Chapter 57
Why does it take so many calories of algae to support so few calories of humans?
(Figure 57.12)
Answer: At each link in the food chain, only a small fraction of the energy at one level is
converted into mass of organisms at the next level. Much energy is dissipated as heat or
excreted.
How can the existence of inverted pyramids of biomass be explained? (Figure 57.13)
Answer: In the inverted pyramid, the primary producers reproduce quickly and are eaten
quickly, so that at any given time, a small population of primary producers exist relative
to the heterotroph population.
Why do streams with trout have more algae? (Figure 57.14)
Answer: Because the trout eat the invertebrates which graze the algae. With fewer
grazers, there is more algae.
What might be the effect if snakes that prey on fish were added to the enclosures?
(Figure 57.15)
Answer: The snakes might reduce the number of fish, which would allow an increase in
damselflies, which would reduce the number of chironomids and increase the algae. In
other words, lower levels of the food chain would be identical for the "snake and fish"
and "no fish and no snake" treatments. Both would differ from the enclosures with only
fish.
How is it possible for the biomass of the primary producers to stay relatively constant as
the primary productivity increases? (Figure 57.17)
Answer: Herbivores consume much of the algal biomass even as primary productivity
increases. Increases in primary productivity can lead to increased herbivore populations.
The additional herbivores crop the biomass of the algae even while primary productivity
increases.
Why is the amount of light an important determinant of carnivore biomass? (Figure
57.18)
Answer: More light means more photosynthesis. More plant material means more
herbivores, which translates into more predator biomass.
How could you devise an experiment on invasibility that didn’t rely on species from
surrounding areas? (Figure 57.19)
Answer: Introduce them yourself. For example, each spring, you could place a
premeasured number of seeds of a particular invasive species in each plot. Such an
experiment would have the advantage of more precisely controlling the opportunity for
invasion, but also would be less natural, which is one of the advantages of the Cedar
Creek study site: the plots are real ecosystems, interacting with their surrounding
environment in natural ways.
a. Why is species richness greatest at intermediate levels of productivity? b. Why do
more structurally complex areas have more species? c. Why do areas with less variation
in temperature have more species? (Figure 57.20)
Answer: (a) Perhaps because an intermediate number of predators is enough to keep
numbers of superior competitors down. (b) Perhaps because there are more habitats
available and thus more different ways of surviving in the environment. (c) Hard to say.
Possibly more stable environments permit greater specialization, thus permitting
coexistence of more species.