Predator-induced plasticity in relation to prey body size: A meta-analysis of Daphnia experiments

Predator-induced plasticity is an adaptive response inprey to predation risks. Considering that size-selective predation affects the body-size distribution of prey populations, we hypothesised that the body size of species determines their expression of predator-induced plasticity. However, as a result of variations in expression among species, an integrated understanding of the relationships between traits remains lacking. We considered the predator-induced plasticity of Daphnia species separately for their body size, defensive morphology and life-history traits. The aim of this study was to compare the predator-induced plasticity of Daphnia against size-selective predators: phantom midge Chaoborus larvae and fish. This meta-analysis included 75 studies and nine plasticity traits for 15 Daphnia species. Daphnia were analysed separately according to phylogeny (subgenera Ctenodaphnia, Daphnia and Hyalodaphnia) and body size (large >3.0 mm, medium 2.0 mm and small 1.0 mm). Size-dependent analyses showed that at least three of the nine plasticity traits were expressed in Daphnia against fish. Among the three subgenera, Daphnia induced the greatest plasticity against both predators (five key traits against Chaoborus and four against fish). The relationship between size and age at first reproduction showed that Daphnia exhibited different degrees of plasticity against Chaoborus and fish. Daphnia had only one set of life-history response to fish, namely, decreased size at maturity and early reproduction. By contrast, Daphnia exhibited three sets of life-history responses to Chaoborus larvae: decreased size and delayed reproduction, increased size and delayed reproduction, and increased size and early reproduction. Overall, predator-induced plasticity was expressed well in small and medium-sized Daphnia, and was more evident in small Daphnia than would be expected from size-selective predation. This study shows that the body size of Daphnia is a key trait influencing its plasticity expression and evolution. We detected novel pattern among life-history traits, but found no strong associations between morphological defences and life-history traits. We suggest that prey size, predation preference and body size are important factors for understanding and explaining the evolution of phenotypic plasticity.