Habitat trumps biogeography in structuring coral reef fishes

As one of the world’s most diverse ecosystems, coral reefs have been the focus of numerous biogeographic analyses. With strong biodiversity gradients across the Indo-Pacific, coral reefs have shed light on the effects of evolutionary history, isolation, and human exploitation on local assemblages. However, there are also strong environmentally driven local gradients in faunal assemblages. We ask, does reef fish community composition and trait space vary to a greater extent across small scales (i.e. along habitat gradients) or across large scales (i.e. across geographic regions separated by up to 12,000 km)? Using a standardized survey method that explicitly includes habitats (i.e. the slope, crest, and flat), we surveyed a highly diverse family of reef fishes (Labridae) in nine regions across the Indo-Pacific, from the Cocos (Keeling) Islands to French Polynesia. We demonstrate that small-scale habitat gradients represent a greater axis of variation, in both the taxonomic and trait composition of fish assemblages, than large-scale biogeographic gradients. Indeed, fish assemblages just 10 m apart, along a habitat gradient, appear to differ more than assemblages in the same habitats separated by over 12,000 km along the world’s largest biodiversity gradient. Essentially, fish assemblages cluster by habitat regardless of their biogeographic region, with habitat associations trumping biogeographic affiliations. This emphasizes the primacy of local environmental factors, such as hydrodynamics, in shaping the ecology of reef fishes. It also raises serious concerns over the use of combined datasets, where data from different habitats are used, without explicit recognition, in global-scale analyses.