Reef-fish community structure and dynamics: An interaction between local and larger-scale processes?

Studies of coexistence in biotic communities have focused largely on local ecological processes. As a result, effects of regionally varying processes on community structure and their interactions with other processes operating locally have received considerably less attention. Here I investigate variation in predator abundance and species richness at a large spatial scale and associated community-wide patterns of species richness and abundance of tropical fishes on Australia's Great Barrier Reef. I constructed reefs from natural substrata to standardize their structure, isolation and history, which could otherwise be confounded between locations. Recruitment, both total abundance and species richness of recruits, to these reefs was greater at the northern location than at the southern one. In contrast, communities of resident fishes that developed on these reefs showed the opposite pattern; species richness and abundance were greater on the southern reefs. Piscivorous fishes were proportionately more abundant on the northern reefs. Therefore, predators were more abundant at the location with greater recruitment but lower abundance and species richness of resident fishes. Also, the declines in species richness and abundance of fishes from the observed maximum in one year to the following observed minimum were related to average predator densities among reefs. These results suggest that one ecological process that varies between distant locations, in this case predation estimated by predator abundance, may override the effects of other ecological processes, in this case recruitment, in determining local patterns of coexistence. Furthermore, they suggest that understanding causes of local patterns of species richness and abundance may require information about processes that determine regional variation in ecological interactions.