Indirect effects: concepts and approaches from ecological theory

Abstract

Indirect effects are changes in the abundance of a population resulting not directly from the action of a causal agent (such as a toxicant) but indirectly through the effects of the causal agent on other species. Examples are presented of simple indirect effects that have been studied by theoretical ecologists. The possible use of both empirical studies and ecosystem models to predict indirect effects of one species on another is discussed. Both approaches have been used to estimate the relative magnitudes of direct and indirect effects in food webs. However, the uncertainty of interspecific interactions may multiply when indirect effects are computed, leading to low confidence in predictions of effects. Environmental and demographic stochasticity, as well as population density-dependent self-regulation, may diminish the influence of indirect effects.