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Effects of hurricane disturbance and feral goat herbivory on the structure of a Caribbean dry forest

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Abstract

Question

Hurricanes are a major factor influencing forest structure and have been linked to higher incidences of multiple-stemmed trees in Caribbean dry forests relative to the continent. In Sept 1998, category 3 Hurricane Georges passed over Mona Island. This island, unlike others in the Caribbean, has had feral goats for five centuries. In this study we addressed the following questions: (i) what are the short-term (4 mo) and long-term (10 yr) responses of Mona Island's dry forests to hurricane disturbance in terms of forest structure and tree mortality; and (ii) is there any effect of goat exclusion on the recovery process and forest structure after the hurricane?

Location

Dry forest of Mona Island, Puerto Rico.

Methods

Permanent fenced and unfenced plots established in 1997 and monitored annually until 2008 were used to evaluate the interplay between goat herbivory and hurricane effects on structural dynamics of the tree community. Within these plots, vegetation data collected include species identity, DBH, successional status and hurricane-related tree mortality.

Results

We found that vegetation responses to hurricane disturbances may have been influenced by the presence of feral goats through at least two mechanisms. First, goats may have led to changes in forest structure and composition that feed back into the recovery dynamics that follow hurricane events. Second, goat herbivory limits the production of multiple stems, a trait that could be an adaptive response in hurricane-prone areas. Feral goats may lead to alternate successional pathways by keeping this dry forest at early-successional stages much longer than expected, and shifting vegetation communities to an alternate state where smaller, shrubbier and perhaps less palatable species dominate.

Conclusions

Differences in structure between Mona Island's dry forest and other tropical dry forests are related to changes in species composition that may have been triggered by introduced goats. Our results highlight the potential complexities that may arise when setting management goals within the context of novel ecosystems. Ultimately, implementation of management goals requires knowing what changes are linked to the ecological integrity of the ecosystem and what cultural values are placed on the current state of the tree community vs its historical condition.

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