Biology 2483A Chapter 21: Energy Flow and Food Webs
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Lecture 20 – Energy Flow and Food Webs
The ecological roles of organisms are determined by their
interactions—what they eat and what eats them.
o This determines the organisms influence on movement of energy and nutrients through an ecosystem.
o Trophic dynamics control energy/nutrient flow.
Trophic levels describe the feeding positions of groups of organisms in ecosystems.
- Most energy going through Detritivores
(feeding on dead material produced by
carnivores). Slide at the same level of
herbivores. Essentially eating energy
produced by autotrophs. Get eaten by
carnivores as well.
All organisms are either consumed by other organisms or enter the pool of dead organic matter (
). In terrestrial
ecosystems, only a small portion of the biomass is consumed, and most of the energy flow passes through the detritus.
- Everything that’s not here =
what’s getting eaten.
- Everything here = everything
not getting eaten.
Up to 60% of the studied looked
at, almost all of the NPP ended
up as detritus.
A lot of terrestrial NPP isn’t going
up through herb/carnivores.
Dead organisms and feces are consumed by organisms called detritivores (primarily bacteria, fungi also worms), in a process
o Detritus is considered part of the first trophic level, and detritivores are part of the second level (level of
Most detritus in aquatic streams, lakes, and estuaries is derived from terrestrial organic matter.
o These external energy inputs are called allochthonous inputs.
Allochthonous inputs can be very important in stream ecosystems (up to 99.8%).
o Energy produced by autotrophs within the system is autochthonous energy.
The importance of autochthonous energy inputs increases from the headwaters toward the lower reaches of
Downstream, allochthonous inputs become a smaller proportion.
Water velocity decreases, and nutrient concentrations increase as you go downstream.
Energy Flow among Trophic Levels
The amount of energy transferred from one trophic level to the next depends on food quality and consumer
abundance and physiology.
The second law of thermodynamics states that during any transfer of energy, some is dispersed and becomes unuseable:
Energy will decrease with each trophic level.
A trophic pyramid portrays the relative amounts of energy or biomass of each trophic level.
o Some of the biomass at each level is not consumed, and some of the energy is dispersed in the transfer to the next
In terrestrial ecosystems, energy and biomass pyramids are similar because biomass is closely associated with energy
- Very large base at the bottom in
terms of primary producers.
- Very sharp drop of as you go up to
herbivories partly because of the
energy losses, also because of the
consumptive losses (they’re not
eating everything ie. berries, fruit).
- Trees have low turnover = high
In aquatic ecosystems, the biomass pyramid may be inverted.
o Inverted because primary producers are phytoplankton with short life spans and high turnover.
o Inverted biomass pyramids are more common where productivity is lowest,
Such as nutrient-poor regions of the open ocean.
o Phytoplankton turnover is high, associated with high growth rate and short life span compared with
phytoplankton of nutrient-rich waters.
- Biomass ratio of
heterotrophs:autotrophs – whether
you’re more top heavy.
- Have a cross over point in figure btw
regular and inverted pyramids.
- As you go down in productivity or
biomass, get more inverted pyramids.
Because in these low nutrient
systems, you have much higher
turnover of phytoplankton.
Herbivores on land consume a much less proportion of autotroph biomass (NPP) than herbivores in most aquatic
o On average, about 13% of terrestrial NPP is consumed; in aquatic ecosystems, an average of 35% NPP is
- There is a positive relationship
between net primary production
(NPP) and amount of biomass
consumed by herbivores.
- This suggests that herbivore
production is limited by the
amount of food available.
Why don’t terrestrial herbivores consume more of the available biomass?
o Several hypotheses have been proposed:
1. Herbivores are constrained by predators, and never reach carrying capacity.
a. Predator removal experiments support this hypothesis in
2. Autotrophs have defenses against herbivory, such as secondary compounds, spines, etc.
a. Plants of resource-poor environments tend to have stronger defenses than plants from resource-rich
3. Phytoplankton are more nutritious for herbivores than terrestrial plants.
a. Terrestrial plants have structural components (effects of gravity more strong than in water)
such wood, which have few nutrients.
b. Freshwater phytoplankton have carbon:nutrient(nitrogen) ratios closer to those of herbivores
than to those of terrestrial plants.
Trophic efficiency: Amount of energy at one trophic level divided by the amount of energy at the trophic level immediately
Trophic efficiency incorporates three types of efficiency:
1. Proportion of available energy that is consumed (consumption efficiency).
2. Proportion of ingested food that is assimilated (assimilation efficiency).
Biomass (secondary production)
3. Proportion of assimilated food that goes into new consumer biomass (production efficiency).
1. Consumption efficiency is higher in aquatic ecosystems than in terrestrial ecosystems.
o Consumption efficiencies also tend to be higher for carnivores than for herbivores (most of the prey are accessible).
2. Assimilation efficiency is determined by food quality and the physiology of the consumer.
Food quality of plants and detritus is low because of complex compounds such as cellulose, lignins, and
humic acids that are not easily digested, and low concentrations of nutrients such as nitrogen and
Animals have carbon:nutrient ratios similar to the animals consuming them.
Assimilation efficiencies of herbivores and detritivores are 20%–50%; carnivores are about 80%.
Endotherms (warm-blooded) digest food more completely (keep body temp stable) than ectotherms
(cold-blooded) and thus have higher assimilation efficiencies.
Some herbivores have mutualistic symbionts that help them digest cellulose (ie. cows).
Ruminants (cattle, deer, camels) have a modified foregut with bacteria and protists that break
down cellulose. They have higher assimilation efficiencies than other herbivores.
3. Production efficiency is strongly related to the thermal physiology and size of the consumer.
o Endotherms allocate more energy to heat production, and have less for growth and reproduction than ectotherms.
o Body size affects heat loss in endotherms. As body size increases, the surface area-to-volume ratio decreases.
o A small endotherm (e.g., a shrew), loses a greater proportion of its heat across its body surface than a large
endotherm, such as a grizzly bear, and will have lower production efficiency.
Large mammals double production
efficiency compared to small.
Endotherms much poorer than Ectotherms
in terms of production efficiency due to
heat generation losses.
Change in food quantity or quality impacts trophic efficiency and can determine consumer population size.
Steller sea lion populations in Alaska declined by about 80% over 25 years.
o Smaller body size and decreased birth rates suggested food quantity or quality might be a problem.
o Looked at gut contents: Prey quantity was not declining but the sea lions had shifted from a diet of mostly herring
(high in fats) to more cod and pollock (half as much fat).
o The change in diet reflected a shift in the fish community.
- Quality of food supply, and not the quantity of food affected their population
Changes in the abundances of organisms at one trophic level can influence energy flow at multiple trophic levels.
- Drastic shifts in population sizes occur in a “domino effect”
What controls energy flow through ecosystems? “Bottom-up” or “top-down” control?
o The “bottom-up” view: Resources that limit Productivity (NPP) determine energy flow through an ecosystem
o The “top-down” view: Energy flow is governed by predator consumption rates at the highest trophic level,
which influences multiple trophic levels below them.
In reality, both controls are operating simultaneously in ecosystems.
o Top-down control has implications for the effects of trophic interactions on energy flow.
What determines # of trophic levels in an ecosystem?
o Limit of tropic levels = ~5.
o The number of trophic levels may change due to addition or loss of a top predator, or a predator in the middle of
the food chain. Or, an omnivore may change food preference.