Long-term understory vegetation dynamics and responses to ungulate exclusion in the dry forest of Mona Island

Mona Island protects one of the most important remnants of Caribbean dry forests and hosts a high diversity of rare and endangered plant and animal species. Feral ungulates (goats and pigs) were introduced to the island ~500 y ago, and their populations may be threatening the conservation of Mona Island’s native biodiversity. In this study, we used permanent fenced and unfenced plots at 4 different depression-forest sites on Mona Island to assess long-term (10-y) understory vegetation dynamics and the potential responses of this vegetation stratum to ungulate exclusion. Depression forests on Mona Island are of particular interest because they hold the highest diversity of tree species on the island and are critical nesting habitats for the endangered Cyclura cornuta stejnegeri (Mona Island Rock Iguana). Overall, our results showed that understory vegetation at depression forests on Mona Island is very diverse and highly dynamic. In terms of composition and abundance, the understory of Mona Island is characterized by a high abundance of grasses and herbs and a high diversity of trees and shrubs. Long-term understory responses to ungulate exclusion were highly variable over time and included significant increases in the abundance of woody species as well as significant increases in the richness of non-woody species. Our results provide valuable information about the dynamics of understory vegetation in Caribbean dry forests that should be useful in planning for the conservation and restoration of these habitats.