Microdistribution of Benthic Species (Ephemeroptera, Plecoptera, Trichoptera, Diptera: Simuliidae) in Lapland Streams

At three localities in central Swedish Lapland (approx. 660N, 16'E), areas of stream bottom were mapped with regard to depth, current and substrate conditions, and the benthic fauna of Ephemeroptera, Plecoptera, Trichoptera and Diptera Simuliidae was sampled in spring and autumn. The three environmental factors were found to be variously associated with each other, three combinations being dominant. The distribution of the benthic species was compared with these combinations, and it was found that most species showed a significant over-representation at sites characterized by one of these complexes. An attempt was made to establish which of the three factors was the most important for the different species. This analysis was complicated by the association between the environmental factors, but certain conclusions could be drawn from interspecific comparisons. Substrate is an especially important determinant of the life conditions of the fauna; besides other functions it usually has to provide food for the animals. Benthic species select their habitats on the basis of factor combinations, rather than isolated factors; in this interplay of environmental factors food was found to have a dominant influence in the biotope investigated. Pe3ioMe