Higher predation rate need not and did not lead to higher risk‐induced trait responses in related zooplankton species

Abstract

Predators can directly affect prey populations both through predation (consumption of prey) and risk‐induced trait responses (RITRs) that reduce predation risk but are often associated with a fitness cost. Thousands of studies make clear that RITRs (also termed anti‐predator or defensive traits) including changes in behavior, morphology, and life history, are employed by numerous taxa across diverse ecological systems, and there is large variation in their magnitude. A natural goal is to elucidate the species and circumstances for which and to what magnitude RITRs are expected. A candidate hypothesis is that prey species that experience higher mortality from a predator will exhibit a higher RITR. This hypothesis is an intuitive extension of the fact that invulnerable animals are not expected to exhibit an RITR, while vulnerable species are. We present an example that clarifies why this relationship is not always expected and when it is expected. Other factors may influence the level of the RITR leading to the possibility that a positive relationship is not expected. We elucidate this problem using a mesocosm experiment with a fish–cladoceran system in which there is large variability in the predation rate on different cladoceran species. Results not only did not show a positive relationship, but rather a negative trend between predation rate and the RITR. In fact, highly preyed upon taxa did not respond, while the least preyed upon taxa had the largest responses. These results clarify how the level of predation risk interacts with many factors to determine the RITR of prey.