Predicting non-consumptive predator effects on multiple prey in a complex natural system

Background/Question/Methods

Predators affect prey fitness through nonconsumptive effects (NCEs) when there are costs associated with a prey's phenotypic response to predators to reduce predation risk. The relative magnitude of phenotypic responses, and associated NCEs, are likely to vary greatly between prey based on factors such as prey vulnerability and differential costs of a phenotypic response.  We examine differences in the magnitude of behavioral responses (vertical migration) of multiple zooplankton prey to Bythotrephes, an invasive predator cladoceran in Lake Michigan. Using data from field surveys of abiotic variables and predator densities, and predation experiments we generate relationships of zooplankton birth and death rates as a function of depth to establish a fitness function. Optimal migration amplitudes are derived from the fitness function, and these predicted values are compared with observed values.  Ensuing NCEs associated with the predicted and observed migrations are then calculated.

Results/Conclusions

We found that some zooplankton prey species were more vulnerable to Bythotrephes than others, and thus differed in their survival gains associated with migration. Further, because colder temperatures affect some prey species more than others, different prey species also have higher growth reduction costs associated with migration. Thus, optimal migration amplitudes differed among prey species. In the field, migration amplitudes varied greatly, from none at all (copepod nauplii) to strong (the cladoceran Daphnia mendotae). Overall, the observed migration amplitudes in the field matched our predicted values quite well. Observed migrations that deviated from predicted values may be explained by constraints on a prey species’ ability to migrate. We also found that the ensuing NCEs associated with predicted and observed migrations were vastly different among zooplankton prey species. These results suggest that in order to understand the role of predator NCEs in a community, we need to compare how prey species with different vulnerabilities, growth constraints, and response abilities respond to the same predator species, rather than assuming that interactions between single predators and single prey species can be generalized to the level of communities.