The Vortex Model of Coral Reef Biogeography

Distributions of reef organisms show high species diversity at the western equatorial boundaries of the ocean basins. This pattern can be produced through transport of new species that have formed in isolated peripheral areas into the centre of high diversity. Strong and persistent west- erly currents dominate surface circulation at tropical lati- tudes. Larvae and rafted organisms are carried into diversity foci at the western edges of the oceans at a rate far more rapid than outward dispersal. A biogeographic model based on differential oceanic exchange rates of transport for larvae and rafted organisms is proposed. The name 'vortex model' was selected to emphasize the importance of currents in the movement of species towards the diversity centre. This steady-state model is consistent with the dispersal model in that it encompasses movement of species away from the di- versity centres. The vortex model agrees with vicariance theory because it accommodates allopatric speciation (by divergence) an isolated peripheral locations. The vortex model does not preclude the formation of additional species in the diversity centre. A computer simulation of the simplified model was developed in which speciation and ex- tinction occur randomly on a matrix of evenly spaced islands in an idealized rectangular ocean. Initial conditions of the simulation include a uniform species distribution with rates of speciation and extinction chosen to produce an in- creasing number of species over time. Dispersal in the simu- lation is controlled entirely by speed, direction and variability of ocean currents. The resulting pattern of species is characterized by high diversity foci at the western equatorial boundary and decreasing diversity with increas- ing latitude.