Grasshoppers (Orthoptera,Saltatoria) on alpine and dealpine riverbanks and their use as indicators for natural floodplain dynamics

Gravel banks with sparse vegetation are typical habitats of braided alpine and dealpine floodplains. They are not only habitats for semiaquatic or hygrophilous organisms, but include also xerophilous elements: 17 species of grasshoppers were recorded in the floodplain of the Upper Isar (Bavaria, FRG). In the alpine region, two species (Bryodema tuberculata, Chorthippus pullus) occur only in floodplains, colonizing young gravel banks with sparse vegetation. Population dynamics and habitat selection of Bryodema tuberculata, studied on the Upper Isar for several years, are characterized by small scale movements of marked individuals between neighbouring gravel banks and fluctuations of the abundance on different sections of the gravel banks. Furthermore, the periodic desiccation of small watercourses is important for the dispersal of the females by walking. Historically widespread, today its distribution corresponds to the few remaining inundation areas. The main causes of decrease and extinction in most localities are man-made changes in floodplain dynamics. The building of retention reservoirs and the diverting of streams for hydropower influenced succession in the downstream floodplains. Most of the gravel banks with sparse vegetation changed into willow thickets or pine forests. All remaining inundation areas should be protected. Bryodema tuberculata is a good indicator for the balance between progressing succession and the reorganization of gravel banks by floods.