Island Biogeography: Studies on the Xeric Shrublands of the Inner Channel Islands, California

Abstract

Ten 0.063 ha samples of xeric shrubland vegetation and associated topographic, soil, climatic and disturbance variables (grazing and fire histories, air pollution) are described on four Channel Islands (Santa Cruz, East Anacapa, Santa Catalina, San Clemente). Floristic variation among island samples responds primarily to a gradient of decreasing moisture availability, inducing a transition from mesophyllous coastal sage shrub to increasingly fleshy coastal succulent scrub vegetation. The trends in species richness among island samples appear to be largely a reflection of the number of species in the associated mainland plant formations, although island samples are more species poor than their mainland counterparts. Previous studies of the California Islands vegetation, sampling more islands and considering island floras as a whole, found positive correlations between island area and species richness. In the present study, samples are limited to a single broad vegetation type, sites of a fixed area are considered, and disturbance factors are minimized. When richness (total or native species only) of these samples is considered against island area and distance to the mainland, low or negative correlations result. When total area of the vegetation type on the island is used as independent variable, the negative correlations persist. These findings suggest that the previous positive correlations reported were due to increasing habitat (including disturbance) heterogeneity with increasing island area, rather than to reduced extinction pressure as postulated by the MacArthur-Wilson model. A third hypothesis, that species increase with area due simply to larger sample size, cannot be filnnly ruled out by these data.