It is well recognized that the identity and incidence of ecologically important species (e.g., keystone, dominant, foundation species) can strongly shapes community structure and regulates ecosystem function. Much less is known about the importance of intraspecific variation in these important species, although growing evidence suggest that genetic and phenotypic variation in important species could be as important as the incidence of those species. This raises the question of how important is intraspecific variation relative to the incidence of important species for communities and ecosystems. Here we use data from a whole lake comparative study and small-scale mesocosm experiment to test the effects of intraspecific variation in the alewife, a keystone predator, relative to the incidence of alewife. Here we compare lakes and mesocosms with landlocked, anadromous or no alewife. We estimate the standardized effect size for intraspecific variation and incidence of alewife on zooplankton prey biomass and composition, species richness and diversity, and algal biomass.
We find that both intraspecific variation and the incidence of alewife had strong effects on zooplankton community structure and algal biomass. Standardized effect sizes for intraspecific variation were roughly equally to the effect size for the incidence of alewife, although effect size varied among response variables. For example, at the whole lake scale, intraspecific variation had as large an impact on algal biomass and the strength of trophic cascades as the incidence of alewife. Results from the mesocosm experiment and whole lake comparative study were similar for most of our response variables. Our results demonstrate that variation within important species can be as important to communities and ecosystems as the incidence of those species. Much more attention should be paid to the processes that generate and maintain intraspecific variation (both within and among populations) and the ecological implications of that variation.