Projected impacts of climate change on spatio‐temporal patterns of freshwater fish beta diversity: a deconstructing approach

ABSTRACT Aim To assess the potential impacts of future climate change on spatio‐temporal patterns of freshwater fish beta diversity. Location Adour–Garonne River Basin (France). Methods We first applied an ensemble modelling approach to project annually the future distribution of 18 fish species for the 2010–2100 period on 50 sites. We then explored the spatial and temporal patterns of beta diversity by distinguishing between its two additive components, namely species turnover and nestedness. Results Taxonomic homogenization of fish assemblages was projected to increase linearly over the 21st century, especially in the downstream parts of the river gradient. This homogenization process was almost entirely caused by a decrease in spatial species turnover. When considering the temporal dimension of beta diversity, our results reveal an overall pattern of decreasing beta diversity along the upstream–downstream river gradient. In contrast, when considering the turnover and nestedness components of temporal beta diversity we found significant U‐shaped and hump‐shaped relationships, respectively. Main conclusions Future climate change is projected to modify the taxonomic composition of freshwater fish assemblages by increasing their overall similarity over the Adour–Garonne River Basin. Our findings suggest that the distinction between the nestedness and turnover components of beta diversity is not only crucial for understanding the processes shaping spatial beta‐diversity patterns but also for identifying localities where the rates of species replacement are projected to be greatest. Specifically we recommend that future conservation studies should not only consider the spatial component of beta diversity but also its dynamic caused by climate warming.