Biological invasions mediated by humans have affected most ecosystems on Earth since the last two centuries. Although the effect of biological invasions on species diversity is well understood, our knowledge on their functional consequences remains scarce. In this study, we test whether biological invasions are a major driver of functional changes in natural assemblages at the worldwide scale. This was done using a worldwide data set gathering native and non-native fish species composition of 1058 river basins dispersed throughout the world. The body size of 8730 out of the 9750 freshwater fish species was also collected.
We showed that non-native fish species are a non-random functional subset of the worldwide set of fish species. Indeed, non-native fish were significantly larger, partly because fish are strongly associated with human diet and activities. We further showed that the average body size of species assemblages increases through invasion. We further demonstrated, using a null model approach, that these changes were opposite from those expected if non-native species were a random subset of the worldwide set of fish species. Finally, we demonstrated that these non-random functional changes were strong enough to affect an eco-geographical rule known as the Bergmann’s rule. Indeed, non-native species modify the worldwide latitudinal gradients of average body size, meaning that biological invasions are a major process explaining nowadays patterns of body size distribution. Overall, we show that, in a two centuries lap time, humans have affected the functional integrity of most worldwide river basins in a highly targeted direction. and that human-assisted invasions are at the cusp of breaking down thousand hundred years of biological shaping. These functional changes could strikingly affect ecosystems properties and functioning.