RIPARIAN GROUND BEETLES (COELOPTERA, CARABIDAE) PREYING ON AQUATIC INVERTEBRATES - A FEEDING STRATEGY IN ALPINE FLOODPLAINS

The food and feeding habits of riparian ground beetles were studied in four alpine floodplains (Bavaria, Germany): a 5th-order stream (the I sar) and three 3rd-order streams. The riparian fauna along the streams mainly consists of predaceous species. Riparian ground beetle densiti es were much higher along the Tsar than along the small streams. Aquat ic invertebrates composed 89% of the potential prey for carnivorous te rrestrial insects along the Isar. Besides aquatic organisms washed ash ore, stoneflies emerging on land are of considerable importance as pot ential prey for terrestrial predators. In contrast, only 34% of the po tential prey organisms collected along the small streams were of aquat ic origin. Food abundance was 9 times higher in the shore region of th e Isar compared to the small streams. Surface drift in the Isar, a pot entially important food source for riparian organisms, was about 10(6) organisms and exuviae per meter stream width in 24 h. The drift densi ty in the Isar was 59 times higher than that in a small stream. Terres trial organisms provided only 3% of the drifting particles in the Isar , but 50% in the small stream. Gut content analysis reveals, that ripa rian ground beetles in the Isar floodplain mainly feed on aquatic orga nisms washed ashore or emerging on land. While small Bembidion species prefer chironomids (larvae and adults) the larger species Nebria pici cornis feeds on emerging stoneflies, terrestrial riparian organisms an d aquatic organisms accumulating along the shoreline. The prey of ripa rian ground beetles in the floodplain of the three small streams mainl y consists of terrestrial species some of which may have been washed a shore.