) Trophic cascades revealed indiverse ecosystems. Trends Ecol. Evol. 14, 483–488

17 Polis, G.A. et al. (2000) When is a trophic cascadea trophic cascade? Trends Ecol. Evol. 15, 473–475

18 Courchamp, F. et al. (1999) Cats protecting birds:modelling the mesopredator release effect.J. Anim. Ecol. 68, 282–292

19 Karl, B.J. and Best, H.A. (1982) Feral cats onStewart Island; their foods, and their effects onkakapo. N. Z. J. Zool. 9, 287–294

20 Fitzgerald, B.M. (1988) Diet of domestic cats andtheir impact on prey populations. In TheDomestic Cat: The Biology of its Behavior(Turner, D.C., ed.), pp. 123–146, CambridgeUniversity Press

21 Crafford, J.E. (1990) The role of feral house micein ecosystem functioning on Marion Island. InAntarctic Ecosystems: Change and Conservation(Kerry, K.R. and Hempel, G., eds), pp. 359–364,Springer-Verlag

22 Bloomer, J.P. and Bester, M.N. (1990) Diet of adeclining feral cat Felis catus population onMarion Island. S. Afr. J. Wildl. Res. 20, 1–4

23 Bloomer, J.P. and Bester, M.N. (1992) Control offeral cats on sub-Antarctic Marion Island, IndianOcean. Biol. Conserv. 60, 211–219

24 Courchamp, F. et al. (2000) Rabbits killing birds:modelling the hyperpredation process. J. Anim.Ecol. 69, 154–164

25 Smith, A.P. and Quin, D.G. (1996) Patterns andcauses of extinction and decline in Australianconilurine rodents. Biol. Conserv. 77, 243–267

26 Courchamp, F. et al. (1999) Control of rabbits toprotect island birds from cat predation. Biol.Conserv. 89, 219–225

27 Van Vuren, D. and Coblentz, B.E. (1987) Someecological effects of feral sheep on Santa CruzIsland, California, USA. Biol. Conserv. 41,253–268

28 Coblentz, B.E. (1978) The effects of feral goats(Capra hircus) on island ecosystems. Biol.Conserv. 13, 279–286

29 Laughrin, L. et al. (1994) Trends in vegetationchanges with removal of feral animal grazingpressures on Santa Catalina Island. In TheFourth California Islands Symposium: Update onthe Status of Resources (Halvorson, W.L. andMaender, G.J., eds), pp. 523–530, Santa BarbaraMuseum of Natural History

30 North, S.G. et al. (1994) Changes in the vegetationand reptile populations on Round Island,Mauritius, following eradication of rabbits. Biol.Conserv. 67, 21–28

31 Morris, I. (1996) Kakadu National Park,Australia, Steve Parish Publishing

32 Scowcroft, P.G. and Conrad, C.E. (1992) Alien andnative plant response to release from feral sheepbrowsing on Mauna Kea. In Alien Plant Invasionsin Native Ecosystems of Hawai’i: Managementand Research (Stone, C.P. et al., eds), pp. 625–665,University of Hawai’i Cooperative National ParkResources Studies Unit

33 Stone, C.P. et al. (1992) Responses of Hawaiianecosystems to removal of feral pigs and goats. InAlien Plant Invasions in Native Ecosystems ofHawai’i: Management and Research (Stone, C.P.et al., eds), pp. 666–704, University of Hawai’iCooperative National Park Resources StudiesUnit

34 D’Antonio, C.M. and Vitousek, P.M. (1992)Biological invasions by exotic grasses, the

grass/fire cycle, and global change. Ann. Rev. Ecol.Syst. 23, 63–87

35 Copson, G. and Whinam, J. (1998) Responseof vegetation on subantarctic Macquarie Island to reduced rabbit grazing. Aust. J. Bot.46, 15–24

36 Klinger, R.C. et al. (1994) Vegetation response tothe removal of feral sheep from Santa CruzIsland. In The Fourth California IslandsSymposium: Update on the Status of Resources(Halvorson, W.L. and Maender, G.J., eds), pp. 341–350, Santa Barbara Museum of NaturalHistory

