Print PageInvasive species and climate change represent two of the most important threats to global biodiversity, especially for island endemics. However, few studies have quantified the demographic consequences of both factors for the same species; one reason is the scarcity of long-term data that captures effects of both invasions and climatic variation on key parameters such as survivorship. This study used monitoring data from mapped and tagged individuals of the endemic plant Jepsonia malvifolia (Saxifragaceae) on Santa Cruz (SCI) and Santa Rosa (SRI) Islands in California, USA. The study goal was to determine the effects of invasive pigs and climate variables on survivorship. Data allowing estimation of four between-year transitions were available for SRI, all from the period after pig removal on this island in 1992. For SCI, three transitions were observed prior to pig removal in 2004, and four after. We used forward stepwise logistic regressions to test for 1) Changes in survivorship on SCI after pig removal, and 2) Effects of climate variables such as mean annual growing season precipitation and temperature on survivorship, independent of pig effects. We also tested for the influence of other factors such as island, plant size, and density on survivorship.
Results/Conclusions
Pig removal was associated with dramatically increased J. malvifolia survivorship on SCI, but large plants (Odds ratio for without pigs: with pigs=5.72) were more strongly affected than small ones (Odds ratio for without pigs: with pigs=1.85). On SCI, years after pigs were removed also had significantly higher mean growing season temperatures, creating the potential for confounding climate effects with those of herbivores. Analysis of data from both islands taken during years when pigs were absent showed that higher temperatures were significantly associated with increased survivorship in both large and small plants. Survivorship also went down significantly with higher J. malvifolia numbers, indicating an important role for density dependence. These results provide further evidence for the devastating negative effects of invasive herbivores on some endemic island plants. This work also illustrates the importance of considering and controlling for factors such as climate variation and density dependence in assessing the effects of invasive species removals and other management actions. Future studies should monitor changes in the spatial distribution of J. malvifolia to determine whether density dependence will limit recovery after pig removal, and continue the censuses necessary to further characterize climate effects and improve predictions of responses to global warming.
