CONNECTING THEORETICAL AND EMPIRICAL STUDIES OF TRAIT-MEDIATED INTERACTIONS

Trait-mediated interactions (TMIs), in which trophic and competitive interactions depend on individual traits as well as on overall population densities, have inspired large amounts of research, but theoretical and empirical studies have not been well connected. To help mitigate this problem, we review and synthesize the theoretical literature on TMIs and, in particular, on trait-mediated indirect interactions, TMIIs, in which the presence of one species mediates the interaction between a second and third species. (1) In models, TMIs tend to stabilize simple communities; adding further biological detail often reduces stability in models, but populations may persist even if their dynamics become mathematically unstable. (2) Short- and long-term changes in population density caused by TMIs depend even more on details, such as the curvature of functional responses and trade-offs, which have rarely been measured. (3) The effects of TMIs in multipredator communities depend in a straightforward way on the specificity of prey defenses. (4) Tritrophic and more complex communities are theoretically difficult; few general conclusions have emerged. Theory needs new kinds of experiments as a guide. The most critical needs are experiments that measure curvatures of trade-offs and responses, and experiments that (combined with theory) allow us to scale from short- to long-term responses of communities. Anecdotal evidence from long-term and large-scale studies suggests that TMIs may affect community dynamics at practical management scales; community models incorporating TMIs are necessary and require closer collaborations between theory and experiment. Corresponding Editor: F. R. Adler. For reprints of this Special Feature, see footnote 1, p. 1081.