Biotic interactions under climate warming: temperature-dependent and species-specific effects of the oligochaete Chaetogaster limnaei on snails

Biological invasions impose severe threats to biodiversity, and the extent of this phenomenon is predicted to intensify under climate warming. Only a few investigators have examined how temperature interacts with other environmental variables to affect competition between native and invasive species. Physa acuta, an invasive freshwater snail in Europe, often outcompetes the indigenous congener Physa fontinalis. Both species are hosts of the oligochaete Chaetogaster limnaei limnaei (CL). We examined how the growth rates of the 2 competing physid species are co-affected by CL infestation (0, 10, 20 CL/snail) and temperature regime (15, 20, 25 degrees C). Physa acuta grew faster than P. fontinalis. Warmer temperatures strongly favored growth rates of P. acuta, whereas P. fontinalis hardly profited from increased temperatures. CL infestation reduced the growth rates of both physids, but affected P. acuta more strongly than P. fontinalis. At the highest CL infestation level, the growth rates of both species were equal at 15 and 20 degrees C, and faster growth of P. acuta occurred only at 25 degrees C. Thus, using only the temperature response of a species to predict its reaction to climate warming is insufficient because biotic interactions can strongly affect species' responses to temperature change. Additional host-preference experiments showed that physids were preferentially infested compared to Radix balthica and Bithynia tentaculata, 2 species that commonly co-occur with the physids. We showed the clear competitive superiority of P. acuta over P. fontinalis, but environmental conditions exist in which their fitness-relevant growth rates do not differ. CL infestation can reduce the competitive superiority of P. acuta, so CL might facilitate the co-existence of the 2 species. In general, its density-dependent effects and different selectivity for snail species may make CL an agent that structures snail communities in the field, a role that has been underestimated.