Most community-based models in ecology assume that all individuals within a species respond similarly to environmental conditions and thereby exert identical effects as consumers or prey. Rather, individuals differ among systems, with important implications for population demographics and community interactions. For widely distributed assemblages made up of poikilotherms with high first-year mortality, species-specific differences in growth reaction norms as affected by both temperature and genotype will influence biotic interactions. For a broadly distributed fish assemblage, first-year growth does not vary with latitude for a planktivorous prey species, but declines with increasing latitude for a terminal piscivore. Size-based competitive interactions between these species are likely to be more intense at high latitudes, as they spend an extended time sharing resources during early life. Such patterns probably are pervasive and must be considered when seeking to understand species interactions. Improving our knowledge of how temperature and local adaptations affect size-based interactions should enhance our ability to manage and conserve widespread assemblages.