Diverse native island flora shows rapid initial passive recovery after exotic herbivore removal on Santa Rosa Island, California

Abstract

Removing exotic vertebrates from islands is an increasingly common and potentially effective strategy for protecting biodiversity. Yet, surprisingly few studies evaluate large-scale effects of island removals on native plants. We surveyed 431 hectares of habitat in 7 canyons on Santa Rosa Island just after exotic herbivore control began (1994–1996), and again after two herbivore species had been eradicated and ~ 90% of herbivores removed (2010–2012). We searched for 68 endemic and/or rare native plant taxa, mapping and recording abundances for the 39 found. Initially most of these 39 species were absent from most canyons (79.9% species-canyon combinations). Nearly 35% of absences changed to presences by 2010–2012, while only 5.5% of presences changed to absences. Thirty-six of these 39 species increased in total area, and 38 increased in total abundance. Graminoids increased more and shrubs less than other life histories, for both apparent colonizations and abundances. Beta diversity and species turnover between canyons was high at both surveys. Although a diversity of the 39 located taxa showed substantial gains, nearly half remained uncommon in 2010–2012. These results reinforce the devastating effects of exotic vertebrate herbivores on island native plants, particularly long-lived, slow-growing species. They also demonstrate significant potential benefits of exotic herbivore removal even without other active restoration, not only for vegetative cover but for a number of rare taxa. Our surveys were more spatially extensive than most post-removal studies; high spatial turnover in these data suggests that larger-scale monitoring may be critical to capture full effects of exotic animal removal.