A REVIEW OF TRAIT‐MEDIATED INDIRECT INTERACTIONS IN ECOLOGICAL COMMUNITIES

In this paper we review the empirical studies documenting trait‐mediated indirect interactions (TMIIs) in food webs. Basic models and empirical approaches that form the foundation of our conceptualization of species interactions generally assume that interactions are an intrinsic property of the two interacting species and therefore are governed by their respective densities. However, if a species reacts to the presence of a second species by altering its phenotype, then the trait changes in the reacting species can alter the per capita effect of the reacting species on other species and, consequently, population density or fitness of the other species. Such trait‐mediated indirect interactions can reinforce or oppose density‐mediated effects and have been largely overlooked by community ecologists. We first briefly develop the case for the broad mechanistic basis for TMIIs and then review the direct evidence for TMIIs in various permutations of simple three‐ to four‐species food webs. We find strong evidence for quantitatively significant effects of TMIIs in a variety of aquatic and terrestrial systems. We further highlight those few studies that address the question of the relative magnitudes of density‐ and trait‐mediated effects and the role of species densities in their transmission. These studies indicate that trait effects are often as strong or stronger than density effects. We conclude that ecological communities are replete with TMIIs arising from trait plasticity and that these effects are quantitatively important to community dynamics. Finally, we synthesize our results and indicate profitable directions for future research. Corresponding Editor: F. R. Adler. For reprints of this Special Feature, see footnote 1, p. 1081.