Trait-initiated indirect effects due to changes in consumption rates in simple food webs

This article explores the potential indirect effects due to changes in consumption-related traits of a focal prey species in each of two simple food webs. The systems considered are: one predator exploiting two independent prey, and one predator exploiting two prey that use a common limiting resource. In each system, the indirect effect of a change in a consumption-related parameter of one prey species on the population density of the other prey is determined. When prey only share a predator, the change in one prey’s density produced by a change in the other prey’s vulnerability to the predator is determined by whether or not that second prey is initially overexploited; i.e., whether its exploiter’s intake decreases with an increase in the attack rate. Increases in the vulnerability of an overexploited prey decrease the predator population and thereby indirectly increase the population size of the second prey. Increases in the vulnerability of a prey that is not overexploited have the opposite effects on both the predator and second prey populations. Both of these outcomes can be reversed by strong direct density dependence in the predator, provided the predator exhibits satiation. In the food web with a shared food and a shared resource, increases in predator vulnerability of the more vulnerable prey species always increase the population size of the more resistant prey species. However, increases in the vulnerability of the more resistant prey can either increase or decrease the population size of the more vulnerable prey species. When the more resistant prey becomes better at capturing the resource, the less resistant may either increase or decrease. Overexploitation also explains these outcomes. General principles for understanding the consequences of changes in vulnerabilities or attack rates are discussed