Aspectos trofodinámicos de la ecología

Cadenas alimentarias y dinámica trófica

• La cantidad de energía que se transfiere entre los niveles sucesivos de una cadena alimentaria es, en promedio, el 10%

Producción secundaria

• La producción primaria es relativamente fácil de medir.

• La producción secundaria, sin embargo, es más difícil de estimar debido a tiempos generacionales más largos, distribución discontinua de las poblaciones, abundancias poblacionales menores.

Producción secundaria

• A través de la obtención de datos de campo sobre la abundancia de zooplancton y peces.

• A través de la obtención de datos experimentales sobre la energética del zooplancton y peces.

• Utilizando estimados de producción primaria y conocimientos sobre trofodinámica:

---estimaciones indirectas: conociendo cuanta energía puede ser transferida entre cada nivel trófico.

Eficiencia ecológica

• Eficiencia con la que la energía puede ser transferida entre niveles tróficos sucesivos.

• Cantidad de energía que se extrae de un nivel trófico λ0 dividida entre la energía que entra al nivel trófico λ1

• Difícil de medir –puede ser estimada a través del uso de las eficiencias de transferencia

Eficiencia de transferencia

• Et = Eficiencia de transferencia• Pt = productividad del nivel trófico λt

• Pt-1 = productividad del nivel trófico λt-1

• Et = Pt/Pt-1

* No todos los organismos se transfieren… Algunos mueren por otras causas distinitas a la depredación (y entran al ciclo del detritus)

Eficiencia de transferencia

• ~20% del fitoplancton a los herbivoros

• 10-15% en niveles sucesivos

• La pérdida de energía entre los niveles tróficos llega a ser del 85 al 90%, principalmente debida a la respiración

How many trophic levels?

• ranges from 2 to 6 levels– less in coastal and/or upwelling areas– more in open ocean (oligotrophic areas)

• number of trophic levels is dependent on the size of phytoplankton– phytos tend to be large in upwelling

regions (WHY?) and small in open ocean areas (WHY?)

Estimating Secondary Productivity

• Once the trophic structure is known, secondary production can be estimated:

P(n+1) = P1En

• P is productivity at the (n+1)th trophic level• n is number of trophic transfers (trophic levels

minus one)

• P1 is annual primary production

• E is ecological efficiency

Weaknesses

• E is very sensitive: by doubling E, secondary production can increase 10-fold

• food chain versus food web - trophic transfer is not as simple as equations imply

AQUÍ VAMOSProductivity

• Productivity refers to biological activity/interaction in the environment

• Measuring productivity– numbers or biomass often measured as

gC/m2/yr• oceanic average productivity = 100 gC/m2/yr

– rates of growth (or excretion, grazing, sinking, etc.)

– organism interactions with the environment and/or each other

Consumer - Food Interactions

• Productivity = growth rate - loss rate– For primary productivity

• growth rate varies with light, nutrients, and temperature• loss rate includes respiration, grazing, sinking, and

death

– For secondary productivity• growth rate varies with ingestion of food• loss rate includes respiration, egestion, excretion, and

death

Productivity

• responsible for most of phytoplankton loss• other loss mechanisms are not really a factor

unless grazing does not occur• grazing can have no impact, prevent a bloom, or

terminate a bloom (depending on timing)• 90% of carbon and energy is lost at each step of

trophic pyramid– material loss due to respiration, DOC, and POC– DOC and POC utilized by microbial loop, detritivores,

etc.

Grazing

Global Patterns of Productivity

Fish production.Fish production.

Measuring Secondary Productivity

• in some cases, primary production may not be a good indicator of production at higher trophic levels– eutrophic systems (PP>>grazing)– HABs/selective grazing

• in such cases, excess primary production may enter the microbial-detritus circuit

Top-down Estimates

• relying solely on fisheries statistics to “fill in the blanks” for lower trophic levels can lead to underestimates– omission of production of competing,

unharvested species

Zooplankton Productivity

• defined as total amount of new production within a time frame, regardless of whether all individuals survive through the whole time frame

• B = Xw

• B = biomass, X = number of individuals, w = average weight of an individual

Zooplankton Productivity

• Pt = (X1-X2)((w1+w2)/2) + (B2 - B1)

• Pt = production between time intervals t1 and t2

• B2 - B1 refers to increase in biomass

• the remainder of the equation refers to biomass produced, but lost, during the time interval

Zooplankton Productivity

• ideally, one would study a single cohort of a population over time– cohort = one identifiable generation of

progeny of a species

• practically impossible to do– cannot follow and sample same water

mass long enough to get meaningful results

Zooplankton Productivity

• cohort studies focus on following changes in relative numbers and weights of distinctive life stages of abundant species (copepods)

Zooplankton Productivity

• productivity may change over time – zooplankton stages grow at different rates– rates vary over the course of a year– in temperate regions, growth will be greatest

in the spring when food is plentiful and zooplankton are young

– productivity may be negative in the winter as individuals utilize food reserves rather than eating

Experimental Biological Oceanography

• laboratory-scale experiments

• enclosed ecosystem experiments

• computer simulations

Laboratory-scale Experiments

• individual organisms in small volumes of water

• food requirements

• transfer efficiencies

• mainly herbivorous copepods (and phytos)

Laboratory-scale Experiments

• G = R - E - U - T

• G = Growth

• R = ration of ingested food

• E = egested fecal material

• U = excretory products (e.g., urea and ammonia)

• T = respiration

Laboratory-scale Experiments

• excretory products (U) are usually negligible, so equation is often simplified to AR = T + G

• A = proportion of food actually utilized

• A = (R - E)/R

Assimilation Rates

• assimilation rates are highest for carnivores (80 - >90%), lower for herbivores (50 - 80%), and lowest for detritivores (<40%)

• WHY?

Feeding Rate Estimates

• a known number of zooplankton (1-10s) and a known concentration of food (phytoplankton) are put into a culturing container (kept in the dark - WHY?) and the zooplankton are allowed to feed

• food particle concentrations are remeasured at a later time to determine grazing rates

Estimating Ingestion (R)

• grazing rates are related to food concentrations

• Michaelis-Menton kinetics

• R = Rmax(1 - e -kp)

• k = grazing constant

• p = prey density

µmax

½ µmax

Kn

gra

z in

g

rate

food concentration

Ko

species Bdominates

species Adominates

[phytoplankton]

gra

zin

g r

ate

Estimating Respiration

• T = respiration rate

• can be determined in closed-bottle experiments

• related to temperature and size of individual

Estimating Egestion (E)

• fecal matter produced

• copepods produce fecal pellets

• collect them, count them, and weigh them!

Estimating Growth

• Growth (G) can be determined once R, E, A, and T are known

• Once G is known, growth efficiency can be estimated:– gross: K1 = G/R x 100%

– net: K2 = G/AR x 100%

Growth Efficiency

• temperature and food concentration will affect growth efficiency

• efficiency changes with age

• net growth efficiency for zooplankton generally vary between 30 - 80%

• terrestrial animals vary between 2 - 5%

Growth Efficiency

• growth efficiency estimations allow us to determine the food required to produce certain animals at different trophic levels

• still laboratory-based