Phenotypes of predator individuals underpin contrasting ecosystem effects

Intraspecific variability has implications for our understanding of ecosystem functioning. Yet, beyond ontogenetic variation in body size, we know little about how phenotypic differences between individuals modulate ecosystem functioning. We addressed this question empirically within a forested stream, by studying the ecosystem effects triggered by conspecific predatory dragonfly larvae that varied in sex, body shape, behaviour and growth rate. With the help of stream enclosures, we were able to evaluate how different predator phenotypes drive cascading effects on litter-dwelling prey, fungal biomass and leaf litter decomposition, in uncontrolled natural conditions. Some individual predators increased leaf litter standing stocks and fungal biomass compared to controls (without dragonfly larvae added), but others had the opposite effect. These contrasting effects were repeatable between the two runs of our experiment. Phenotypic effects were related to the sex and the position of individuals along a life history-behavioural continuum, and were possibly mediated by differential top-down control of prey, or by inter-individual variability in non-consumptive effects. Several phenotypic dimensions are likely to mediate individual variability in predator effects on ecosystem processes, even in a complex and stochastic natural context such as a forested stream. We invite further investigations that seek to understand the mechanisms (e.g., non-consumptive effects) driving individual variability in predator effects on ecosystem processes.