Testing predictions of island biogeography theory in tropical premontane forest fragments

Abstract

Habitat fragmentation is a major threat to biodiversity and a prevalent disturbance in tropical premontane forest landscapes, yet fragmentation impacts in premontane forests are poorly understood. To understand plant responses to fragmentation, I examined fragmentation impacts using a traditional conceptual framework, island biogeography theory, along with more recent concepts about fragmentation impacts, edge effects and matrix influences. In my first study, I tested island biogeography theory predictions for herbaceous plant and tree species richness in fragments of varying in size and isolation distance from 1 to 209 hectares, and 0.5 to 6.7km, respectively. Contrary to predictions of the theory, I found that there was no relationship between species richness and fragment size or species richness and isolation distance. Examination of the plant community composition revealed an increase in pioneer tree species in small size classes indicating a possible shift of the forest fragment community to a more successional and less mature forest after fragmentation. In my second study, I address the impact of the surrounding matrix on the microclimate and plant composition from the edge to the forest interior. I observed a narrow edge effect in microclimate and species composition extending 10m into the forest fragment. Within 10m from the edge and into the fragment I observed more weedy herbaceous species and a greater proportion of pioneer trees, indicating a shift in plant composition near edges as a result of fragmentation. My final study examines fragmentation using a landscape approach to understand the relationship between matrix (landscape) characteristics and plant composition in fragments. I used spatial analysis software to characterize the matrix surrounding the focal fragments of this study. The influence of matrix characteristics and configuration on species composition data was determined using principle components analysis. The landscape was described a patchy landscape made of irregular patches of primary and regenerating forest, pasture and croplands, based on landscape metric analysis. Principle components analysis demonstrated that landscape metrics such as fragment shape and isolation are highly correlated with both tree species diversity and species component, particularly the proportion of pioneer tree species. These studies demonstrate that plat community responses to fragmentation can be better understood in the context of the landscape than by examination of the fragment in isolation.