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 VAMOS Productivity

•  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.

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

graz

ing

rate

food concentration

Ko

species B dominates

species A dominates

[phytoplankton]

graz

ing

rate

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