Predators drive prey population dynamics and trophic cascades through a combination of consumptive and non-consumptive effects (CEs and NCEs, respectively). Theory predicts that prey naivete-- the lack of eco-evolutionary experience with a predator-- magnifies total predator effects and increases CEs relative to NCEs. Empirical tests of this theory are lacking, particularly in systems with diverse prey communities like freshwater lake zooplankton. Furthermore, the effects of prey naivete on trophic cascades within these diverse systems is poorly understood.
We used a pond mesocosm experiment in which naive and experienced zooplankton prey (from fishless lakes and lakes with abundant trout predators, respectively) were exposed to one of three treatments: free-swimming trout predators (NCEs + CEs), caged trout (predation risk only, NCEs), or predator-free controls to test the following hypotheses: (1) Experienced prey suffer elevated NCEs, reflecting the costs of anti-predator traits, (2) but lower CEs and total predator effects, reflecting the benefits of these adaptive traits; and (3) Trophic cascades are stronger in systems with naive prey relative to experienced prey.
Results/Conclusions
We found that predation risk (NCEs) affected experienced zooplankton abundance, body size, and community biomass, but had no detectable effect on the naive zooplankton community, consistent with elevated costs of anti-predator traits. Predation by trout (CEs + NCEs) had a greater impact on the taxonomic composition and size structure of the naive than the experienced zooplankton community. However, contrary to our predictions, predation decreased zooplankton community biomass to a greater extent in the experienced than the naive community. Furthermore, the positive impact of trout on phytoplankton abundance (i.e. the trophic cascade) was approximately twice as strong in the experienced than the naive community.
These results suggest that the eco-evolutionary experience of zooplankton prey affects the balance of consumptive and non-consumptive predator effects. While experience confers resistance to changes in community composition, it does not reduce the impact on prey community biomass, and in fact magnifies the strength of trophic cascades.