Prey subsidy or predator cue? Direct and indirect effects of caged predators on aquatic consumers and resources

The non-consumptive effects of predators on prey can affect prey phenotypes, potentially having important consequences for communities due to trait-mediated indirect interactions. Predicting non-consumptive effects and their impacts on communities can be difficult because predators can affect resources directly through nutrient cycling and indirectly by altering prey resource use, which can lead to complex interactions among resources and consumers. In this study we examined the effects of caged dragonfly predators on aquatic resources in the presence and absence of two focal herbivores, the tadpoles of Neotropical tree frogs Agalychnis callidryas and Dendropsophus ebraccatus. We crossed the presence/absence of caged dragonflies with four tadpole treatments: no tadpoles, each tadpole species alone, and both species together to examine interactions among tadpole composition, predator presence, and time on tadpole growth, resources, and zooplankton abundances. Predator effects on growth changed through ontogeny and was species-dependent. Predators initially reduced then dramatically increased A. callidryas growth, but had no effect on D. ebraccatus. Predators also increased the abundances of both periphyton and phytoplankton. However, there was no evidence of a trait-mediated trophic cascade (i.e., tadpole by predator interaction). Instead, nutrients from prey carcass subsidies likely played an increasingly important role in facilitating resources, and shaping tadpole growth, competitive interactions, and zooplankton abundances through time. In nutrient-poor aquatic systems the release of nutrients via the consumption of terrestrially derived prey items by aquatic predators may have important impacts on food webs by facilitating resources independent of the role of trait-mediated trophic cascades.