Island species are especially susceptible to the impacts of invasive species, and most documented extinctions due to invasions have occurred on islands.  A number of large-scale invasive eradication programs have been implemented on islands recently.  These efforts often yield clear and strong benefits, particularly for native species affected by introduced predators. However, few detailed monitoring data are available to evaluate the potentially complicated responses to these large-scale community changes or determine whether total eradication of invaders is necessary to reverse declines of native species.  California’s Channel Islands are centers of native plant diversity, home to 145 endemic species, but have been altered by a number of different introduced vertebrate herbivores, many now under management.  On Santa Rosa Island, cattle were removed completely and deer and elk numbers reduced in 1998, but the consequences of these changes for native plants and the effects of ongoing damage were not clear.  We collected and analyzed demographic data and herbivore damage records from 1995-2006 for over 1000 individuals of Castilleja mollis, an endangered plant endemic to Santa Rosa Island.  Our goal was to characterize the response of C. mollis to past cattle removal and ongoing impacts of introduced deer and elk.  

Results/Conclusions

Damage due to ungulate scraping of the ground dramatically increased both plant mortality and dormancy and reduced flowering and growth.  Stem browsing damage did not affect survivorship but significantly reduced plant growth.  Larger plants with more flowers were significantly more likely to experience stem damage, and browsed inflorescences produced 30% fewer flowers than unbrowsed ones.  Damage related to ungulate scraping and stem browsing declined steeply between 1997 and 2000 and remained relatively low, although persistent, since.  Castilleja mollis abundances rose sharply after 1997 in parallel to these changes, but began to decline steadily again after 2003.  Multivariate regression indicates that both the decline in scraping damage and a decrease in mean growing season temperatures were linked to increasing C. mollis populations between 1997 and 2002, while 2003 to present declines appear to be caused by higher temperature conditions.  Mean growing season temperatures in the region have risen significantly over the last 50 years, suggesting C. mollis populations may be increasingly threatened by warm temperatures.  Moreover, climate impacts may confound the positive effects of herbivore removal as well as reduce the resilience of C. mollis to lowered rates of herbivore damage.