Relationship of Eastern Gulf of Mexico Reef-Fish Communities to the Species Equilibrium Theory of Insular Biogeography

Abstract

A 1971 summer red tide (Gymnodinium breve Davis) and associated stress conditions resulted in mass mortalities and near extirpation of reef biotas from at least 1536 km2 of central West Florida Shelf. An estimated 77% of the resident fish species perished at shallow-water (12-18 m depths) reefs. Reef-fish colonization was monitored irregularly (1-6 month intervals) at two reefs for 3 years after the red tide. In addition, species censuses were taken 4 years after defaunation at one reef and 5 years later at both reefs. Since eastern Gulf of Mexico reefs occur as isolated patches, reef-fish coloniz- ation data were analysed in light of MacArthur and Wilson's species equilibrium model developed for insular biotas. Certain features of reef-fish colonization appeared consistent with the MacArthur-Wilson model: (1) an increasing convex colonization curve, (2) an observed immigration (colonization) rate de- creasing with time, (3) differences between colonization and decolonization (observed extinction) rate decreasing through time, and (4) attainment and maintenance of a rather stable species richness after 15 months colonization similar to the pre-defaunation level. However, other aspects of reef-fish coloniz- ation did not seemingly fulfil basic requirements of the model: (I) an erratic decolonization rate indicating no tendency to increase through time, (2) an observed species turnover rate considerably less than theoretical predictions, and (3) development of a compositionally stable community nearly identical with that existing prior to the red tide. It is proposed that eastern Gulf reef-fish communities develop according to well-defined successional rather than chance colonization processes. The eventual stability in species richness and composition probably represents attainment of a 'climax' community rather than a dynamic equilibrium resulting from continual species turnover. Possible reasons for failure of these communities to conform to the species equilibrium theory are presented.