The Habitat Persistence Hypothesis: A New Perspective on the Distribution of Coral-Reef Organisms

We propose the “Habitat Persistence Hypothesis” (HPH) to explain patterns of diversity on tropical coral-reefs. We reviewed distributions of organisms inhabiting tropical coral reefs, and formulated the HPH to account for biogeographical patterns on both shallow (<30m) and deep (30–150m) coral-reef habitats. Species occurring on deep reefs appear to show higher rates of endemism and less eastward diversity attenuation than their counterparts on shallow reefs. The HPH stipulates that shallow-reef habitats persist across glacio-eustatic sea-level changes in regions with sloped bathymetry (e.g., continental regions and large islands), but are largely extirpated during low sea-level stands in regions with steep bathymetry (e.g., coral atolls); in contrast to deep-reef habitats, which persist in all regions. The HPH suggests that habitat persistence results in higher rates of endemism, and attenuating diversity with distance from centers of species richness are shaped by recolonization from areas with shallow-habitat persistence. Whereas existing hypotheses that attempt to explain biogeographical patterns rely on observations limited to shallow (<30m) coral-reef habitat and invoke processes operating on speciation time-scales (~10⁷), the HPH includes patterns on the remaining 80% of coral-reef habitat (30–150m) and invokes processes operating on time scales associated with sea-level changes (~10⁵yr). The HPH posits seven specific predictions that can be directly tested to distinguish its role from existing coral-reef biogeographic hypotheses.