The effect of chital (Axis axis) on the structure, composition and functional traits of plant communities in the forests of the Andaman Islands

Abstract

Herbivory has been shown to have a major impact in driving community structure and species composition in various ecosystems around the world. Although ungulate-vegetation interactions have been prevalent for millennia, the last few centuries have seen rapid changes in ecosystems globally, due to human-mediated introductions and range expansions of ungulates. In the Andaman archipelago, an introduced mammalian herbivore, the spotted deer or chital (Axis axis) has been reported to cause extensive damage to vegetation. Chital, being browsers in this ecosystem with no native mammalian herbivores or predators, have the potential to alter successional trajectories and community and ecosystem dynamics. This study attempts to quantify the community-level effects of varying densities of this introduced ungulate on the evergreen forests in these islands. I predicted that, in response to increasing ungulate habitat use, plant communities would show shifts in a) the size class distribution of trees, b) the species composition, richness and relative species abundances of both understory and adult tree communities, and c) community-level leaf functional traits, such that modified communities would reflect a decrease in the abundances of species that are susceptible to herbivory and the functional traits that characterise such species. Contrary to my expectations, I found no density-dependent effect of introduced chital on the size class distributions of trees or the abundances of understory and adult tree communities. However, intensity of habitat use by chital has a small but consistent negative effect on the species richness of the understory. A detrended correspondence analysis of community compositions, however suggests that understory community compositions are relatively similar across sites, while the compositions of adult tree species differ across sites. There appear to be community-wide shifts in SLA and leaf thickness along a gradient of increasing habitat use intensity by chital. Rather than resulting from shifts in species compositions, these shifts appear to result from within-species shifts in leaf functional traits along a gradient of herbivory. Species thus showed an unexpected degree of plasticity in these responses. Sampling needs to be done in islands with intermediate to high intensities of herbivory and more functional traits like leaf fibre and tannin content need to be quantified for a more robust understanding of the system.