Interspecific competition and coexistence in a guild of coral-dwelling fishes

We investigated the effects of interspecific competition on abundance, habi tat partitioning, and coexistence of six closely related species of gobies (genus Gobiodon) that inhabit a range of coral species at Lizard Island, Gr eat Barrier Reef, Australia. After documenting the extent of overlap in hab itat use among pairs of species in the field, we used a combination of fiel d and laboratory experiments to investigate the relationship between these patterns and the occurrence of interspecific competition. Experiments in aq uaria tested the ability of five of the species to compete against Gobiodon histrio, the apparent competitive dominant, including the effects of body size and prior residency. A manipulative field experiment, in which abundan ce of G. histrio was reduced, tested whether competition with this species limits the abundance of the other five species. Two species competed for space with G. histrio in the field, yet overlap in habitat use with G. histrio was high for one of these species (G. axillari s) and low for the other (G. brochus). In aquaria, G. axillaris and G. hist rio preferred the same species of coral and had equivalent, size-based, com petitive abilities. The coexistence of G. axillaris and G. histrio at the s cale of tens of meters on the reef can thus be explained by a competitive l ottery model. However, differential distributions of these two species acro ss the reef flat and reef crest suggest that resource partitioning or habit at selection at larger spatial scales may also be important to their coexis tence. In aquaria, G. brochus was an inferior competitor to G. histrio and could only gain access to the preferred species of coral through an advanta ge in body size or prior residency. Low overlap in habitat use between G. b rochus and G. histrio in the field appears to result from niche shifts by t he subordinate competitor only. The field experiment indicated that the other three species did not compete for space with G. histrio. Experiments in aquaria demonstrated that G. riv ulatus and G. histrio exhibited low overlap in habitat use and did not comp ete, because they preferred different species of coral. In contrast. G. uni color and G. histrio exhibited high overlap in habitat use but did not comp ete, because they were able to cohabit the same coral colonies without affe cting each other. In aquaria, G. quinquestrigatus and G. histrio preferred the same coral species' however, in a field recolonization experiment, cora l colonies previously occupied by G. quinquestrigatus were rarely recoloniz ed by G. histrio, indicating that these species coexist because they use di fferent types of coral colonies in the field. The study demonstrates that t here is no single relationship between overlap in resource use and the occu rrence of interspecific competition and that species within a guild can coe xist by a diversity of mechanisms.