37 Wenner, A.M. and Thorp, R.W. (1994) Removal offeral honey bee (Apis mellifera) colonies fromSanta Cruz Island. In The Fourth CaliforniaIslands Symposium: Update on the Status ofResources (Halvorson, W.L. and Maender, G.J.,eds), pp. 513–522, Santa Barbara Museum ofNatural History

38 Wehtje, W. (1994) Response of a Bishop pine(Pinus muricata) population to removal of feralsheep on Santa Cruz Island, California. In TheFourth California Islands Symposium: Update onthe Status of Resources (Halvorson, W.L. andMaender, G.J., eds), pp. 331–340, Santa BarbaraMuseum of Natural History

39 Wenner, A.M. et al. (2000) Removal of Europeanhoneybees from the Santa Cruz Island ecosystem.In Proceedings of the Fifth California IslandSymposium (Browne, D.H. et al., eds), SantaBarbara Museum of Natural History

40 Merlin, M.D. and Juvik, J.O. (1992) Relationshipsamong native and alien plants on Pacific islandswith and without significant human disturbanceand feral ungulates. In Alien Plant Invasions inNative Ecosystems of Hawai’i: Management andResearch (Stone, C.P. et al., eds), pp. 597–624,University of Hawai’i Cooperative National ParkResources Studies Unit

41 Eckhardt, R.C. (1972) Introduced plants andanimals in the Galapagos Islands. Bioscience 22,587–590

42 Rutherford, C. and Chaney, S. (1999) Islandplants gain new lease on life. Fremontia 27, 3–5

43 Van Riel, P. et al. (2000) Eradication of exoticspecies. Trends Ecol. Evol. 15, 515

44 Murphy, E.C. et al. (1998) Effects of rat-poisoningoperations on abundance and diet of mustelids inNew Zealand podocarp forests. N. Z. J. Zoology25, 315–328

45 Fischer, J. and Lindenmayer, D.B. (2000) Anassessment of the published results of animalrelocations. Biol. Conserv. 96, 1–11

46 Short, J. et al. (1992) Reintroductions ofmacropods (Marsupialia, Macropodoidea) inAustralia: a review. Biol. Conserv. 62, 189–204

47 Hobbs, R.J. (1999) Restoration of disturbedecosystems. In Restoration of DisturbedEcosystems (Walker, L., ed.), pp. 673–687,Elsevier Science

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Trabajo Práctico 9 ESTUDIO DE LA INVASIÓN DE ARDILLAS EN BUENOS AIRES

