Predator-mediated resource limitation shapes body and head size variation in stickleback populations

Predator and prey communities are important putative drivers of phenotypic variation in consumers. However, in natural food webs, we often lack an understanding of the ecological interactions by which the species community affects consumer traits. In a comparative study, we explore how phenotypic variation of 34 Greenlandic threespine stickleback populations is affected by the presence and absence of an intraguild predator (Arctic char). In the presence of char, we find that stickleback have a larger body size but a similar head size, resulting in allometric changes (i.e., change in relative head size) that are consistent with resource mediation of ontogenetic growth trajectories observed in other fishes. Using path analysis, we show that the observed shift in the relative head size of stickleback can be partly explained by predator-mediated effects on the species composition and biomass of zooplankton. Our study suggests that top predators can indirectly affect resource limitation of consumers via changes in the prey communities of consumers, and thereby influence the allometry of consumer traits, likely via allocation tradeoffs. These results have important implications for interpreting the putative causes of allometric variation among populations and for using comparative studies to discern the ecological causes of phenotypic variation in natural populations.