How Natural Habitat Patchiness Affects the Distribution of Diversity in Californian Serpentine Chaparral

Components of diversity in the woody plant community were compared among three sets of sites in the Coast Ranges of Northern California: 24 small patches (0.5–3 ha) of serpentine soil, found in four sets of 5–7 patches; 24 equivalently spaced sampling sites within four large (6–55 km2) continuous areas of serpentine; and 24 sites on the nonserpentine soils adjacent to the small patches. On patchy compared with continuous serpentine, alpha diversity (local) was 32% lower, gamma diversity (total) was 17% higher, and beta diversity (differentiation in species composition among sites) was 72% higher. Of the increased beta diversity on patches, most (62%) occurred at the “local” scale of patches 50–3200 m apart, and only 10% occurred at the scale of clusters of patches 16–45 km apart. Complementarity, an alternative measure of beta diversity, showed the same patterns. Woody plants on nonserpentine soils showed diversity patterns similar to those on patchy serpentine, i.e., lower alpha and higher beta and gamma diversity relative to continuous serpentine. Nonserpentine plants showed a stronger response to environmental gradients than did serpentine plants. For the woody plant community. on serpentine soils, the natural large-scale patchiness of the habitat leads to lower local diversity but higher differentiation and regional diversity. The increased regional diversity appears to be almost entirely the result of edge effects, rather than of “patchy coexistence” as described by metacommunity models.