Synergistic effects of predation and parasitism on competition between edible and inedible phytoplankton

Abstract

Predation and parasitism are trophic interactions that strongly influence food web structure and community composition. It is well known that both, predation as well as parasitism influence competition within prey/host communities. Many studies focus on only a single type of trophic interaction, however, in natural systems both are co-occurring and strongly interlinked. While impacts of predation/parasitism on the competitive abilities of prey/host species have been extensively studied, the combined effects of parasitism and predation remain still poorly understood. Here, we address this crucial research gap by examining experimental plankton communities in which parasitism and predation simultaneously impact competitive interactions among host and non-host species. We constructed an experimental plankton food web consisting of two competing phytoplankton species, a zooplankton grazer, and a parasitic aquatic fungus (chytrid). We experimentally assessed the single and combined effects of predation and parasitism on competition between edible non-host and inedible host phytoplankton for the shared nutrient pool via their impact on phytoplankton dynamics. In isolation, chytrid infection increased, while zooplankton predation decreased the relative abundance of edible to total phytoplankton. In combination, however, the relative abundance of edible phytoplankton was lower than predicted from the individual effects of zooplankton predation and chytrid parasitism, respectively, suggesting a synergistic effect. This was verified by comparing experimental results to model simulations. While such a synergistic effect had been theoretically predicted, to our knowledge, this is the first experimental study that demonstrates this effect. To improve our understanding on community dynamics, future studies should address the complexity of interaction types in more detail and help to assess the presence of synergistic/antagonistic effects in combination.