Population dynamics and food webs

We begin with the two clear patterns from Chapter 4.

• The trophic level pattern: diversity declines at higher trophic levels, so much so that some levels entirely lack species.

• The omnivory pattern: fewer omnivorous species exist than we expect.

Ecology has known the first of these patterns for a long time. It was first enunciated by Charles Elton (1927), a founder of modern ecology. He pointed out the limit to the number of trophic levels in animal communities in all sorts of biomes. Although I have not found a place where he explicitly noted that diversity declines as we approach the top level, it is a trivial deduction from what he did say.

Stuart Pimm and John Lawton discovered the second pattern while investigating a hypothesis to explain all food web patterns. Their hypothesis is called the dynamical stability hypothesis. Because the dynamical stability hypothesis underlies their treatment of the trophic level pattern as well as the omnivory pattern, I will begin this chapter by explaining it.

Pimm and Lawton propose that if associations of species deviate from their usual patterns, they become dynamically unstable and simply do not last. Instead, deviant associations lose some of their species and thus change to other associations.