Aquatic vegetation promotes prey diversity but not the diversification of predator phenotypes

Freshwater fish have evolved to forage in a range of habitats, from open water in lakes to benthic shorelines and streams. Much research has focused on how prey differences, such as zooplankton versus benthic macroinvertebrates, might select for divergence in predator traits across this habitat gradient. However, major structural components of this habitat contrast, such as aquatic vegetation, may fundamentally constrain foraging success. Here, we used a mesocosm experiment to test for interactive effects of vegetated habitat and fish density on the survival and growth of phenotypically variable juvenile sticklebacks. We investigated two scales of phenotypic variation: distinct lake and stream ecotypes and F2 lake-stream hybrids. We found that macrophytes strongly increased invertebrate biomass and functional richness, which increased average fish condition. Surprisingly, the relative performance of contrasting ecotypes and genotypes depended only on fish density. Stream ecotypes had higher survival than lake ecotypes, as well as higher condition. In parallel, F2 hybrids had higher growth when homozygous for stream alleles at three quantitative trait loci. Together, these findings suggest that adaptive divergence among habitats depends on the ways that prey type, habitat structure, and genetic variation for feeding and growth traits combine with competition to limit the abundance of accessible prey.