Biogeographic patterns in major marine realms: function not taxonomy unites fish assemblages in reef, seagrass and mangrove systems

We examine the effects of different biogeographic histories on assemblage composition in three major marine habitats in two biogeographically distinct marine realms. Specifically, we quantify the taxonomic and functional composition of fish assemblages that characterise coral reef, seagrass and mangrove habitats, to explore the potential effects of biogeographic history and environment on assemblage composition. The three habitats were surveyed in the Caribbean and on the Great Barrier Reef using a standardised underwater visual census method to record fish size and abundance data. The taxonomic composition of assemblages followed biogeographic expectations, with realm-specific family-level compositions. In marked contrast, the functional composition of assemblages separated habitats regardless of their biogeographic locations. In essence, taxonomy characterises biogeographic realms while functional groups characterise habitats. The Caribbean and Indo-West Pacific have been separated for approximately 15 million years. The two realms have different taxonomic structures which reflect this extended separation, however, the three dominant shallow-water marine habitats all retain distinct functional characteristics: seagrass fishes are functionally similar regardless of their taxonomic composition or biogeographic location. Likewise, for coral reefs and mangroves. The results emphasise the advantages and limitations of taxonomic vs. functional metrics in evaluating patterns. Taxonomy primarily reflects biogeographic and evolutionary history while functional characteristics may better reflect ecological constraints