Chapter 21: Energy Flow and Food Webs Flashcards
The Artic is…
one of the msot remote and pristine areas on Earth
How much more PCB concentration were in breast milk from Inuit women than Quebec women?
7 times higher
PCBs
type of industrial chemicals called persistent organic pollutants (POPs)
Can POPs remain in the envrionment for a long time?
Yes
What form are POPs produced in at low latitudes?
they are produced in gaseous form and enter the atmosphere
Atmospheric circulation patterns
they carry them to the Artic, where they condense to liquid and fall from the atmosphere
What has been banned in North America but some developing countries still use them?
Manufacture and use of POPS
What is the correlation between POPs and diet?
-Inuit communities that ate mostly marine mammals had the highest levels of POPs
-Communities that ate herbiovorous caribou had lower levels
-Inuit community diets were centered around different trophic levels
Trophic Levels
describe the feeding positions of groups of organisms in ecosystems
The ecological roles of organisims are determined by…
their trophic interactions (what they eat and what eats them)
What is trophic level based on?
the number of feeding steps by which it is seperated from autotrophs
What does the 1st trophic level consists of…?
autotrophs or primary producers
Autotrophs
generate chemical energy from sunlight or inorganic chemical compounds
What trophic level generates most of the dead orgnaic matter in an ecosystem?
autotrophs
2nd Trophic Level
Herbivores that consume autotrophs; also includes detritivores that consume dead organic matter
3rd and higher trophic levels
Carnivores that consume animals from the level below
Ominvores
feed at multiple trophic levels
Omnivores Example
coyotes are opportunistic feeders, consuming vegetation, mice, other carnivores, and ond leather boots
Detritus
all arganisims are either consumed by other organisims or enter the pool of dead organic matter
What portion of biomass is consumed in terrestrial ecosystems?
small portion
Where does most of the energy flow pass through in the trophic levels?
detritus
What do detritivores consume?
dead organisims and feces
What are detritivores?
primarily bacteria, archaea, and fungi
What tropic level is detritus considered to be part of?
1st trophic level
What trohic level are detritivores part of?
2nd level
Where is much of the detritus in streams, lakes, and estuaries derived from?
terrestrial organic matter
Allochthonous Inputs
external energy inputs from detritus
Autochthonous Energy
energy produced by autotrophs
Where are allochthounous inputs most important?
stream ecosystems
River Continuum Concept
states that the importance of autochthonous energy inputs increases from the headwaters toward the lower reaches of a river
What increases and decreases as you go downstream?
-water velocity decreases
-nutrient concentrations increase
How is the amount of energy transferred from one trophic level to the next?
depends on the food quality and consumer abundunce and physiology
Second Law of Thermodynamics
states that during any transfer of energy, some is dispersed and becomes unusable
Available energy _____ with each trophic level
decrease
What portrays the relatice amounts of energy or biomass in each trophic level?
trophic pyramid
Is all of the biomass at each trophic level consumed?
no. Some of the energy is dispersed in the transfer to the next level
Why are energy and biomass pyramids similar in terrestrial ecosystems?
biomass is closely associated with energy production
What does the biomass pyramid look like in aquatic ecosystems?
Inverted
What are the primary producers?
phytoplankton
What is the life span of phytoplankton?
short life spans and high turnover
Where are inverted biomass pyramids most common?
productivity is lowest, such as nutrient-poor regions of the open ocean
Factors that determine how much energy flows from one trophic level to the next:
-Amount of NPP at basse of food web
-Proportion of each trophic level consumed by the one above it
-Nutritional content of autotrophs, detritus, and prey
-Efficiency of energy transfers
Do land herbivores or aquatic herbivores consume a lower proportion of autotrophs?
land herbivores consume a much lower proportion of biomass
How much terrestrial NPP is consumed on average?
about 13%
How much NPP is consumed in aquatic ecosystems?
35% on average
What kind of relationship does NPP and amount of biomass consumed by herbivores have?
positive relationship
What may be limited by the amount of food available?
Herbivore production
Hypotheses for why terrestrial herbivores consume less of the available biomass?
-Herbivores are constrained by predators and never reach carrying capacity
-Autotrophs have defenses against herbivory, such as secondary compounds, spines, etc.
-Phytoplankton are more nutritious for herbivores than terrestrial plants
What hypothesis is suppourted by predator removal experiments?
