Density-Dependent Predation of a Dominant Species does not Facilitate Increased Diversity in Treeholes

Abstract

The effects of predation on water-filled treehole communities in North Carolina were examined using mesocosm experiments and observations in natural treeholes. The presence of the predator Toxorhynchites rutilus and leaf litter abundance were manipulated in mesocosms to examine interactions between resources and predation. Long-term examination of interactions in unmanipulated treeholes provided data on natural variation in water volume and predator density. Toxorhynchites rutilus preys upon two common treehole insects, Aedes triseriatus and Culicoides guttipennis. We predicted that T. rutilus would act as a keystone predator and reduce the density of these dominant species. This would allow other species to coexist and lead to an increase in diversity. We also predicted that effects of predation would be reduced in habitats with high levels of resources, due to either increased refugia or decreased competition. The results did not entirely support the predictions. In both mesocosms and treeholes T. rutilus depressed densities of the most abundant prey type, A. triseriatus. In treeholes, the presence of T. rutilus depressed densities of the midge C. guttipennis, and predator densities were positively associated with insect diversity in treeholes. Strikingly, higher diversity was also associated with high densities of the dominant prey in treeholes. In addition, mesocosms showed no relationship between T. rutilus presence and species diversity. The relationship between the predator and diversity in treeholes appears to be unrelated to predation on the dominant competitors and is instead caused by some other habitat characteristic. While there were effects of resources on densities and diversity in both mesocosms and treeholes, neither of the resources analyzed, leaf litter or water, appear to be the sole characteristic that mutually allows for large populations of T. rutilus and high species diversity. We conclude that though T. rutilus is an aggressive predator, it does not cause an increase in prey species diversity as a keystone predator would. Further research is needed to determine the conditions that favor the presence of predator and high prey diversity.