Extinction, turnover and species diversity in an experimentally fragmented California annual grassland.

Many natural populations are subdivided among partially isolated habitat patches, but the influence of habitat patchiness per se on species immigration, extinction, and the resulting patterns of species diversity, has received virtually no experimental study. In an experiment designed to test the effects of habitat subdivision on local community structure, we compare the diversity and annual turnover of flowering plant species in 3 treatments of the same total area, but subdivided to different degrees. We experimentally fragmented a California winter annual grassland into isolated plots, two of 32 m2, eight of 8 m2, and 32 of 2 m2, each treatment representing a combined area of 64 m2. Insularization of the experimental habitat fragments is provided by grazing sheep. The effects of plot area on species diversity, extinction, and turnover are consistent with the MacArthur-Wilson model. Species richness increases with the degree of habitat subdivision. Extinction, immigration, and turnover, however, are relatively independent of the degree of subdivision. These experimental results contrast with predictions that habitat subdivision necessarily results in greater rates of extinction accompanied by reduced species diversity.