Ausbreitung terrestrischer Wirbelloser durch Fließgewässer

Fragmentation and isolation are considered prominent reasons for the loss of biodiversity in modern landscapes. Principle conclusions concerning fragmentation, population linkage, and risk of extinction however are questionable if they are based on data of active dispersal only. Distances of passive dispersal of terrestrial invertebrates by vectors, like rivers, exceed maximum active mobility as shown by field experiments at the Lahn river (near Marburg, Hesse) and the Elbe river (near Dessau, Saxony-Anhalt) in 2000 to 2002 and experiments in the laboratory. Marked Arianta arbustorum (Gastropoda, Helicidae) were released at the Lahn river. Individuals travelled a maximum distance of 2.1 km. At the Elbe river Helix pomatia drifted up to 19.8 km downstream. In the laboratory A. arbustorum still floats on the water surface for up to 32 h (+17,5°C water temperature). This would allow for a transport of more than 173 km at a flow speed of 1.2 m s-1. Thus, passive dispersal by rivers could be a key process of recolonisation of dynamic habitats of floodplains and cultural landscapes. This hypothesis is further supported by high numbers of terrestrial invertebrates transported alive by the running water during flood conditions. At the Rhine river (Kühkopf-Knoblochsaue, near Darmstadt) drifting plant debris carried 4170 (SE +/-738) individuals of terrestrial invertebrates per 100 liter debris (flood conditions, March 2001). At the Lahn river 2191 (SE+/-279) and 1011 (SE+/-157) individuals per 100 liter plant debris were captured by a drift sampler (January and February 2001). Habitats along rivers are arranged longitudinally. Thus, the chance for washed-away individuals to find a suitable habitat is rather high. This probability is increased as Arianta arbustorum has been shown to be able to search actively for suitable habitats near landing locations.