Influence of an omnivore, nutrients, and site heterogeneity on stream algal and invertebrate communities.

Abstract

Multiple factors influence the distribution and abundance of organisms in natural communities. In stream ecosystems, factors such as resource availability (e.g., phosphorus concentration) and consumers (e.g., predators) interact to affect species distributions. This dissertation examines how an omnivorous consumer (the crayfish Orconectes propinquus) affects algal communities across a nutrient gradient (nitrogen and phosphorus), and how this consumer influences aquatic invertebrate populations. Caging experiments conducted in the Maple River (a cool water river in northern, lower Michigan U.S.A.) demonstrated that O. propinquus was associated with a decrease in diatom abundance on clay tiles ($\sim$8 crayfish m$\sp{-2}).$ Despite O. propinquus' negative effect on diatom abundance, the composition of the diatom community was unaffected by crayfish. Overall, crayfish had few detectable effects on insect communities. Other studies in the Maple River that manipulated crayfish density and nutrient availability (addition of both nitrogen and phosphorus), showed that crayfish did not affect algal biomass and community composition. However, algae were nutrient limited. Clay pots containing nutrients had a five-fold greater algal chlorophyll a concentration. Diatom diversity declined at the highest nutrient levels. This decline was accompanied by a decrease in the evenness of the community. The most distinct differences in diatom species distributions were associated with the three study sites. Natural densities of O. propinquus in the Maple River were less than one crayfish per square meter. The examination of artificial rock piles in the Maple River showed that aquatic invertebrate populations were largely unaffected by manipulations of crayfish abundance. The percentage abundance of grazing invertebrates was greater in areas without crayfish. The invertebrate community differed considerably among the three study sites in the Maple River. O. propinquus' omnivorous feeding behavior and low density ($\sim$0.8 crayfish m$\sp{-2})$ could limit its impact on algae and invertebrates in the Maple River. By feeding on organisms from many trophic levels, omnivores may have fewer effects on any particular resource. This dissertation does not support the hypothesis that the crayfish O. propinquus functions as a keystone species in all ecosystems.