Once energy captured by primary producers is fixed as NPP, consumption by heterotrophs transfers a fraction of that energy to higher trophic levels (net secondary production). Terrestrial systems have lower trophic efficiencies than aquatic systems, where the rapid rate of primary production supports greater secondary production. The flow of energy in food webs is influenced by the number of trophic levels and the relative contribution of bottom-
learning outcomes
You should be able to:
Compare and contrast, with examples, the production efficiencies of ectotherms and endotherms.
Analyze why trophic efficiencies are higher in aquatic than in terrestrial ecosystems, and explain how this affects biomass throughout the food web.
Analyze, both qualitatively and quantitatively, the effect that length of a food chain has on NPP in an ecosystem.
Explain how omnivory can change the transfer of energy in a food web.
Provide and explain hypotheses relating to control of the length of food chains.
Suppose there are two food webs, one in a forest, the other in a lake. All else being equal, which food web would have the higher trophic efficiency, and why? Comparing different species, which would have the higher production efficiency, a bear or a crayfish? Explain your answer.
The lake food web would have the higher trophic efficiency. Forest ecosystems have lower trophic efficiency than aquatic systems because much of the NPP in forests is in the form of wood and is unavailable to consumers. In contrast, the lake food web is based on phytoplankton and algae, which are more easily converted to secondary production. Estimates suggest that, on average, only 13 percent of terrestrial biomass is consumed by herbivores, compared with 35 percent in aquatic ecosystems.
The crayfish, which is an ectotherm, would have a higher trophic efficiency than the bear, which is an endotherm. Endotherms maintain higher metabolic rates than ectotherms and thus have less energy left over to devote to growth and reproduction.
Now compare two food webs in different lakes. One has three trophic levels, and the other has four. All else being equal, which would have the higher NPP? Why? If you added omnivory to the top level of the food web with four trophic levels, how would it differ in NPP compared with the food web without omnivory?
The lake with three trophic levels would have the higher NPP. A three-
Omnivory can also change the way energy is transferred in food webs by essentially “collapsing” trophic levels on one another. A four level food web with a top consumer feeding on both the herbivore and the primary producer should have lower NPP than a four-
Compare a tropical rainforest food web that has five trophic levels with a food web in a desert having three trophic levels. What hypothesis might best explain the difference in trophic level length between the two ecosystems?
One hypothesis, which focuses on the amount of NPP entering an ecosystem, suggests that bottom-
We’ve considered how energy flows from primary producers to consumers, and the implications of that flow for food webs. Besides energy, a key component of the metabolism and growth of organisms is nutrients (including water). Earlier in the chapter you read that nutrients continually cycle between living organisms and the abiotic environment via nutrient cycling (see Figure 57.1C). We will consider nutrient cycling next.