Blinded by the bright: How species‐poor habitats contribute to regional biodiversity across a tropical seascape

Aim Conservation efforts have traditionally addressed the threat of biodiversity loss by prioritizing regions and habitats with high endemic species richness. However, species-poor habitats often harbour distinct, valuable and/or functionally unique species that contribute to regional diversity. In the tropical marine realm, the 'mangrove-seagrass-coral reef continuum' has dominated both the scientific literature and marine spatial planning. Bare sediment habitats are also part of this continuum and often comprise the majority of bottom area, as they do in most regions worldwide, but are understudied and rarely considered in conservation. To address this information gap, we partition the relative contributions of five habitats in the coastal mosaic (forereefs, patch reefs, seagrass, mangroves and bare sand) to fish diversity across a tropical seascape. Location Carrie Bow Cay, Belize Barrier Reef. Methods Using diver visual censuses, we assessed fish community composition at three replicate sites from each of the five habitats annually for 5 years. We then partitioned diversity into within (alpha) and between (beta) habitat contributions, and further partitioned beta-diversity into components attributed to species replacement and richness difference. Finally, we determined the local contributions to beta-diversity (LCBD) by each habitat and identified the key individual species behind these contributions. Main Results Tropical sand flats contributed uniquely and disproportionately to regional fish species diversity, despite having the fewest species overall. Sand flats also had higher-than-expected local contributions to beta-diversity (LCBD) and had the highest percentage of species not found in other habitats. Main Conclusions Our results demonstrate that previously understudied sand habitats harbour a relatively unique assemblage of species compared with other nearby habitats. Marine spatial planning aimed at maximizing the preservation of total biodiversity (including unique species) in the tropics should consider sand habitats.