The influence of discharge and temperature variation on macroinvertebrate community diversity patterns in headwater streams

The diversity of a community of organisms in an ecosystem depends on the interactions between the community members and their environment. It is widely accepted that in most circumstances more variation in the environment yields greater species richness and species evenness in a community. Two stream invertebrate communities were studied to determine if either greater or less variation in temperature and discharge promoted greater diversity. The two sites were located 160 m apart on Lookout Creek in the Oregon Cascades. One site was more variable in temperature and discharge than the other, due to the confluence of Lookout and Cold Creeks between the two sites. All other stream parameters were nearly identical. The stream invertebrate communities were sampled at the two sites and the organisms were identified to families. Ephemeroptera, Plecoptera, and Trichoptera represented 42% of the community above Cold Creek and 58% of the community below Cold Creek. After calculating the diversity of the communities using a number of indices (Shannon's and Simpson's), the stream invertebrate community from the site with less variability in discharge and temperature was found to have a significantly greater taxonomic diversity. This research calls into question current hypotheses regarding environmental variability and diversity with respect to stream invertebrate communities.