DOES THE QUANTITY OF RESOURCES IN THE ENVIRONMENT ALTER THE IMPACT OF MULTIPLE PREDATORS ON THEIR PREY?

Abstract

Predation is an important biological process affecting prey populations and most prey in nature are exposed to multiple predator species. Much research has revealed that the combined effect of multiple predators on their prey can be substantially different from that which would be expected if the predators operated independently of each other. Some reasons for this discrepancy include interference among predator species and/or anti-predator responses of prey to one predator that alter the effectiveness of the other predator. In systems where one of the predators is an omnivore, an increase in the availability of primary producers via nutrient enrichment could alter the extent to which the combined impact of multiple predators on their prey differs from that which would be predicted if predators were assumed to forage independently of each other. We conducted an experiment in artificial ponds to examine this idea. Our experiment manipulated the occurrence of two common pond predators, omnivorous crayfish (Procambarus acutus) and carnivorous dragonfly larvae (Anax sp.), in environments that varied in the amount of nutrients available to support algal growth. We measured the mortality, mass at metamorphosis, and larval period of herbivorous tadpoles (Hyla squirella). We saw no statistical difference between observed and predicted proportion of prey consumed in multiple predator treatments. Predicted proportion of prey consumed was derived using a model which assumes the effects of each predator are independent, while observed proportion of prey consumed was the proportion of prey metamorphosed in each multiple predator treatment. Even though there were no statistically significant results, we did observe a trend of prey mortality being 28% less than predicted in high nutrient treatments with both Anax sp. and P. acutus, compared to low nutrient treatments with both Anax sp. and P. acutus. We also observed prey mortality being 25% less than predicted in high nutrient treatments with two Anax sp. predators and a 20% greater than predicted in low nutrient treatments with two Anax sp. present. These differences are rather large and could disrupt ecosystem processes by changing prey abundances. We saw that growth of H. squirella varied among predator treatment, but saw little variation depending on nutrient availability. Our experiment shows that multiple predators can have unexpected impacts on regulating prey densities.