Herbivores are constrained by predators and never reaching carrying capacity
What is the main difference in resource-poor environment plants and resource-rich environment plants?
resource poor plants have stronger defenses
What do unicellular algae generally lack?
chemical and structural defenses
Why are terrestrial plants not as nutritious as phytoplankton?
terrestrial plants have structural components, such as wood, which have few nutrients
What is an indicator of food quality?
the ratio of carbon to nutrients such as N and P
What has a closer carbon to nutrient. ratio to herbivores?
freshwater phytoplankton
Trophic efficiency
amount of energy at one trophic level divided by amount of energy at the trophic level immediately below it
Three components of trophic efficency
-proportion of available energy that is ingested (consumption efficiency)
-proportion of ingested food that is assimilate (assimilation efficiency)
-proportion of assimilated food that goes into new consumer biomass (production efficiency)
Consumption efficency
-higher in aquatic ecosystems than in terrestrial ecosystems
-consumption efficencies also tend to be higher for carnivores than for herbivores
Assimilation efficiency
is determined by food quality and consumer physiology
Why is the food quality of plants and detritus low?
because of complex compounds such as cellulose, lignins, and humic acids, which are not easily digested, and low concentrations of N and P
Animals have __________ similar to the animals consuming them
carbon to nutrient ratios
Assimilation efficencies of herbivores and detritivores
20-50%
Assimilation efficiencies of carnivores
about 80%
Endotherm vs Ectotherm digestion
-Endotherms digest food more quickly than ectotherms and thus have higher assimilation efficiencies
What do mutualistic symbioints help herbivores digest?
cellulose
How do ruminants break down cellulose?
they have a modified foregut with bacteria and protsists that break it down
What has higher assimilation efficencies than all other herbivores?
ruminants
What is production efficeiency strongly related to?
thermal physiology and size of the consumer
What do endotherms allocate more energy to than ectotherms?
heat production, so they have less for growth and reproduction
What size endotherm loses a grerater proportion of its heat across is body surface?
small endotherm lose a greater proportion of its heat than large endotherms
-lower production efficiency
What can impact trophic efficiency can can determine consumer population size?
change in food quantity or quality
How much did Steller sea lion populations in Alaska decline by over 25 years?
about 80%
What seemed to be the cause of the smaller body size and decreased birth rates of Steller sea lions in Alaska?
food quantity or quality problems (decline in trophic efficiency)
What did the change in diet of Stellar sea lions reflect in the fish community?
it reflected a shift in the fish community
What can influence the flow of energy at multiple trophic levels?
changes in the abunduances of organisims at one trophic level
The “bottom up” view
resources that limit NPP determine energy flow throught an ecosystem
The “top down” view
-energy flow is governed by rates of consumption and other interactions at the highest trophic level
-this influences multiple trophic levels below them
What control affects energy flow in an ecosystem?
Both controls are operating simultaneously (bottom up and top down)
What control has implications for the effects of trophic interactions on energy flow?
top-down control
Trophic cascade
series of trophic interactions that result in changes in biomass and species composition
Example of a trophic cascase
Predation by a top carnivore (4th level)
decreases abundance of 3rd level
carnivores, which leads to an
increase in herbivores (2nd level),
and then a decrease in primary
producers.
Where do many examples of trophic cascades come from?
accidental introductions of non-native species or extinctions of native species
Flecker and Townsend (1994)
-compared the effects of brown trout and native galaxies using artificial steam
-brown trout reduced total invertebrate density more than galaxias did
-algal abundance also increased more with brown trout present
-showed a trophic cascade since brown trout had a greater effect on primary production than native fish did
What type of ecosystems are more complex and what does this mean for them in terms of trophic cascades?
terrestrial ecosystems are more complex than aquatic ecosystems making trophic cascades unlikely
Why are trophic cascades unlikely in one species in terrestrial ecosystems?
a decrease in one species would be compensated for by increases in similar species that were not being consumed as heavily
Dyer and Letourneau (1999)
-tropical forest trophic cascade study with four trophic levels
-insecticides were used to kill all ants and beetles were introduced to some plots. Also varied the soil fertility and light lecels to test for bottom up effects
-Found that the trophic cascade was the only significant influence on leaf production by Piper trees
-The predatory beetles decreased ant abundance, which increased rates of herbivory, and decreased leaf area per tree.
What factors have a signifciant effect on leaf production?
light levels and fertility, but the top-down effects of predation are still persistant
What determines the number of trophic levels in an ecosystem?
-the number of levels may change due to addition or loss of a top predator or a predator in the middle of the food chain
-omnivore may change food prefrence
Factors that control the number of trophic levels
-amount of energy entering via primary production
-frequency of distribution
-ecosystem size
Why does the amount of energy that enters the ecosystem via primary production affect the number of trophic levels?
-more production should allow for more trophic levels
-this appears to be important in resource-poor ecosystems
Why does the frequency of disturbance effect the number of trophic levels?
-higher trophic levels depend on lower levels and take time to reestablish after disturbance
-if disturbance is frequent, higher levels may never become established
Why does the ecosystem size effect the number of trophic levels?
larger ecosystems support larger populations, have more habitat heterogeneity, and have higher species diversity
Takimoto et al. (2008)
-a study of 36 islands in the Bahamas found that island size was correlated with number of trophic levels
-disturbance frequency did not impact trophic levels, but the top predator species changed
Food webs
-conceptual models of the trophic interactions of organisims in an ecosystem
-diagram showing the connections between organisims and the food they consume
-shows qualitatively how energy flows from one component of this ecosystem to another
What is often ignored in food webs?
-other interactions like pollination
-the role of microorganisims
Interaction strength
measure of the effect of one species on the population size of another species
Removal experiments
can determine interaction strengths, but can’t really be done for all links in a food web
What can be used to predict strength of interactions?