Por RUBEL DIANA; FISCHER SYLVIA; THOMPSON GUSTAVO; LOETTI VERÓNICA

OBJETIVOS - Que los alumnos puedan aplicar los modelos de crecimiento poblacional en el contexto de la problemática de las invasiones biológicas. - Introducir a los alumnos en los conceptos básicos de la ecología del paisaje. INTRODUCCIÓN Un caso de invasión biológica que se ha estudiado desde etapas relativamente tempranas, es el caso de la ardilla de vientre rojo, Callosciurus erythraeus (Pallas, 1779) en la provincia de Buenos Aires. Esta ardilla es un roedor arborícola y diurno de tamaño medio (Phylum: Chordata, Clase: Mammalia, Orden: Rodentia, Familia: Sciuridae) originario del noreste asiático (Bangladesh; Camboya; China; India; Laos; Malasia; Birmania; Taiwán; Tailandia y Vietnam). No presenta dimorfismo sexual de tamaño ni pelaje, y la coloración típica del pelaje es marrón oliváceo con una banda negra en el dorso que normalmente se extiende desde la base de la cola hasta la cruz (Cassini & Guichón, 2009). El área de influencia media por individuo en su área de distribución original varía entre 0,3 ha y 0,5 ha para las hembras y entre 1,4 ha a 2,2 ha para los machos (Tamura et al., 1989. En CABI, 2011). Su alimentación es variada, se alimenta de diferentes partes de especies de plantas y también de hongos, insectos, huevos y caracoles. El hábitat en su área de distribución natural consiste principalmente en bosques subtropicales, aunque en China también está presente en bosques subalpinos de coníferas o en bosques mixtos de coníferas y frondosas en altitudes superiores a los 3000 m sobre el nivel del mar (Smith and Xie 2008, en UICN 2011). En Taiwán habita desde bosques de bambú de las zonas bajas hasta los bosques de coníferas que llegan a 3000 m s.n.m. (Hu y Yie, 1970). Dado que esta ardilla es una especie invasora en Japón, Francia, Bélgica, Holanda y Argentina, en las áreas de introducción se localiza en varios tipos de áreas arboladas (bosques naturales, plantaciones de coníferas, cultivos, arbustos y parques urbanos) pero prefiere zonas mixtas de frondosas en Japón (Okubo et al., 2005, en CABI, 2011) y Francia. La invasión en Argentina La invasión se origina en la localidad de Jáuregui, cuando son liberados unos pocos ejemplares por no adaptarse al cautiverio en el que los mantenía un poblador. Guichón y Doncaster (2008) proponen un modelo espacialmente explícito que predice el comportamiento de la invasión de esta especie en nuestro país a partir de la liberación, y los datos que usaremos en el trabajo práctico se basan en dicho modelo, aunque se han simplificado para fines didácticos. Al igual que en el modelo propuesto por Guichón y Doncaster (2008), se tomaron cuatro categorías de hábitat - pastizal, urbano, suburbano y bosque - con diferente capacidad de carga para la población de ardillas, que se presentan en el mapa de la Figura 1 para el área de liberación y sus alrededores. La información demográfica sobre esta población se presenta en la Tabla 1, en tanto que la información sobre el comportamiento de dispersión se puede observar en la Tabla 2. Propuesta de trabajo En base a la información disponible, la propuesta es modelar la invasión en las décadas siguientes a la liberación de los dos primeros ejemplares. Más específicamente predecir cuál será el estado de la población en cuanto al tamaño poblacional y el área de distribución, después de 30 años de la liberación de los primeros ejemplares. Para ello, contarán con el mapa que figura en el anexo, y les proponemos utilizar como guía el cuestionario que sigue y los datos poblacionales que figuran en las Tablas 1 y 2.

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a) ¿Cuál es la capacidad de carga estimada para el parche en el que se produce la liberación inicial de ardillas? b) ¿Cuánto tiempo llevará que la población alcance la capacidad de carga del parche? c) ¿En cuánto tiempo se espera que comience la colonización de nuevos parches? d) ¿Cuál es el escenario esperado en 30 años? ¿Qué población total de ardillas se espera? ¿Qué área de ocupación tendrá la población? e) ¿Qué suposiciones hay que hacer para poder hacer este análisis? f) ¿Qué etapas de la invasión estamos observando en estos análisis? g) Sobre la base a todo lo visto en la clase de hoy: ¿Qué estrategias se podrían proponer para limitar la invasión?

Categoría en el mapa 0 1 2 3 Tipo de hábitat pastizal urbano suburbano bosque Capacidad de carga (ind/ha) 0 2 4 8 Tasa finita de incremento = 1,53/año

Tabla 1: Información demográfica sobre la ardilla de vientre rojo proporcionada a los alumnos

Densidad Menos de 50% de K

Entre 50% y 75% de K

Entre 75% y 90% de K

Más de 90% de K

Nº de parcelas desfavorables que puede cruzar

0 (no se dispersa)

1 2 3

Metros de pastizal que pueden recorrer

0 100 200 300

Tabla 2: Datos sobre el comportamiento de dispersión. K representa la capacidad de carga. Se supuso que los parches eran colonizados por 4 individuos.

Bibliografía Cassini G and Guichón ML, 2009. Variaciones morfológicas y diagnosis de la ardilla de vientre rojo,

Callosciurus erythraeus (Pallas, 1779), en Argentina. Mastozoología Neotropical 16(1):39-47. Guichón ML and Doncaster CP, 2008. Invasion dynamics of an introduced squirrel in Argentina.

