Intra-and Interregional Comparisons of Numbers of Species on Marine Hard Substrate Islands

Intraand interregional comparisons of the number of species (S) on hard substrate islands were made, and factors affecting S were examined. Contrary to previously published data, we found substantial variation in S with no clear geographic pattern. In fact within one region the range in S was as great as found among regions. Regression analyses suggested that immigration rate, realized pool size (i.e. the total number of species observed locally), and island size all had significant positive effects on S. Other factors such as latitude, temperature, salinity and regional pool size had no significant effect on S. Analysis of the data nested by habitat, ocean and latitude gave the same results. This suggests that local factors affecting the ability of a species to reach an island, i.e. distance from a source of immigrants and species dispersal abilities, have a greater effect on S than regional factors that might determine the total number of potential colonizing species. Given the apparent importance of local factors in determining the number of species on hard substrate islands, we question the relevance of interregional comparisons in assessing factors which may influence the distribution and abundance of species.