ALBERT

Description: Multidimensional niche differentiation might increase the stability of coexistence by reducing overall niche overlap which might have implications on the dynamics of biological invasions. The oligohaline Baltic Seais inhabited by three native and one invasive corophiid amphipod species. These filter feeding crustaceans differ in their substrate preferences and salinity optima which could potentially allow their robust coexistence. However, recent theory predicts that competing species must diverge across all non-substitutable resources (e.g., food and space). We have measured the filter mesh size in the four species (i.e., the distance between bristles on the filtering setae), revealing considerable differences among the three natives (Apocorophium lacustre, Corophium multisetosum and C. volutator), whereas the invasive Chelicorophium curvispinum showed strong overlap with A. lacustre. Theory suggests that the four species cannot coexist robustly due to their overlap in food particle size irrespective of differences in their salinity optima and substrate preferences which is in accordance with observations of local extinctions of A. lacustre. Nevertheless, the stability ensuing from the multidimensional niche differentiation might delay competitive exclusion; i.e., the spatial separation by salinity and substrate types might decrease the intensity of competition for food. Our data for co-occurring populations indicate that A. lacustre might be able to decrease its filter mesh size overlap with C. curvispinum by character displacement and its broader salinity tolerance also might help the native species to persist in the region. However, the niche shift of the species might increase its overlap with C. multisetosum.