The phenotypic variation of consumer populations is jointly shaped by their predators and communities of prey. In natural food webs, we often lack a mechanistic understanding of the ecological interactions that affect consumer phenotypes, making it challenging to identify the causes of natural selection. In a comparative study of 75 Greenlandic lakes, we explore how stickleback phenotypic variation is affected by the occurrence of arctic charr (a predator of stickleback), as well as by variation in lake morphology, abiotic environmental conditions, and zooplankton prey community composition. In a first step, we test how the food web configuration of fish (i.e. presence and absence of stickleback and charr) can affect zooplankton community composition. In a second step, we examine how variation in both prey and predator environments, along with other environmental gradients (e.g lake size, productivity), influence functional trait variation of stickleback among lakes. This integrative comparative approach revealed novel insights into the likely underlying ecological causes of stickleback phenotypic variation.
Results/Conclusions
In this study, we found that stickleback profoundly affect the composition of the zooplankton community, relative to both fishless lakes and lakes with only charr. When alone, stickleback shift the community composition of zooplankton to be dominated by the most evasive prey species (i.e diaptomus copepods). However, when charr are present the communities include larger zooplankton species and resemble more closely to lakes with only charr or no fish. This suggests the presence of charr impacts the prey community of stickleback via indirect ecological effects (i.e. by reducing the effects of stickleback on zooplankton). Additionally, these effects likely influence the phenotypic variation of stickleback. For example, the dominance of evasive copepods is associated with the relative head size of stickleback – a trait that has been associated with food limitation in other fishes. At the same time, we find that traits underlying habitat-specific feeding performance are correlated with lake size, and hence with the availability of pelagic versus benthic habitats. Our work suggests that the phenotypic variation of consumer populations is not only driven by landscape-level variation in habitat availability (e.g. lake morphology), but also by the complex ecological interactions in natural food webs.
