Environmental tolerance governs the presence of reef corals at latitudes beyond reef growth

Aim: Understanding mechanisms that enhance survival outside the usual range of a taxonomic group is key to understanding diversity gradients, how range boundaries operate and how environmental change might influence geographical distributions. Our aims were: (1) to identify species-level traits associated with reef-building coral species whose ranges extend into high-latitude, non-reefal habitats (i.e. ‘beyond reef’), (2) determine if these patterns were consistent in different global regions, and (3) determine if traits associated with living beyond reef were related to enhanced dispersal or tolerance to harsher environmental conditions. Location: Three regions of the Central Indo-Pacific with tropical–temperate gradients from 35° N to 36° S. Methods: Scleractinian coral richness was collated and analysed as a function of latitude, region and whether or not reef accretion occurred. Species traits that are hypothesized to contribute to living beyond the environmental limits of carbonate reef accretion were compiled; these included traits associated with dispersal potential and environmental tolerance. The contribution of species traits to presence beyond reefs was then quantified using a binomial generalized linear mixed effects model. Results: In each region, coral species richness dropped by up to 73% where reef accretion ceased. Compared with corals restricted to coral reefs, the subset of species extending beyond reefs shared common traits related to environmental tolerance (larger depth ranges, more robust morphologies and tolerance of turbid water), but not to dispersal potential. Patterns were mostly consistent among regions. Main conclusions: We show for the first time that coral species living successfully in beyond-reef habitats share common characteristics that are consistent in three global regions. Environmental tolerance appears to play the dominant role in determining which species successfully establish beyond reefs. Our trait-based approach sheds light on how species assemblages and ranges might be altered by environmental change or loss of reef habitats.