Ecologists increasingly acknowledge the potential impact of rapid and fine-scale evolution on ecological processes. However, the impact of ecological context on the pace and direction of contemporary evolution remains unclear. To experimentally investigate ecological constraints on the rapid divergence (<150 years) of stickleback ecotypes in central Europe, we used a combination of large-scale mesocosms, morphological traits, and genomic data. Specifically, we tested whether habitat complexity and competition intensity promote or constrain diet differentiation among ecotypes using replicated mesocosms with factorial combinations of low and high density of macrophytes and low and high density of juvenile sticklebacks. All fish were lab-raised, and either a mixture of wild-type lake and stream fish or second generation (F2) lake-stream hybrids. Using these two types of focal populations in parallel allows us to test for divergent ecotype performance at the scale of the entire genome and at individually recombining genetic markers. We measured differences in diet and performance among juveniles using stable isotopes and body condition, relating these to morphological and genomic characteristics of individual fish. We then assessed the magnitude of these differences across ecological treatments.
Results/Conclusions
We found that habitat complexity and fish density interactively affected the potential strength of selection, with vegetated habitats increasing survival rates at high density. Selection in resource-limited conditions favored stream fish over lake fish, and preliminary genomic analyses also suggest selection for F2 hybrids with stream rather than lake alleles at certain loci. Most importantly, both fish density and habitat complexity impacted within-mesocosm variance in individual stable isotope signatures (δ13C and δ15N) even after accounting for differences in survival and growth. This indicates impacts of ecological context on realized variation in dietary niche. Variation in δ13C was interactively determined by fish density and habitat structure, and variation in δ15N was additively affected, with resource limitation due to increasing fish density and decreasing habitat complexity leading to greater variance in δ15N values. These findings demonstrate central roles for ecological processes such as resource supply and population growth in promoting and constraining niche divergence, and provide a novel test of the role of ecological selection in generating divergence patterns observed in the wild.