Figure 14.2 Food webs for (a) an unpolluted and (b) a polluted marsh/estuary. (Based on Mattson and Vallario, 1975.)

Figure 14.2 (Continued)

hypothetical food webs for a similar marsh, one that is unaffected by human activities, and a second that is affected heavily by pollution.

The more species there are in an ecosystem, the more possible interactions that could exist. Specifically, if there are s species, there could be up to s(s — 1)/2 pairwise interactions. However, in reality, each species tends to have only about two interactions on average. Another generalization that can be made about food chains is that there tend to be about two to three prey species for each predator, no matter how many total species are present.

The flow of energy described in this section goes one way. It starts with the solar input, part of which is captured in gross productivity; what does not get used by the producers forms net productivity; successive trophic levels harvest part of this and produce their own biomass. At each step a majority of the energy is lost. Ultimately, all is returned to the environment as heat, after doing some useful work in producing biomass at each level. Although the ecosystem may try to optimize its use of energy, energy cannot be recycled. The second law of thermodynamics prevents the waste heat from being collected and reused for useful work.

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