Island biogeography theory as a predictor of species diversity in tropical premontane forest fragments

Background/Question/Methods

Forest fragmentation is prevalent in tropical landscapes and poses a threat to biodiversity conservation. Extensive deforestation and fragmentation has occurred in premontane forests of Costa Rica over the last 40 years leaving behind less than 30% of the original forest cover. Forest fragments now persist as island remnants in a “sea” of various non-forest land uses such as pasture and cropland. The Equilibrium Theory of Island Biogeography provides a conceptual framework within which the effects of fragmentation can be examined. Specifically, the theory predicts an increase in the number of species with increasing area of the island and decreasing distance from the mainland, in this case, the nearest continuous forest.

We tested these predictions in fragmented premontane tropical landscapes at an elevation of 1100-1500m in southern Costa Rica. Species richness was measured for both trees and herbaceous plants in 8 fragments of varying sizes from 1- 209 hectares in area. To further examine biodiversity patterns in the landscape, species diversity, community dominance, and similarity for both herbaceous and tree communities within fragments were determined.

Results/Conclusions

We did not observe a positive relationship between fragment area and species richness for either tree or herbaceous species as predicted by the Equilibrium Theory of Island Biogeography. Additionally, species diversity and patterns of dominance did not vary among fragments. Patterns in herbaceous and tree community similarity were explained by changes in elevation between fragments, but not by fragment proximity to each other or distance from mainland. Our results indicate that fragment area and isolation are poor predictors of species diversity of mid-elevation Costa Rica forest after 40 years of fragmentation. This may be a result of prolonged relaxation time, the time taken for species to regain equilibrium after disturbance. Our results have implications for the role of small fragments in conservation practices in general. Small fragments may serve as refuges of species diversity for a considerable time after disturbance and may harbor individuals which may provide propagules for reforestation. Long relaxation times may also serve as a buffer during which conservation measures can be implemented to mitigate biodiversity loss.