Is predation by turtles sufficiently strong to affect invertebrate biodiversity?

Background/Question/Methods

              Ecologists have long known that predation can have a strong effect on the diversity and abundance of prey present in ecological communities.  Much evidence on the importance of predators in aquatic systems has stemmed from studies where individuals manipulate the abundance or occurrence of particular predator species or groups of predator species that are thought to play an important role.  In freshwater aquatic systems, fish are often thought of as the most important predator in permanent ponds while salamanders and insects are often thought of as the most important predator in ephemeral ponds that dry. Turtles represent a taxonomic group of predators that is completely overlooked in studies assessing the importance of predation.  This is surprising as turtles can consume a wide array of prey species and can be locally abundant.  We conducted an exclosure experiment in an ephemeral pond that lacks fish to examine whether turtles play an important role in controlling the distribution and abundance of amphibian and invertebrate prey within the pond.     

Results/Conclusions

             The ability of turtles to graze in some plots but not others did not alter the number of prey species present within study plots or the evenness of prey species present within study plots.  A comparison of species accumulation curves reveals, however, that turtles tended to reduce the total number of prey species found across all study plots where they had the ability to graze in contrast to that observed across all of the areas where they did not have access.  PERMDISP results suggest that scale dependent differences in the effect of turtles on species richness are the result of turtles homogenizing the kinds of prey species that are present in areas where they could graze.  Our results also suggest that turtles reduced the abundance of large invertebrates, especially dragonflies.  The abundance of insects in areas that turtles had access to was approximately 75% of that observed in areas where turtles did not have access.  Our results demonstrate that turtles can have an important affect on the abundance of invertebrates present in ponds and can cause a reduction in the total diversity of insects present in a pond by homogenizing species composition of different localities.