Community structure in some southern English streams: the influence of physicochemical factors

SUMMARY. 1. Invertebrates and fish were surveyed during October 1976 in thirty‐four stony riffle stream sites in Ashdown Forest, Sussex. 2. A variety of physicochemical factors were also measured in an attempt to assess the importance of each in determining the distribution of species and the structure of communities. 3. Three analytical techniques—stepwise multiple regression analysis, ordination and community classification—revealed that the structure of these communities was strongly related to variation in stream pH. Acid sites had low numbers of individuals, low species richness and low equitabilities. Summer temperature and stream discharge also appeared to play significant roles. The pattern of catchment land use was shown to have an important bearing on stream pH. 4. In the most acid sites only collectors, shredders and predators occurred. In more basic sites the number of species in collector and predator categories increased and these were joined by grazer/scrapers and filter feeders. 5. A theoretical basis for explaining stream community structure is discussed.