-predator and prey body size
-this is because feeding rate is realted to metabolic rate, which in turn is governed by body size
Paine (1966)
-removal of Pisaster sea star in rocky intertidal zone
-mussels become dominant after removal and species richness went from 15 to 8
-called Pisaster a keystone species because it has a greater influence than its abundance or biomass would predict
Keystone species concept
implies that protecting a keystone species may may be critical for protecting the many other species that depend on it
Keystone speices
-tend to be top predators but not always
-can be keysonte species in only part of their range which suggest that interaction strength depends on environmental context
What can removal experiments provide for determining the strength of indirect effects?
can provide estimates of the net effect of a species
Net effect
sum of the direct effect and all possible indirect effects
What does a predator have a direct and indirect effect on?
-direct efect on its prey
-indirect effect on other species that compete with, facilitate, or modify the enviornment of the prey species
When is the potential for indirect effects to offset or reinforce the direct effect of a predator?
when the direct effect is weak
Berlow (1999)
-tested the potential for indirect effects to offest or reinforce the direct effect
-Whelks have a direct negative effect on mussels by eating them
-Barnacles facilitate mussels by providing crevices for mussel larvae to settle in
-when barnacle density is high, they do not attach well and are easily knocked off the rocks, along with the mussel larvae
-when barnacle density is low, whelk predation on barnacles has a negative indirect effect on mussels by providing them with a more stable settlement substrate
-when barnacle density is high, thinning by whelks has a positive indirect effect on mussels by providing them with a more stable settlement substrate
If a predator’s effect on a prey species depends on the prescence or absence of other species…
the potential for the predator to elimanate that prey species is less
What does variation associated with weak interactions promote?
coexistence of multiple prey species
How is stability measured?
how much the populations change over time
Charles Elton and Eugene Odum
argued that simpler, less diverse food webs should be more easily perturbed
Robert May (1973)
used random assemblages of organisims to demonstrate that food webs with higher diversity are less stable using mathematical analyses
What did strong trophic interactions accentuate in May’s model?
population fluctuations
The more interacting species there were in May’s model, the more likelu that population fluctuations would reinforce one another. What does this lead to?
extinction of one or more species
What are the factors that allow naturally complex food webs to be stable?
-As shown by Below, weak interactions can stabilize trophic interactions
-newer models and experiments suggest that some food webs may have a structure that allows increased species diversity to have a stabilizing effect
-buffering influence of weak interactions and behavioral or evolutionary changes in prey choice can help reduce population fluctutations
Why is the identity of species in a food web important?
some species exert a disproportionally greater influence on stability, and others are more likely to go extinct
Where is pant production often higher and why is this important?
-plant production is often higher in diverse communities
-more diverse communities recover better from disturbances
How does increased plant diversity affect other species?
-arthropod communities were more stable
-specialist herbivores had lower stability
-all herbivores showed greater stability
Mechanisims for how plant diversity influences arthropod community stability
-Greater and more stable plant biomass
-increased diversity in the arthropod communities
What is higher plant diversity linked to?
greater predator abundance and diversity due to its influence on habitat diversity
-these predators may exert top-down effects on herbivore and plant abundances
Portfolio effect
variation in one species’ population cancels out variation in another so that overall abundance in the communiuty remains the same
What would a greater diversity among arthropods lead to?
greater probability of the portfolio effect
Bioaccumulation
some chemicals are not metabolized or excreted, and become progressively more concentrated in tissues over an organisim’s lifetime
Biomagnification
concentration of these compounds increases in animals at higher trophic levels, as animals at each trophic level consume prey with higher concentrations of the compound
Rachel Carson (silent spring 1962)
-described the devestating effects of pesticides, espically DDT, on nontarget bird and mammal species
-Silent spring talked about the dangers of bioaccumulation and biomagnifaction of POPS
DDT
-was thought to be a “miracle” in the 1940-1950 and was used extensively on crops and to control mosquitoes
-but it was building up in top predators, contributing to the near-extinction of peregrine falcons and bald eagles
-eventually banned after Carson’s publication
How does biomagnification apply to the Inuit people?
-Inuit people that consume marine mammals have higher concentrations of POPs. These animals occupy 3rd,4th, or 5th trophic levels
-Inuits who consume mostly caribou (herbivores) have lower POP levels
Where have pollutants been reported on Earth?
all environments
What concentration of pollutants are found in animals in remote areas?
high concentrations espically in top consumers
How are ecosystems connected?
by the movement of energy and materials
How are POPs and other pollutants transported>
atmospheric circulation and animals
Krummel et al. (2003)
-sampled sockeye salmon in eight lakes in southern Alaska. Also collected sediment cores to anylze for PCBs
-sedimentary PCB correlation was positively correlated with salmon density
Blais et al. (2005)
-found that mercury and POPs are transported by northern fulmars (pelagic fish-eating seabirds) from the ocean to small ponds near their nesting colonies
-showed how behaviors can exacerbate problems of biomagnification
