Community structure in some southern English streams: the influence of species interactions

SUMMARY. 1. The results of a survey of thirty‐four stream sites, differing in pH and invertebrate species richness, indicated that the pool of locally available, suitably adapted species was smaller in the acid streams. This plays a part in determining the general pattern of lower species richness at more acid sites. 2. Diversity of feeding categories increased with species richness, indicating that a greater range of food resources was available in the less acid, more species‐rich communities. 3. The pattern of predation varied with pH and species richness. The numbers of large insect predators were lower in the less acid, more species‐rich communities and this was correlated with the presence of fish. 4. A detailed study of the guild of detritivorous stoneflies in four streams differing in species richness provided evidence that density compensation occurs, niche width decreases and niche overlap declines as species richness increases. 5. We discuss the roles that competition and predation play in determining the structure and richness of stream invertebrate communities.