Prey State and Experimental Design Affect Relative Size of Trait- And Density-Mediated Indirect Effects

Indirect effects, in which one species affects another through an intermediate species, can occur by changes in either the density or the traits of the intermediate species. Ecologists have focused primarily on density-mediated indirect effects, but have become interested in quantifying the relative sizes of trait- and density-mediated indirect effects. We examined how state-dependent prey behavior and experimental protocols affect the sizes of measured trait- and density-mediated indirect effects in a three-species chain (predator, prey, and resource). We found that the size of trait-mediated indirect effects relative to the size of density-mediated indirect effects increases as the level of resources increases. We also found that the relative contributions of trait- and density-mediated indirect effects depend on the timing of manipulations in relation to the state of individuals and their vulnerability to predators. In addition, we found that trait-mediated indirect effects that have been measured during a portion of a season may diminish or disappear when measured across a whole season because of behavioral compensation. These results show that the relative contributions of trait- and density-mediated indirect effects in a system will be variable, and experiments need to be designed to account for dynamic systems. Corresponding Editor: F. R. Adler. For reprints of this Special Feature, see footnote 1, p. 1081.