Ecography 31:211-220. Hu Y and Yie ST, 1970. Some biological notes on the Taiwan squirrel. Plant Protection Bull. Vol. 12,

No. 1:21-30 (In Chinese). Okubo M, Hobo T and Tamura N, 2005. Vegetation types selected by alien Formosan squirrel in

Kanagawa Prefecture. Natural History Report of Kanagawa 26: 53-56. Smith and Xie, 2008 citado en: http://www.iucnredlist.org/details/136490/0 Tamura N, Hayashi F, Miyashita K, 1989. Spacing and kinship in the Formosan squirrel living in

different habitats, Oecologia 79:344-352.

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Trabajo práctico 10 PATRONES ESPACIALES Y ESCALAS OBJETIVO GENERAL

Analizar la estructura espacial de la heterogeneidad y el efecto de la escala sobre la percepción de la heterogeneidad en un gradiente urbano-suburbano. INTRODUCCIÓN

Los sistemas naturales son heterogéneos, y esta heterogeneidad puede darse a distintas escalas espaciales. A una escala dada, la heterogeneidad puede formar distintos patrones: si el ambiente va cambiando en forma continua y gradual a lo largo del espacio hablamos de gradiente, cuando las porciones distintas se ubican en forma contigua, de mosaico ambiental, mientras que si podemos distinguir porciones distintas dentro de un ambiente predominante, se habla de una estructura de parche- matriz (Figura 1). Esta estructura espacial, es decir, cómo se distribuye la heterogeneidad en el espacio, está determinada principalmente por la heterogeneidad del sustrato (por ejemplo, un gradiente de altura, o distintos tipos de suelo), por disturbios naturales (por ejemplo, inundaciones periódicas, incendios) o por las actividades humanas (uso de la tierra, urbanizaciones).

Figura 1. Esquemas de patrones de heterogeneidad: (a) parches dentro de una matriz, (b) mosaico ambiental

La ecología del paisaje es la rama de la ecología que estudia cómo la estructura espacial afecta los procesos ecológicos y, a su vez, cómo estos procesos determinan la estructura espacial. Un paisaje está formado por un conjunto de ecosistemas que se repiten en el espacio (a una escala de entre 10 y 100 km de diámetro), y puede ser descripto por sus elementos: tipos, forma y tamaño de parches, forma, longitud y ancho de corredores, y tipo de matriz. La matriz es el tipo de parche más frecuente, y puede ser más o menos extensa, dispersa o agregada, y estar más o menos perforada. Por otro lado, un paisaje también se caracteriza por el arreglo espacial de los elementos (grado de agregación) y el grado de diferencia entre los distintos tipos de parches (contraste), Figura 2.

Un conjunto de paisajes forma una región, que es un área geográfica mayor determinada por sus características geológicas, el macroclima y una actividad humana común, sus distintas partes están conectadas por transporte, comunicación y cultura. Tanto los paisajes como las regiones son escalas espaciales “humanas”, no corresponden a niveles de organización biológica. Como hemos dicho antes, las características y los procesos que suceden a escala de paisaje están determinados por sus elementos, es decir, por las características y procesos a una escala menor. Pero también están influidas por las características de la región donde están inmersos los paisajes. Es decir, lo que sucede a una escala dada, está determinada por los niveles superiores e inferiores.

a b

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Figura 2. Componentes de la heterogeneidad: contraste y agregación

Según la escala en que estemos trabajando, vamos a poder distinguir la heterogeneidad a distintos niveles, como cuando se cambia el aumento de un microscopio cambian los detalles de algunas estructuras, o cuando un avión se va acercando a tierra va cambiando el nivel de detalle con que uno ve los objetos, pero dejamos de percibir otros niveles de heterogeneidad a mayor escala. Por otro lado, según el ambiente, la heterogeneidad puede variar según la escala de observación debido al tamaño de los parches. Si los parches son muy grandes (grano grueso, Figura 3) trabajar en una escala pequeña (de detalle) no los percibimos debido a que abarcamos un solo tipo de parche, mientras que si los parches son pequeños (grano fino, Figura 3) al trabajar a escalas grandes no son distinguibles.

