The Role of Fish Predation and Spatial Heterogeneity in Determining Benthic Community Structure

The effects of predation and spatial heterogeneity on the benthic macroinvertebrates in the littoral zone of a pond were investigated in a year—long caging experiment. Bluegill sunfish stocked at three densities within the cages were used as the predator. Heterogeneity was manipulated using artificial macrophytes. The presence of plants led to increases both in species richness and in density of most macroinvertebrates. Uncovered benthic sediments had greater species richness and higher macroinvertebrate densities than did sediments covered by macrophytes, suggesting that certain benthic species, notably several chironomid groups, migrate onto the macrophytes when this habitat becomes available. Fish predation had a significant impact on both the number of benthic species and the densities of certain benthic macroinvertebrate groups. The predatory chironomid Clinotanypus pinguis showed a strong negative response to bluegill predation, with fish exclusion resulting in higher densities throughout the year. A group of herbivorous chironomids exhibited a response to fish predation that varied over the year, with predation resulting in elevated densities in the fall and winter but reduced densities in the summer. Many of the other species found in the littoral zone showed no significant response to bluegill predation. In addition, fish predation had no significant effect on densities of most macroinvertebrates occurring on the macrophytes. These complex responses of the various macroinvertebrates to fish predation are explainable in terms of seasonal changes in predation intensity and the importance of invertebrate vs. vertebrate predation. In the fall and winter bluegill feeding is often reduced. By concentrating on a few preferred prey (predaceous chironomid species), their feeding might alleviate competitive or predatory relationships among the macroinvertebrates and thus lead to increased densities of many species. The high bluegill feeding intensity known to occur during the summer is severe enough to limit the numbers of most invertebrate prey. As fish predation was not effective on invertebrates in the macrophytes, it seems that the added physical complexity serves as a refuge from predation.