Bioregions are predominantly climatic for fishes of northern lakes

Aim: Recurrent species assemblages integrate important biotic interactions and joint responses to environmental and spatial filters that enable local coexistence. Here, we applied a bipartite (site–species) network approach to develop a natural typology of lakes sharing distinct fish faunas and provide a detailed, hierarchical view of their bioregions. We then compared the roles of key biogeographical factors to evaluate alternative hypotheses about how fish communities are assembled from the regional species pool. Location: Ontario, Canada and the Upper Midwest, USA. Time period: 1957–2017. Major taxa studied: Freshwater fishes. Methods: Bipartite modularity analysis was performed on 90 taxa from 10,016 inland lakes in the Southwestern Hudson Bay, Mississippi River and St. Lawrence River drainages, uncovering bioregionalization of North American fishes at a large, subcontinental scale. We then used a latent variable approach, pairing non-metric partial least-squares structural equation modelling with multiple logistic regression, to show differences in the biogeographical templates of each type of community. Indicators of contemporary and historical connectivity, climate and habitat constructs were estimated using a geographical information system. Results: Fish assemblages reflected broad, overlapping patterns of postglacial colonization, climate and geological setting, but community differentiation was most linked to temperature, precipitation and, for certain groups, lake area and water quality. Bioregions were also marked by non-native species, showing broad-scale impacts of introductions to the Great Lakes and surrounding basins. Main conclusions: The dominant effects of climate across broad spatial gradients indicate differing sensitivities of fish communities to rapidly accelerating climate change and opportunities for targeted conservation strategies. By assessing biological variation at the level of recurrent assemblages, we accounted for the non-stationarity of macroecological processes structuring different sets of species on the landscape and offer novel inference on the assembly of inland fish communities.