INTERSPECIFIC COMPETITION AND COEXISTENCE IN A GUILD OF CORAL-DWELLING FISHES

We investigated the effects of interspecific competition on abundance, habitat partitioning, and coexistence of six closely related species of gobies (genus Gobiodon) that inhabit a range of acroporid coral species at Lizard Island, Great Barrier Reef. After documenting the extent of overlap in habitat use among pairs of species in the field, we used a combination of field and laboratory experiments to investigate the relationship between these patterns and the occurrence of interspecific competition. Experiments in aquaria tested the ability of five of the species to compete against G. histrio, the apparent competitive dominant, including the effects of body size and prior residency. A manipulative field experiment, in which abundance 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. axillaris) and low for the other (G. brochus). In aquaria, G. axillaris and G. histrio preferred the same species of coral and had equivalent, size-based, competitive abilities. The coexistence of G. axillaris and G. histrio at the scale of 10’s metres on the reef can thus be explained by a competitive lottery model. However, differential distributions of these two species across the reef flat and reef crest suggests that resource partitioning or habitat selection at larger spatial scales are also important to their coexistence. In aquaria, G. brochus was an inferior competitor to G. histrio and could only gain access to the preferred species of coral through an advantage in body size or prior residency. Low overlap in habitat use between G. brochus and G. histrio in the field appears to result from niche shifts by the subordinate competitor only, indicating coexistence via an included niche model. The field experiment indicated that the other three other species did not compete for space with G. histrio, and these species exhibited either low (G. rivulatus) or high (G. quinquestrigatus and G. unicolor) overlap in habitat use with G. histrio. Experiments in aquaria demonstrated that G. rivulatus and G. histrio did not compete because they preferred different species of coral. In contrast, G. unicolor and G. histrio exhibited high overlap in habitat use but did not compete because they were able to co-habit the same coral colonies without affecting each other. In aquaria, G. quinquestrigatus and G. histrio preferred the same coral species and G. quinquestrigatus was an inferior competitor, so these species were expected to compete for space in the field. In a field recolonization experiment, coral colonies previously occupied by G. quinquestrigatus were rarely recolonized by G. histrio, indicating that these species coexist because they use different types of coral colonies in the field. The study demonstrates that there is no single relationship between overlap in resource use and the occurrence of interspecific competition, even among closely related species, and that species within a guild can coexist by a diversity of mechanisms.