Species richness of resident and transient coral-dwelling fish responds differentially to regional diversity

Aim: To determine whether the diversity of resident and transient coral-dwelling fish responds differentially to gradients in regional species richness. Location: Three regions in the Indo-Pacific (Red Sea, western Indian Ocean, Great Barrier Reef) which contain increasingly larger regional diversities of reef fish. Methods: I surveyed fish residing within branching coral species. Fish species were a priori categorized as resident or transient based on the degree of affiliation between the fish and live coral. To compare among regions that differ in coral diversity I used a modified species-volume relationship (SVR). Each point in the SVR represents the total number of fish species, resident or transient, found within the cumulative volume of a specific coral species. Empirical SVRs were further compared with random-placement null models. Results: For transient species, I found that the observed SVRs did not differ consistently from those expected from random samples drawn from the corresponding regional species pools. In addition, for a given volume of coral, more fish species were found in richer regions, indicating strong regional influences on local diversity. In contrast, resident richness was lower than that expected from random samples of the species pool, and richness in rich regions was reduced comparably more than in poor regions. The SVRs of resident species were similar among regions with different regional diversities. Main conclusion: These results suggest that, within coral species, transient fish richness is mostly influenced by stochastic allocation of species from the regional pool. Conversely, richness of resident species within a coral species is limited, making it independent of regional diversity. Since higher regional diversity of resident fish was not accompanied by higher richness per coral species or by decreased niche breadth, higher regional diversity of resident fish species must be rooted in higher coral richness. Consequently, ecological interactions between functional groups (coral and fish) can be powerful drivers of regional biodiversity.