Figura 3. Grano de un paisaje de acuerdo al tamaño promedio de los parches

Poco contraste

Mucho contraste

Poca agregación Mucha agregación

Grano grueso Grano fino

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Es importante diferenciar entre la escala ecológica y la escala cartográfica. Esta última se refiere a la relación entre la distancia que hay entre dos puntos dados en su representación en un mapa respecto a su distancia real (Por ejemplo 1:50000, 1:250000). La escala ecológica se refiere a la dimensión espacial y/o temporal de un objeto o proceso. Un proceso de gran escala abarca un área grande y/o dura un tiempo largo (por ejemplo, las glaciaciones ocuparon grandes áreas y se produjeron durante largos períodos de tiempo). Por otro lado, un proceso a pequeña escala ocurre en dimensiones limitadas o en tiempos relativamente cortos (por ejemplo, perturbación producida por la caída de un árbol).

La escala se distingue por su grano y su extensión. El grano es el tamaño de la mínima unidad distinguible (como el tamaño de los granos de arena o la rugosidad de una lija) y la extensión es el tamaño total abarcado (Figura 4). Si uno está parado en un punto, hasta dónde puede abarcar con la vista sería la extensión y los detalles que puede percibir son el grano. Entre ambos puede haber un número variable de niveles intermedios. El grano y la extensión dependen de las características de cada especie, principalmente su tamaño y movilidad. Las especies más grandes y/o que recorren mayores distancias tienden a tener una extensión mayor que las especies pequeñas y sedentarias.

Figura 4: Para un organismo dado, la extensión es el mayor tamaño al que puede distinguir la heterogeneidad, el grano el menor tamaño. En la figura se distinguen 3 niveles, los parches más chicos representan el grano y el área más grande representa la extensión. DESARROLLO Área de estudio: En este trabajo práctico vamos a analizar el efecto de los cambios de escala en la percepción de la heterogeneidad en la región metropolitana de Buenos Aires y alrededores, hasta la zona del Río Luján- Otamendi, Partido de Campana. Se tratará de determinar a cada escala los factores que determinan la heterogeneidad, teniendo en cuenta que ésta es el resultado y la expresión de procesos que actúan a distintas escalas y son de distinto origen, en particular en la región estudiada, la heterogeneidad observada resulta de la interacción entre procesos naturales y las transformaciones que introduce el hombre al hacer uso del suelo.

El área estudiada comprende un mosaico de comunidades donde están representadas tres ecorregiones determinadas principalmente por el clima y la topografía: la ecorregión Delta e Islas del Paraná, representada por el bosque ribereño, la región Pampeana, representada por los pastizales de la zona alta, y la región del Espinal, representada por los talares que se desarrollan sobre albardones de conchilla en la barranca. La ecorregión Delta e Islas del Paraná corresponde a los valles de inundación de los trayectos medios e inferiores de los río Paraná y Paraguay, e incluye al

Extensión

Grano

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Delta del Paraná. Se trata de un paisaje de islas bajas e inundables. La ecorregión de la Pampa constituye el sistema de praderas más importante de la Argentina. Su relieve es relativamente plano y está expuesta a anegamientos permanentes o cíclicos. Las praderas estuvieron originalmente dominadas por gramíneas entre las que predominaron las del género Poa, Stipa, Piptochaetiun y Aristida. Es la ecorregión que más transformación ha sufrido debido a procesos de urbanización y a la implementación de la agricultura y ganadería extensiva. El Espinal desde el punto de vista arbóreo está caracterizado por la presencia del género Prosopis (algarrobos, ñandubay y caldén). En la provincia de Buenos Aires los talares son los representantes típicos del espinal.

Figura 5. Unidades del paisaje en la región metropolitana de Buenos Aires

Las características propias de cada ecorregión han sido modificadas por las actividades humanas, dando lugar en el área a cinco grandes unidades de paisaje en la Región Metropolitana (Atlas Ambiental de Buenos Aires; Figura 5):

• La Planicie Pampeana y la Franja Costera ocupada por la urbanización, a la que

se denomina Área Metropolitana de Buenos Aires (AMBA) • La Planicie Pampeana no ocupada por la urbanización, a la que se denomina

Pampa • La Franja Costera no ocupada por la urbanización, a la que se denomina Costa • El Bajo Delta del Río Paraná, que se denomina Delta • El Estuario del Río de la Plata que se denomina Río La división entre el AMBA y la Pampa es dinámica y arbitraria, y entre ellas se presenta una

interfase, comúnmente denominada “periurbano”, en la cual se registran simultáneamente manifestaciones urbanas y rurales.

