Biological interactions as determinants of distributions of benthic invertebrates within the substrate of stony streams

Stainless steel screen cages were buried in the substrate of Otter Creek, Wisconsin, during spring 1976 and 1977 and the East River, Colorado, during summer 1977 and stocked with known densities of benthos, Cages were retrieved after 3 days of colonization and attrition from a given direction and species densities and distributions compared to those initially present. With increasing initial benthic density, the net change in numbers of invertebrates per cage decreased linearly, the net immigration decreased linearly, and the net emigration increased linearly. Benthic invertebrates preferred low density areas of substrate to high density areas of comparable physical‐chemical quality. Theoretical equilibrium cage densities indicated that given a consistent habitat, a consistent number and assemblage of benthic invertebrates will colonize. The most abundant species in both streams were categorized according to their observed immigration patterns as density‐dependent or density‐independent species; overall trends showed that most species were density‐dependent.