THE ISLAND AND MAINLAND BIOGEOGRAPHY OF RESIDENT ROCKY-SHORE FISHES IN THE GULF OF CALIFORNIA

Abstract

Resident rocky-shore fishes were quantitatively sampled and visually censused on islands and mainlands in the Gulf of California. Patterns of biogeographic distribution were investigated in relation to the MacArthur-Wilson model of island biogeography, the lottery hypothesis of reef fish community structure. Analyses of the collections show regular patterns of distribution and relative abundance of rocky-shore fishes. Most of the variation in species number and species diversity is explained by latitude and the associated gradient of physical and oceanographic parameters. The correlation of species number and diversity is higher for island than for mainland samples. The best graphical (numerical clustering) evidence of distinct biogeographic regions in the Gulf is obtained using a similarity measure which includes proportional abundance of species (Horn's measure). Groups corresponded to upper, central, and lower Gulf areas. Greater species number, biomass, and numerical (H'(n)) and biomass (H'(b)) species diversity on islands is paralleled by greater water clarity and volume of ocean near collecting sites. This is true for the entire Gulf, the central Gulf, and for Isla San Pedro Nolasco versus the adjacent mainland rocky shoreline in the central Gulf. Community differences between island and mainland areas were greatest in the upper Gulf. 'Gamma' diversity, a proposed measure of community variance (site to site species turnover within habitats) is highest for mainlands and lowest for islands indicating more predictable community species composition on islands. Visual censuses show a good species-area curve for patch reefs and very small nearshore islands and lower species turnover in more exposed near shore habitats. There is a trend toward smaller body size of fishes on islands and an increase in body size with latitude. It is suggested that both phenomena are the result of thermally regulated growth rather than genetic population differences. High endemism in the resident rocky-shore community (the small sedentary, blennioid and gobioid fishes) is attributed to their relatively poorer means of dispersal (demersal eggs and short-lived pelagic larvae) as compared to the more mobile larger resident reef fishes with greater means of dispersal (pelagic eggs and long-lived pelagic larvae). Insular biogeographic processes in marine reef habitats are mediated by physical and biological oceanographic conditions and processes. Distance appears to be no significant barrier to dispersal for Gulf rocky-shore fishes. Immigration rates may be high in rocky habitats in general, but in inshore mainland areas physical and biological disturbance suggest higher extinction rates and consequent lower diversities. The results of these studies do not agree well with either the MacArthur-Wilson model of island biogeography or the lottery hypothesis of reef fish community structure, however, disturbance does seem to play some role in regulating diversity. Considering dispersal and colonization, oceanographic conditions (e.g., water quality, current patterns) may be important ecological factors that influence the evolution of this community.