Larval fish‐induced phenotypic plasticity of coexistingDaphnia: an enclosure experiment

Abstract

Summary1. The indirect effects of predators on lower trophic levels have been studied without much attention to phenotypically plastic traits of key food web components. Phenotypic plasticity among species creates phenotypic diversity over a changing environmental landscape.2. We measured the indirect effects of planktivorous larval walleye (Stizostedion vitreum) on phytoplankton biomass through their effects on the dominant herbivore species,Daphnia pulicariaandD. mendotae.3. Fish had no effect on phytoplankton biomass or overallDaphniadensity. We observed a compensatory response to predation by functionally comparable species within a trophic level in the form of shifting dominance and coexistence ofDaphniaspecies. We hypothesized that this phenotypically plastic response to predation decoupled a potential trophic cascade in this freshwater pelagic system.Daphnia pulicariadensity decreased over time with fish predation, butD. mendotaedensity increased over time with fish predation.4. Phenotypically plastic life history trait shifts and reproductive rates differed between species in fishless and fish enclosures, accounting for population trends.Daphnia pulicariawere also proportionally higher in walleye larvae stomachs than in the enclosures, indicating that walleye preferred to feed onD. pulcariaoverD. mendotae. The resultant shift in dominance may partially explain the overall benign effect of fish on grazers and supports the hypothesis that trophic level diversity can decouple a trophic cascade.