Gammarids in Lake Constance: habitat segregation between the invasive Dikerogammarus villosus and the indigenous Gammarus roeselii

Over the past decades the invasive Ponto-Caspian gammarid Dikerogammarus villosus spread throughout large rivers of central Europe, thereby rapidly displacing most of the native gammarid species. In 2003, D. villosus was fi rst observed in Lake Constance and was predicted to out-compete the native Gammarus roeselii. However, today the two species coexist at several sites. We hypothesized that a higher variability of substrates in the littoral zone of Lake Constance, compared to that found in most European waterways, allows coexistence of these two species. To analyze substrate choice of these two gammarid species we exposed them individually and together in enclosures to the littoral zone in the lake. These enclosures contained a combination of the following substrates: bare stones, stones covered with Dreissena polymorpha, Chara sp., shells of Corbicula fl uminea, leaves, and sand. D. villosus strongly preferred structure-rich hard substrates like stones while avoiding sand and leaf litter. G. roeselii barely differentiated among substrates occupying even sandy habitats. To compare these results with natural gammarid habitat choice we measured gammarid densities on the substrates mentioned above in the littoral zone of Lake Constance. In contrast to enclosures, G. roeselii was virtually displaced by D. villosus on stones covered with D. polymorpha in the lake. Even four years after the invasion of D. villosus, high densities of G. roeselii could still be found in Chara and leaf litter. From these data we hypothesize that G. roeselii takes refuge from D. villosus by colonizing substrates not occupied by the invader. These substrate types only exist in natural waterbodies and are lacking in regulated rivers which generally have poor substrate richness.