Coral species richness: ecological versus biogeographical influences

Abstract

Species richness in communities varies with habitat area, productivity, disturbance level, intensity of species interactions, and regional/historical effects. All of these factors influence coral richness but their effects vary with spatial scale, position on the reef, and regional location. Species richness of corals along depth gradients shows a unimodal, hump-shaped curve that peaks at intermediate depths. Moreover, the peak of the curve is higher in regions with larger species pools. This “regional enrichment” of the local community appears in line transect samples as small as 10 m in length. The pattern suggests that ecological factors operating over scales of tens of meters and regional/historical factors operating over thousands of kilometers can both affect local richness. Regional factors probably include differences in speciation relative to extinction rates among regions and proximity of local sites to richness hotspots. Plausible factors operating at the local scale are species interactions, disturbance, and productivity which combine in different ways to produce the unimodal pattern. Shallow areas support few species because extinction rates are high due to frequent disturbance or because of environmental extremes. In addition, high productivity encourages rapid growth and thus the potential for intense interspecific competition. In areas where branching acroporids are abundant, exclusion by these dominant competitors is possible. Deep areas may be depauperate because few species can tolerate the low light levels found there. Areas of intermediate depth have the richest communities because they are open for colonization by many species and because extinction rates are low. Several theories may explain this “openness” and species persistence:

1. Occasional disturbance coupled with low growth rates results in glacially slow exclusion by the dominant competitor.

2. Aggregation of corals creates spatial variation in the intensity of competition and thus refuges from competition within a spatial landscape. Inferior competitors persist because they are superior at dispersal and refuge colonization.

3. Specialist predators focus on high-density juvenile populations near the parent, creating ecological space for colonization by non-prey.

4. Coral competitive abilities are roughly equal and recruitment into the community is a probabilistic event. The community thus exhibits random drift and exclusion is an extremely lengthy process. Based upon empirical evidence, these theories are listed in order of plausibility, but still need to be rigorously tested.