Talk, Vulnerable islands in the sky: science and management of tropical island alpine & sub-alpine ecosystems, Hawaii, USA: 2012-08-03 - 2012-08-08
Although high-elevation ecosystems on oceanic islands still largely possess natural or semi-natural habitats, their resilience to human induced disturbances such as introduced mammalian herbivores or altered fire regime is low. In the absence of mammalian herbivore pressure (e.g. through the European rabbit, Oryctolagus cuniculus, and the feral goat, Capra hircus) during evolutionary times, most endemic plant species have lost their defense mechanism to herbivory (ecological naiveté sensu Brown & Van Vuren (1997) and Lomolino et al. (2010)). Consequently, it has been shown that introduced mammalian herbivores have had detrimental effects on island vegetation and biodiversity. Superimposed on introduced herbivores human induced fires may fundamentally alter high-elevation ecosystems. Here, we present long-term experimental data on the effects of introduced herbivores and altered fire regime on species richness, species composition and the recruitment of native species in the presence versus absence of both disturbances. An 11 year exclosure experiment established by the National Park Caldera de Taburiente (La Palma, Canary Islands, Spain) in the summit scrub on the outer flanks of the caldera was used spanning from about 1800 to 2400 m asl and facing in three directions (NW, NE, E). 18 woody plant species either island endemic and/or endangered were sown with the establishment of the exclosures in 2000. Seedling establishment (i.e. seedling number and diversity), browsing damages and competitive success after a fire event in 2005 were measured with regard to presence and absence of herbivores in equal sized plots (20x20m) inside and outside the exclosures. Herbivore presence significantly reduced the number of seedlings and seedling diversity (Fig. 1), whereas the fire event had the opposite effect on recruitment success. We could not detect a difference in seedling establishment between the upper limit of Canary Pine forest and the summit scrub zone. Endangered species showed highest browsing marks and had a lower growth performance outside of the fenced areas, whereas abundant species profited from reduced competition with presence of herbivores. The current presence of intro-duced herbivores is the main driver for the low species diversity in the upper limit of the Canary Pine forest and the summit scrub. A fire event, however, led to a higher species number in seedlings of native and endemic species. Evidently, herbivores severely modify the species composition of the natural high-elevation vegetation. We suggest introduced herbivores in high elevation ecosystems of oceanic islands to be one of the emerging research frontiers in island biogeography given their global impact regarding effects on biodiversity and ecosystem services.