Effects of grazing on diversity of annual plants in the Sonoran Desert.

A two-yar survey of winter-germinating annual plants in southern Arizona indicates that species diversity declines consistently as a function of increasingly recent grazing by cattle. This finding conflicts with reports that predators enhance prey species diversity in some marine and terrestrial systems. Consideration of equilibrium and nonequilibrium models suggests, however, that enhanced diversity should occur only for open, multi-celled prey populations experiencing intermittant predation. These general conditions appear not to hold for the cattle-annual plant system.