Species Richness of Coral Assemblages: Detecting Regional Influences at Local Spatial Scales

Coral assemblages are generally open to immigration from regional-species pools and thus are regionally enriched rather than saturated with species. Previously we have documented the sensitivity of local richness in these assemblages to a number of regional factors including the size of the regional species pool. Here we focus on the local scale used to determine the local–regional richness relationship. We examine the sensitivity of local richness to regional richness and to two environmental variables (depth and habitat type) across a range of locality sizes. In general, local richness is sensitive to the environmental variables regardless of locality size. In contrast, it is relatively insensitive to regional richness in very small 1-m2 quadrats but highly sensitive when sampled with 10-m line transects. Thus our analysis suggests a spatial threshold somewhere below 10 m for the detection of regional enrichment in coral assemblages. There are four biological mechanisms that may allow regional effects to be expressed at this spatial resolution: (1) disturbance and slow recovery towards competitive equilibrium, (2) heterogeneous biological interactions involving aggregated resources, neighborhood competition, or nonrandom dispersal among local neighborhoods, (3) spatially variable predation by specialists, and (4) probabilistic recruitment coupled with equal competitive ability. Empirical tests are now needed to discriminate among these mechanisms.