Competitive coexistence of coral-dwelling fishes: The lottery hypothesis revisited

Evidence for competitive lotteries among reef fishes has remained elusive despite this being the group of organisms for which the lottery model was first developed. I used a combination of laboratory and field experiments to test the mechanisms of coexistence between two closely related species of coral-dwelling goby, Gobiodon histrio and G. erythrospilus, that occur in similar abundance at Lizard Island on the Great Barrier Reef. These two species exhibited similar patterns of habitat use and nearly identical ability to compete for vacant corals. Furthermore, there was a priority effect where the first species to occupy a vacant coral excluded an interspecific intruder of similar body size. The relative abundance of recruit and juvenile G. histrio and G. erythrospilus in the field matched the relative abundance of adults, as expected where there is no post-recruitment displacement by a competitive hierarchy. Finally, a reciprocal competitor-reduction experiment confirmed that G. histrio and G. erythrospilus compete for vacant space, with the removal of either species leading to an increase in the abundance of the other species. Therefore these two species are nearly ecologically equivalent and appear to coexist by means of a competitive lottery for vacant space.