Durante los últimos años, la Región Metropolitana de Buenos Aires ha experimentado una intensa transformación del territorio y de los usos dominantes. Por ejemplo, en algunos sectores de los valles de inundación de los ríos Luján, Reconquista y Paraná de las Palmas puede observarse que amplios sectores de la franja periurbana y de numerosos espacios intersticiales han pasado de un uso rural a otro urbano, especialmente residencial, recreativo y comercial.

Extendiéndose más hacia el norte de la Región Metropolitana, donde disminuye la urbanización, se puede distinguir la transición de los pastizales en la zona alta hacia los talares de la barranca, los

Río

AMBA

Pampa

Costa

Delta

Costa

Pamp

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humedales en la zona baja hasta la selva en galería a lo largo de arroyos y del río Paraná. En esta zona tenemos representadas comunidades vegetales asociadas tanto a ambientes terrestres como a ambientes acuáticos. Podemos encontrar diferentes comunidades de herbáceas, bosques, ríos, arroyos y canales. A pequeña escala no es el clima el que determina el desarrollo de diferentes comunidades vegetales, sino que los factores principales son la hidrología y el tipo de suelo, ambos asociados al relieve (presencia de barrancas, zonas de inundación) y microrelieve. Las comunidades terrestres que se desarrollan en cada zona dependen del relieve y las características físico–químicas del suelo (principalmente la salinidad y la humedad), nivel de la napa freática, frecuencia de inundaciones y período durante el cual el suelo permanece inundado. Puede observarse, entonces, un gradiente de las comunidades vegetales asociado al relieve y a la hidrología (Figura 6).

Figura 6. Esquema de localización de las diferentes comunidades según su posición en la barranca de río

Desarrollo del Trabajo Práctico: Trabajo en laboratorio con GoogleEarth: Describir la heterogeneidad espacial a distintas escalas cartográficas e identificar los distintos elementos que se pueden distinguir en cada caso. Google Earth es un programa informático que existe bajo este nombre desde mayo 2005 y que permite visualizar el planeta entero a través de un mosaico de imágenes de satélite o fotografías aéreas. El programa permite visualizar la superficie terrestre desde diferentes alturas, lo que implica diferentes escalas de la imagen que se obtiene. Actividades: 1) Descripción de la estructura del paisaje a distintas escalas en tres puntos ubicados a distancia

creciente de la ciudad de Buenos Aires:

Se trabajará con los puntos geográficos 1 y 5 especificados en la Tabla 1. Para ubicarlos en el mapa debe escribir las coordenadas geográficas en el recuadro que dice “volar a” del Google Earth (recordar que latitud Sur y longitud Oeste se representan con valores negativos). En cada punto se trabajará a tres escalas diferentes. Para ello se ubicará el punto en el centro de la pantalla y se buscará que la escala que muestra el Google Earth en el extremo inferior izquierdo, coincida con 4 km, 1 km y 100 m.

Para cada punto y a cada escala especificar: Tipos de unidades distintas que se distinguen, tamaño promedio de las unidades, proporción del área con cada unidad y contraste entre unidades. Comparar los distintos puntos en cuanto a la heterogeneidad, y a cómo varía ésta según la escala. Los criterios a usar para identificar las unidades de paisaje, así como los distintos ambientes son: la forma, el color y la textura de la imagen.

2) Descripción de la estructura del paisaje a lo largo de una transecta que va desde la ciudad de

Buenos Aires hacia el norte, abarcando unos 50 km. hasta la zona del río Luján:

Talar Pastizal Parte baja

del humedal

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Los cinco puntos especificados en la Tabla 1 se encuentran a lo largo de una transecta. Se ubicarán esos puntos en el mapa a partir de sus coordenadas y se visualizará la transecta. Luego se trabajará con cada uno de ellos a una escala 1:1000, analizando una superficie de aproximadamente 4 Km2 (2 X 2 Km). Para cada punto se cuantificará sobre la imagen de Google Earth la composición relativa de distintos componentes del paisaje, y los resultados serán volcados en la planilla adjunta (Tabla 2).

