Chronic browsing by an introduced mammalian herbivore in a tropical island alters species composition and functional traits of forest understory plant communities

Mammalian herbivores have large-scale impacts on vegetation, altering
structure and species composition, especially in tropical grasslands and
savannas. However, there is limited understanding of the potential impacts
of mammalian herbivores in tropical wet forests, where they are typically
less abundant. We investigated the effects of an introduced mammalian
herbivore chital (Axis axis) on vegetation structure, composition and leaf
functional traits of tropical evergreen forests of the Andaman Islands,
India. Across seven islands, representing a gradient of herbivore
densities, increasing chital presence was associated with decreased
understory richness, understory density and adult tree richness, but was
not related to adult tree density or size class distributions. We also
found a significant decrease in community-level leaf palatability traits
(specific leaf area decreased and leaf thickness increased) with
increasing chital habitat use. This community-level shift in leaf trait
values was better explained by intra-specific variation in leaf traits
across islands rather than changes in species composition. In summary, we
show persistent long-term impacts of an introduced mammalian herbivore on
understory tropical tree communities although there is little impact on
adult tree communities.  Our results also show that functional
traits of species can be altered in response to novel herbivory, even at
herbivore densities where there are no detectable impacts on adult forest
structure or composition. Such altered functional traits may potentially
alter ecosystem functioning in these forests even without changes in
vegetation structure.