Distribution of the Rare Serpentine Sunflower, Helianthus exilis (Asteraceae): The Roles of Habitat Availability, Dispersal Limitation and Species Interactions

Abstract

Helianthus exilis, an annual forb endemic to California's North Coast Ranges, occurs in isolated microhabitats within serpentine outcrops, which are themselves insular. Extensive field surveys and transplant experiments showed that nearly all suitable habitats, although widely separated, are occupied by H. exilis. Small serpentine patches (< I ha) contain little or no suitable habitat, thus explaining the complete absence of H. exilis from these sites. Experiments also showed that H. exilis is an obligate outcrosser and that natural populations were somewhat pollen limited. Nevertheless, isolation of local populations within large serpentine outcrops did not significantly reduce pollination rates. Seed predation rate by a tephritid fly decreased with increasing patch isolation, but seed predation by a moth was not significantly correlated with isolation. In metapopulation terms, our results suggest that H. exilis may be relatively resilient to a reduction in the number of habitat patches or populations, because of its high persistence within the most suitable microhabitats.