Tabla 1. Localización de los puntos a observar durante el trabajo práctico

Localización de los puntos

Punto 1 Tercer puente después de tomar Panamericana Latitud -34.533176° Longitud -58.502494°

Punto 2 Bifurcación del ramal tigre, justo pasando el primer puente. Latitud -34.488707° Longitud -58.570718°

Punto 3 Panamericana y Ruta 197 Latitud -34.474717° Longitud -58.661775°

Punto 4 Av. Benavidez (Ruta 27). Latitud -34.420476° Longitud -58.717830°

Punto 5 Primer puente peatonal después del arroyo escobar. Latitud -34.367515° Longitud -58.774746°

Punto 6 Cruzando el Río Luján, aproximadamente km 60 Latitud -34.295539° Longitud -58.884219°

Tabla 2. Planilla para describir la estructura del paisaje en los 6 puntos a lo largo de la transecta Buenos Aires–Río Luján.

Punto

1 Punto

2 Punto

3 Punto

4 Punto

5 Punto

6 Tipos de uso de la tierra

a) % Residencial (total)

a1) % Viviendas multifamiliares (edificios y departamentos)

a2) % Viviendas unifamiliares (casas y ph)

a3) % Barrios cerrados, countries

b) % Industrial/Comercial (incluye: escuelas, cementerios, clubes, etc)

c) % Espacios verdes no naturales/ parques (no pertenecientes a countries)

d) % de terreno agrícola

e) % de terreno natural o no agrícola (pasturas, pastizales naturales)

f) % de rutas o autopistas

Otros indicadores % total de la superficie construida o pavimentada (sin incluir calles o rutas)

% total de la superficie con vegetación

N° de cuadrados con rutas o caminos

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BIBLIOGRAFÍA

Atlas Ambiental de Buenos Aires. http://www.atlasdebuenosaires.gov.ar/aaba

Begon, M., Harper, J.L.& Towsend, C.R. 1987. Ecología: Individuos, poblaciones y comunidades. De. Omega, Barcelona.

Bonfilds, C. G. 1962. Los suelos del Delta del río Paraná. Factores generadores, clasificación y uso. INTA (RIA). Num. 16 (3) 370 pp. Buenos Aires.

Bonfilds, C. G. 1962. Los suelos del Delta del río Paraná. Factores generadores, clasificación y uso. INTA (RIA). Num. 16 (3) 370 pp. Buenos Aires.

Brown A., Martínez Ortíz U., Acerbi M., Corcuera J., 2005. La situación ambiental argentina 2005. Fundación vida silvestre argentina.

Chichizola S., 1993. Las comunidades vegetales de la Reserva Natural Estricta Otamendi y sus relaciones con el ambiente. Parodiana 8 (2): 227-263.

Forman RTT, (1999) Landscape ecology, the growing foundation in landuse planning and natural-resource management. In: Kovar P (Ed.) Nature and culture in landscape

Iriondo MH y Scotta E. 1978. The evolution of the Paraná River Delta. Proceedings of the International Symposium on coastal Evolution in the Quaternary: 405-418. INQUA,San Pablo, Brasil

Kandus P., Malvárez A.I. y N. Madanes. 2003. Estudio de las comunidades de Plantas Naturales de las Islas del Bajo Delta del Río Paraná. (Argentina). Darwiniana 41 (1-4): 1-16. Nacional.ISSN 0011-6793

Kandus P., Quintana R D., Bó R., 2006. Patrones de paisaje y biodiversidad del bajo delta del río Paraná. http://www.ambiente.gov.ar/default.asp?IdArticulo=5505

Krebs, Charles, J. 1986. Ecología. Análisis experimental de la distribución y abundancia. Ed. Pirámide. Madrid.

Madanes, N. 2008. Humedales de la Reserva Natural Otamendi. En Guía de la Flora de la Reserva Natural Otamendi. Editora Liliana Goveto. Publicación de la Administración de Parques Nacionales. En prensa

Malvárez, A.I y R.F. Bó. 2002. Cambios ecológicos en el Delta Medio del Río Paraná debidos al evento de El Niño 1982-1983”. Publicación especial del Taller “El Niño, sus impactos en el Plata y en la Región Pampeana. J.A. Schnack (Ed.). La Plata, Buenos Aires.

