Population Persistence and Community Diversity in a Naturally Patchy Landscape: Plants on Serpentine Soils

Abstract

Serpentine soils in California are patchily distributed at multiple spatial scales and support a distinctive flora. In a region of 10 x 30 km in Northern California, I compared plant species richness on 24 small (<5 ha) serpentine outcrops and 24 equally-spaced sites within large (> 1 km 2 ) outcrops. For serpentine endemic plants, alpha (local) diversity was lower but beta (differentiation) diversity was higher on small outcrops compared to sites within large outcrops. For alien plants, local diversity was higher on small outcrops than on sites within large outcrops. Experimental work examined mechanisms underlying these patterns. Two alien grasses (A vena fatua and Bromus hordeaceus) appeared to be more prevalent on small outcrops because of edge effects rather than because of habitat quality. One serpentine endemic herb (Calystegia collina) exhibited lower reproductive success on small outcrops, evidently because of a shortage of compatible pollen. Another en demic herb (Helianthus exilis) was absent from small outcrops because of the absence of seeps, a specialized habitat found within large serpentine outcrops. Direct evidence for local extinction and recolonization was found in five herbaceous species that inhabit serpentine seeps. These results illustrate the importance of large- scale landscape structure for population persistence and community diversity in plants.