In local reef fish communities, species richness increases with increasing reef area. At Ishigaki Island, Japan, species richness is much lower on large reefs in the shallow back reef than that expected from random placement model simulations (RPMS). As three aggressive territorial herbivorous damselfish, Stegastes nigricans, Stegastes lividus and Hemiglyphidodon plagiometopon, coexist only on such large reefs, we focused on these species and examined patterns of their distribution and abundance on 84 patch reefs of various sizes (area and height). We also examined their aggressive intra- and interspecific behavioural interactions and habitat use on the two large reefs (the largest complex patch reef and the large flat patch reef) among the 84 patch reefs. While the abundance of both S. lividus and H. plagiometopon was highly correlated with patch reef area, that of S. nigricans was closely correlated with patch reef height. For S. nigricans and S. lividus, interspecific interactions occurred significantly more frequently than intraspecific interactions on the large flat patch reef. However, there was no significant difference in frequencies of the two interaction types on the largest complex patch reef, where they three-dimensionally segregated conspecific territories. This study suggested that reef height as well as reef area influence the distribution and abundance of these territorial herbivorous damselfish in the shallow back reef. As large patch reefs cannot be tall allometrically in shallow back reefs, relatively flat patch reefs may not have the high species richness expected from RPMS based on reef area.

Tepuis are uplifted mountains surrounded by forest or savannas in the Guayana Shield in northern South America. Large areas of the summits of these mountains are characterized by bare rock with vegetation isolates (‘islands’) that vary in size from less than 1 m2 to over 100 m2. The species composition, species–area relationships and the factors involved in the formation of these vegetation islands were quantified on Roraima Tepui, Canaima National Park, Venezuela. A total of 40 species were present on the islands, 35 of which were angiosperms. Orchidaceae was the family with the highest diversity, but Bonnetiaceae, Poaceae and Rapateaceae achieved higher coverage. A positive correlation between island size and species richness was found. This relationship was not linear and the best fit model between island size and richness was exponential. Of the environmental variables studied, the highest correlation was found between the log of soil volume and species richness. Soil nutrient availability, and not water, might be the limiting factor determining the species composition and diversity of these communities.