Predator group composition indirectly influences food web dynamics through predator growth rates

Considerable theoretical work predicts that intraspecific trait variation can have profound ecological consequences by altering species interactions. Because of their high flexibility, behavioral traits may be especially relevant in mediating how species respond to one another, thus affecting food web dynamics and ecosystem functioning. However, empirical evidence supporting this idea is limited. Here we generated predator groups where we manipulated the composition of behavioral types within the groups to assess effects on predator growth rates, prey communities, basal resources and ecosystem functioning in replicated outdoor ponds. Using European perch (Perca fluviatilis), we created three types of predator groups where all individuals expressed either bold or shy phenotypes, or individuals were a mix of the two behavioral types. Bold perch grew faster in mixed groups, indicating that predator growth depends both on each individual’s behavioral type and that of its group members. However, there was no evidence that the behavioral composition of perch directly altered the dynamics of lower trophic levels. Instead, final perch biomass, not behavioral composition, had the strongest influence on lower trophic levels. Thus, the central question may not be whether predator behavior matters at all for trophic dynamics, but rather when behavioral effects will predominate over effects of other influences such as predator biomass variation.