Intraspecific trait variation and its effects on food chains

Traits such as skill at foraging and investment in anti-predator defense may vary among individuals within a species population. This intraspecific variation has implications for community dynamics. The implications of intraspecific variation of a consumer in the intermediate level of a tritrophic food chain are explored for the case in which two different phenotypes exist within the consumer population having tradeoffs in traits with respect to foraging for resources and resisting predation. The topology of such a web is similar to that of the diamond-shaped food web. An important result of prior studies on diamond-shaped webs is that conditions for equilibrium coexistence of two competing consumer species can be found, but the transient oscillations would make it likely for one competing species to become extinct. In the case of two phenotype subpopulations within a single consumer species, however, switching between the two subpopulations can occur, which is stabilizing. As a result, it is feasible for two distinct phenotype subpopulations of the consumer to exist between resources and predators in a tritrophic chain. The occurrence of two stably coexisting phenotype populations changes the nature of the bottom-up and top-down effects in the chain. The predator exerts a top-down effect on the resource, not the consumer subpopulations, and changes in the resource carrying capacity causes changes in the consumer subpopulations, but not the populations of the predators or the resources themselves.