Microhabitat availability in Welsh moorland and forest streams as a determinant of macroinvertebrate distribution

Abstract

SUMMARY. 1 Eighteen streams in mid‐Wales were sampled for macro‐invertebrates in both riffles and margins in April 1985–87. Stream macro‐flora, substrata and marginal habitats were surveyed in May 1988. 2. TWINSPAN classification of the macroinvertebrate data indicated three major stream groups. One was distinguished by circumneutral pH and had a flora and fauna typical of such conditions. The other two groups consisted of acidic streams with moorland and conifer afforested catchments respectively. The forest streams were the more acidic but the two groups also differed significantly in the composition of their marginal habitats. 3 The acidic moorland streams had more vegetation (‘soft’ features) in the margins and supported several invertebrate taxa which were relative more abundant there than in the riffles. These taxa may be excluded from forest streams because the margins are ‘hard’ due to greater erosiveness and shading. 4. In view of the increasing cover by conifer afforestation in Britain, it is clearly necessary to elucidate all its effects on stream ecosystems, which include changes to the physical environment.