Regulation of Freshwater Community Structure at Multiple Intensities of Dragonfly Predation

We examined the role played by predaceous dragonfly nymphs, Celithemis fasciata (Odonata: Libellulidae), in the regulation of the community structure of a benthic macroinvertebrate assemblage in Par Pond, an 1100—ha reservoir in South Carolina. Effects of predation intensity on species richness, evenness, and density wer evaluated by adding zero, two, four, and eight large dragonfly nymphs (antepenultimate and penultimate instars) to previously sieved (0.85—mm mesh) bottom sediment containing benthic macroinvertebrates. Predator and prey assemblages were then placed in individual field microcosms that consisted of polyethylene trays surrounded by underwater screens (mesh <2 mm diagonally) and suspended 15 cm below floating platforms. Twelve replicates of each treatment level were run during each of three 6—wk experimental periods: April—May 1980, August—October 1980, and January—February 1981. In addition, colonization of microcosms by invertebrates was quantified, and samples from natural, unenclosed benthic fauna were collected seasonally along a transect for comparison with experimental assemblages. We tested whether predator enhanced, depressed, both increased and decreased, or had no effect on the complexity of the community structure. The dual effect of predation on community structure is predicted by Connell's "intermediate disturbance hypothesis." Results showed that dragonfly nymphs can significantly influence the structure of the benthic community. However, the results did not show that invertebrate predation is the sole or even the primary regulator of community structure. Species richness was significantly greater at intermediate treatment levels (thus supporting Connell's general hypothesis), but the increase was not great (a range of °10%). The mechanisms by which species richness is maximized at intermediate intensities of predation are not entirely evident, but are probably a combination of prey refuges and nonselective predation with patch switching. In contrast, species evenness, as measured by equitability and by Simpson's index, was greatest at the highest predation level (which does not support the intermediate disturbance hypothesis). Dragonflies appeared to exert a greater influence on prey density than on community diversity.