High b diversity in the flora of Californian serpentine ‘ islands

Identification of global biodiversity hotspots is a leading conservation priority (Mittermayer et al. (1999) Hotspots. Conservation International, Washington, DC; Stein et al. (2000) Precious Heritage: The Status of Biodiversity in the United States. Oxford University Press, Oxford, UK), but we know little about the nature and structure of such hotspots. Botanical richness in one recently identified hotspot, the California Floristic Province, has long been attributed by evolutionists to edaphic and climatic heterogeneity. Here we demonstrate that the flora restricted (endemic) to serpentine soil, the state’s most botanically distinctive substrate, shows unusually high ‘b diversity’ – i.e., spatial variation in species composition – among regions, although ‘a’ diversity within any given region is low. Conservation of an endemic-rich flora requires a network of sites to capture this among-region or spatial component of diversity.