The effects of introduced ungulates on native and alien plant species in an island ecosystem: Implications for change in a diverse mesic forest in the Hawaiian Islands

In island ecosystems, where endemic species have long been viewed as competitively inferior and more susceptible to herbivory relative to continental species, the impacts of invasive species may be especially pronounced. The sensitivity of island species to invasive species may lead to threshold effects causing irreversible changes in these ecosystems. Predictions that native, mostly endemic plant species are more adversely affected by ungulates than introduced alien plant species, and that competitive interactions between understory native and alien plant species are more severe when ungulates are removed were examined in a 15 year longitudinal study in a diverse mesic forest on Kaua'i (Hawaiian Islands). Over this time period the density of both native and alien overstory canopy trees declined more in unfenced control plots than in plots fenced to exclude ungulates; alien and native trees in the canopy responded similarly to ungulate exclusion via fencing. A differential effect of ungulates on native vs. alien plant species was pronounced in the understory, where the density of small individuals of native canopy species declined sharply in unfenced relative to fenced plots. In contrast, the density of small individuals of alien canopy species was less affected over time by ungulate exclusion. Native herbaceous and understory shrub species also decreased in density over time in unfenced plots relative to fenced plots, while alien understory species increased greatly in some fenced plots relative to unfenced plots. Despite the increase in alien understory plants, no evidence for competition between these alien species and small individuals of native canopy species in the understory could be detected, as numbers of plants in these two understory categories covaried similarly in the presence or absence of ungulates. The contrast between large numbers of native species in the canopy and the severe disruption of the understory in this forest suggests that ungulate removal implemented now or in the near future might prevent loss of the canopy and lead to regeneration of these species. The near absence of recruitment of any endangered species not already present in protected plots indicates that active restoration will be essential to prevent loss of many of the characteristic species of these forests.