Intraspecific variation in mating behaviour modulates the effects of mosquitofish introduction on prey communities

Intraspecific variation is recognized as a driver of ecological processes, yet prior studies have overwhelmingly focused on the ecological effects of intraspecific trait variation driven by natural selection. Far less is known about how variation in sexually selected traits influences ecological outcomes. In this study, we tested whether intraspecific differences in reproductive behaviours affect the ecosystem- and community-level impact of western mosquitofish (Gambusia affinis) introductions. Male mosquitofish frequently harass females through persistent mating attempts. We manipulated the intensity of male harassment and then used a mesocosm experiment to measure the ecological consequences of these differences in harassment. Although this behaviour manipulation was short-lived, our results suggest that differences in harassment intensity affected zooplankton communities. Compared to fish-absent controls, mesocosms with high-harassment males had smaller zooplankton body size and reduced abundance in several dominant taxa. These effects were not observed in low-harassment treatments, suggesting a threshold response in which only high-harassment males elicited community changes. The presence of mosquitofish also caused a trophic cascade to impact phytoplankton biomass; however, the strength of this cascade was not altered by harassment intensity. Our findings demonstrate that sexual selection can shape community composition and shows that reproductive behaviour can modulate the impact of species introduction.