Post on 05-Mar-2020
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