Species Richness in Streams of Different Size from the Same Drainage Basin

Abstract

The species richness of stream benthic invertebrates was studied along a longitudinal profile of the Salmon River, Idaho, during spring, summer, and autumn. Sampling was done using replicate rocks and the analytical approach of Stout and Vandermeer (1975) was used to calculate theoretical number of species present, relative immigration rate, and relative spatial heterogeneity. Species richness varied with stream size, being highest in midorder streams and lower in headwater and high order streams. This downstream shift in species richness conforms to the river continuum concept (Vannote et al. 1980). Possible cause is associated with varying temperature regimes in the different-sized streams as well as other factors. Species richness also varied with season depending on stream size. Low order streams were more individualistic, probably because of a greater influence of local (terrestrial) environmental conditions, but showed a higher richness in summer than in autumn. Larger streams were more similar and showed a higher richness in autumn than in summer. Other community-level indicators, such as relative immigration rate and extinction rate, also showed seasonal differences. We conclude that different macroinvertebrate community types having characteristics of either nonequilibrium (density-independent, opportunistic) or equilibrium conditions can be found in streams from the same drainage basin depending on location along the river continuum and time of the year. These findings have important implications for future studies of stream communities as well as for those of predation and competition in streams. Among other things, we propose that the interactive/noninteractive model of Wilson (1969) may be more appropriate for stream invertebrate communities than those assuming strictly nonequilibrium or equilibrium conditions. We further suggest that any attempt to classify or compare stream communities, such as was done for tropical versus midlatitude streams by Stout and Vandermeer (1975), must take seasonality and position along the downstream gradient into account.