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Biological invasion represent one of the major threats to global biodiversity as alien species often displace indigenous species. However, knowledge of the mechanisms behind such displacements and the driving factors of the competitive superiority of the alien species still remain rare. In our study we combined analysis of field data and laboratory experiments examining species interactions, to investigate the impact of temperature in the case of the alien freshwater snail Physa acuta that is held responsible for the decline of indigenous snail Physa fontinalis in Europe. From field data, we identified higher temperature as the most important difference between sites populated by alien P. acuta and those where indigenous P. fontinalis occurred. Results of the species interaction experiment conducted at 15, 20, and 25°C confirmed the hypothesis that the competitive superiority of P. acuta over P. fontinalis increases at warmer temperatures. In single species treatments, increasing temperature stimulated both species to grow faster and reach greater shell heights. Coexistence treatments revealed an asymmetric competitive interaction between the two snail species. In both species, the density of conspecifics did not affect snail growth; however, density of heterospecifics affected the growth. At 15°C, the presence of heterospecifics stimulated the growth of both species, while at higher temperatures the presence of heterospecifics stimulated the growth in P. acuta, but inhibited in P. fontinalis. Our study shows that temperature can be a powerful driver of the outcomes of alien and indigenous species’ competition by driving asymmetric interaction. Further our results point up that the environmental context cannot be disregarded when investigating the interaction between alien and indigenous species, and predict alien species success and impact.
There are many studies showing that alien species displace native species, and this process has been recognized as a major threat to biodiversity. However, only few studies exist that addressed the mechanisms behind this displacement of native by alien species, and the role of the environmental context in this process. In the present study we analyze how the presence of conspecifics and heterospecifics at different densities alter the growth rates of two competing snail species, the native Physa fontinalis and the alien Physa acuta, on a temperature gradient from 15 to 25°C. We found asymmetric competitive interaction between the species. At 15°C, the presence of heterospecifics stimulated the growth of both species, while at higher temperatures the presence of heterospecifics stimulated the growth in P. acuta, but inhibited in P. fontinalis. Hence, temperature was shown to drive the competitive superiority of P. acuta.