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Biogeographic and environmental drivers of reef fish diversity in coastal islands of the Gulf of Thailand

Felipe M. G. Mattos*, Thamasak Yeemin, Makamas Sutthacheep, João L. L. Feitosa

*Corresponding author:

ABSTRACT: Islands, critical environments for marine biodiversity, have endorsed several classical ecological theories. However, most studies on marine communities in islands are restricted to oceanic settings; whether premises of island biogeography apply to coastal islands is still unknown. In the Gulf of Thailand (GoT), coral reefs occur in nearshore coastal islands, subject to a wide range of environmental conditions, diverging from the most-studied oceanic counterparts. We investigated fish communities of 51 reefs in 10 sites spanning ~1,500 km of the GoT coast, to answer whether diversity and species composition were (1) more influenced by area and isolation than by local environmental conditions and/or (2) affected by the geographical position. As predicted by island biogeography models, diversity decreased with Island isolation, however, for rare species only; reef area increased diversity but solely for common and dominant species. The effect of the geographical position of reefs within the GoT was negligible. Small-scale differences in local conditions of turbidity and depth affected diversity and composition more than isolation and SST. Within-island variability in these conditions yielded a high local turnover of species, enhancing within-site gamma richness, even when the mean alpha richness per site was low. Trophic group composition was similar among most sites, but extreme conditions of environmental constraints (e.g. very high/low turbidity) rendered changes in the dominant trophic group. Results suggest island biogeography models apply to the reef fish communities in coastal islands to some extent; still, the broader range of local conditions are the primary drivers of their diversity and composition.