Stream habitat fragmentation — a threat to biodiversity
Biodiversity is undisturbed rhithral streams in central Europe is high, with about 1000 resident metazoan species; over 600 insect species occur in the Fulda river (Germany). Longitudinal downstream shift of dominance from rheobiontic to rheophilous and finally to ubiquituos rheoxenic taxa in the potamal is described. Present downstream importance of ubiquituous species probably results from replacement of original potamal communities, present faunas being surrogates. Species losses through human impact are well documented for fish. The case of Plecoptera (10 potamal species either altogether extinct, extinct in Central Europe or extremely endangered) suggests that potamal invertebrates suffered as severe losses as did fish.Human impact on major rivers was so severe also because they occur at distances beyond average dispersal capacity of the fauna, i.e. are widely separate ecological islands, with known risk of species losses. In contrast, faunal exchange between adjacent headwater streams in mountains with intact stream nets is easy, certainly for amphibious insects. However, damage to rhithral streams is becoming increasingly frequent. This fragments stream nets, turning also upper parts of drainage systems into ecological islands, with danger of extinctions. Rhithral biodiversity is thought to be much more endangered by human impact than is presently recognized.