Marchetti B, Ruiz L., Madanes N., Sartori G. y P. Cichero. 1988. Relevamiento del medio natural y una propuesta de plan de manejo para la futura Area Natural Protegida " Ing. Rómulo Otamendi". Informe Interno Administración Parques Nacionales: 1-53. Biblioteca Administración de Parques Nacionales

Matteucci, S. y Colma A. 1982. Metodología para el estudio de la vegetación. O.E.A. Serie de Biología.

Matteucci, Silvia D.; Jorge Morello; Gustavo D. Buzai; Claudia A. Baxendale; Mariana Silva; Nora Mendoza; Walter Pengue y Andrea Rodríguez. 2006. Crecimiento urbano y sus consecuencias sobre el entorno rural. Orientación Gráfica Editora. Buenos Aires. (350 páginas).ISBN 978-987-9260-45-6

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DEJAR ESTA HOJA EN BLANCO

Trabajo práctico 10 - Patrones espaciales y escalas

TURNO: GRUPO:

Extension aproximada Tipo de elemento Caracterizacion Proporcion de(metros cuadrados) (matriz, parche, corredor) (nombre que le pondrian) cobertura

1/ 250 m

5/4 km

5/ 1 km

5/ 250 m

1/ 1 km

Punto/altura

1/4 km

38

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Ecología General

42


Ecología General

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Ecología General

46

Ecología General

47

arriba-izquierda] A B C A C G A J A A J A A B B E B B A B B G F E A A K E D I J B B I H A A G E B A B M A B A D A C L B F B E L B I A F E A N D B M B C B A A C B G C A C A D B F E A L B C H C A F D G C A A C C H F A C C A C G I G A A A J A B N J L C A A B A B C J N L C E L H I B D E H E C L D D D E I B C E A M C G G N A F J D I C J G C C I G J D J C A C C B F D J B J C G G F C M G M C D H H C C C B C B J D D C G I E J B C A H A B A H A B F G B G J B A A B B A A A A A L A G J G I J J D K A A A A D N A B A J B A G A G B A A G B F G J L A H B D H

i

Ecología General

48


Ecología General

49

[arriba-derecha] B A J A A D D B B B B B A B A A A G C G B A B A A A E B E D A A C A B A D C A I B A I A I A B A B A A A C B E A D A A E A B B B A B C J A C C C C A D K B B A C A C B N E E C B A C B A A L A D A B I A B B A B A C B A A A A C N D B D C A C C J J D G C A C C C C C C C C C J B J C C B C A A C C C B G C B C C A C M B C C E E A E E J D C D C C B B J D A K A J B A C C A B J B B A B B B A A A C C C C E A G B A C C J C K J C C F A G A A A C K B A J B B C A A G C C J C H B A C H D

ii

Ecología General

50


Ecología General

51

i G A B G A B A A B B A C A D J A A H F A B H A B B J L F L M A B D D L G I G G A B J A A B A B A F A B E D F J D A A H D E A I J E A E A I G D J E A A A M F E M D L F N B A B A B A B C G H A K F E L F A B B I A A I H A A A B D A I M F E A A B A F A F G B D D D B E B A E H E A B B N I A A B M E M I M N N F A B I A B D B I B K F E F M A B D C B I A E F F E E F A B B A A A A G G B A B J L B D B J A B A A A I L B E A A H E A I J E F E A D G G H B B I

[abajo-izquierda]

Ecología General

52


Ecología General

53

ii B J J A B J A B J A B A D J F B L A L A A A A B J A B B A A A A A A D A A I J E F E E E J A A M A M A A B A A A N F E E A A F G E I F F A A I E F K A M E F E F A A B B F E A A M M A B A A I I L F I A F A E E E A A A A A G A B J A A A A A A I E J E E E M

[abajo-derecha]

Guía de Trabajos Prácticos - Parte II - ege.fcen.uba.ar · ensamble de poblaciones de plantas,...

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Transcript of Guía de Trabajos Prácticos - Parte II - ege.fcen.uba.ar · ensamble de poblaciones de plantas,...