How can the study of marine environments deepen our understanding of local vs. regional richness relationships?

Background/Question/Methods and Results/Conclusions

It is widely acknowledged that regional propagule supply, in addition to local interactions among coexisting species, has a strong influence on species richness in ecological communities. Much of the work leading to this conclusion was performed in terrestrial communities. However the regional species pool is often hard to define in terrestrial communities because of habitat heterogeneity and idiosyncratic dispersal barriers. If species in the region are specialized to other habitats or, because of undiscovered dispersal barriers, cannot get to the habitat of interest, they will not be members of the true species pool. Such issues may have in part led some terrestrial ecologists to deemphasize the importance of propagule supply and to focus more on local processes as controls on local species richness. Marine ecologists have had less difficulty than terrestrial ecologists in thinking about species pools, perhaps because dispersal barriers are less obvious in marine environments. The concepts of the propagule “bath” and supply-side ecology were developed by marine ecologists. Ideally, if we had a habitat where all species in the propagule bath could survive, and we could manipulate propagule supply, we could more easily test for the effects of the species pool on local diversity. Island biogeography theory provides a framework for such a test. By increasing the dispersal distance between similar habitats and a propagule source, ever greater numbers of species in the bath could effectively never reach the more distant habitats, reducing the species pool. We studied a gradient of regional coral species richness across the Indo-Pacific, which approximates this behavior. We show that as regional richness declines eastward from the source pool in Indonesia, local richness on island fringing reefs declines accordingly at three spatial scales, indicating a strong influence of the propagule supply on the local ecological community. The special attributes of the marine environment have thus helped to clarify our thinking about local vs. regional richness relationships.