San Clemente Island, one of California's Channel Islands, has a vascular flora containing 13% endemic species, a high number for a small island (145 square km) located close to the mainland (79 km). From the mid-1800s to the early 1990s, San Clemente Island experienced overgrazing by sheep, pigs, cattle and goats, resulting in the rarity and endangerment of many native species. I have conducted surveys of allozyme variation in 15 endemic plant species that are rare or endangered. Although average genetic diversity is low, a broad range of results has been observed. Many species have maintained relatively high levels of genetic variation, including ones whose numbers were severely reduced by overgrazing, and one species exhibits levels of genetic variation among the highest reported for plants. The wide range of results observed for this set of species that has occupied the same, small geographic area and has experienced similar threats for over a century, suggests that prediction of levels of genetic variation based on geographic range and rarity is likely to be generally unsuccessful. However, the species that are most rare, and are currently known to be represented by fewer than five populations or fewer than 100 individuals, have virtually no detectable allozyme variation.

Key words: allozymes, conservation, endemic, genetic diversity