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Abstract

Size, weight and abundance of eleven species of caddisfly collected in four emergence traps from 1983–1991 at the Breitenbach (Germany) were analyzed for spatial and temporal patterns. Scrapers were abundant and small, shredders rare but large, predators were in between. Individual weights of most species increased downstream. No weight change along the stream was detected in A. fimbriata, T. rostocki, P. conspersa and R. fasciata. At low discharge, specimen numbers of A. fimbriata and C. villosa decreased, but it increased in P. conspersa. No correlation between discharge and any weight measure occurred. Intra- and interspecific competition was detected only between the scrapers A. fuscipes, A. fimbriata and D. annulatus, with negative effects on size and weight of other species and of competitors. Similar findings in the shredder C. villosa were effects of site specific temperature patterns, not of direct competition. It was evident that dependencies or interactions discovered at individual sites generally did not occur at all sites simultaneously. Inter- and intraspecific competition for a limited food source was detected only in scrapers during periods of low flow. No effects of food shortage was discovered in shredders or predators. Thus, based on the individual and population success of caddisflies, it is argued that the stream community in the Breitenbach was mainly driven by physical forces, temperature and discharge. Only under particular spatial and temporal conditions competition effects do become evident.