Assessing the ecological impact of non-native species: single-species manipulation fails to reveal impact of invading shrimp

To experimentally assess the ecological impact of non-native species, their presence or abundance is usually manipulated under simplified conditions. Yet, their impact may depend on interactions with native species, whose presence and abundance then also need to be considered. We examined whether the ecological impact of a recent invader in the Baltic Sea, the shrimp Palaemon elegans, is shaped by interactions with two native mesopredators: the shrimp P. adspersus and the threespine stickleback. Using mesocosm experiments, we show that single-species manipulations fail to capture the full ecological impact of the non-native shrimp and that multi-species manipulations are required. In single-species treatments, the effect of the non-native shrimp on lower trophic levels-grazers and primary producers-was similar to those of the native species; all fed on animal prey, which increased algae biomass. However, in multi-species treatments, where shrimps competed with the stickleback for prey, the consumption of algae increased. Notably, the non-native shrimp increased its consumption of algae less than the native shrimp, indicating stronger competitive ability. Thus, the replacement of the native shrimp by the non-native could relax the control of algae biomass. This suggests that the future state of the ecosystem may depend on the absolute and relative abundances of the three species and whether the non-native shrimp will displace or coexist with the native one. These results emphasize the importance of accounting for species interactions when investigating the impact of non-native species on ecosystems. Single-species manipulations may fail to capture full ecological consequences.