Regional turnover and fluctuation in populations of five plants confined to serpentine seeps

Spatial patterns in population turnover were analyzed for five plant species confined to a patchy and isolated habitat, serpentine seeps: annuals Helianthus exilis and Mimulus nudatus and perennials Senecio clevelandii, Astragalus clevelandii, and Delphinium uliginosum. A 3-year survey showed that, over a preceding 15-year period, 32 of 132 known populations in a 4200-ha area disappeared, and 65 new populations appeared. Populations that disappeared were significantly more isolated from conspecific populations than were surviving populations. Populations that disappeared were also marginally significantly closer to human-caused disturbances than were populations that survived. Vacant sites in which new populations appeared were significantly less isolated from conspecific populations than sites remaining vacant. Spatial patterns in population density were analyzed for three of the species for 3 years at 50 sites in a 20 x 40 km region. From 1997 to 1998, population densities of each species changed synchronously throughout the region. From 1998 to 1999, one species (Mimulus nudatus) again showed synchrony, and populations of the other two species (Helianthus exilis, Delphinium uliginosum) remained asynchronous and spatially uncorrelated. The five species we studied all appear to be persisting well at the regional scale, despite recent disturbance to the study region. These results suggest that for rare plants in isolated habitats, the spatial configuration of populations may have an important influence on local population persistence.