Determinants of avian density across a fragmented landscape

Background/Question/Methods

Habitat fragmentation is an increasingly global threat to the persistence of many bird species. Understanding why certain species are more susceptible to effects of fragmentation can help direct future conservation efforts. This study uses a model system of forest fragments in Hawaiʻi, isolated by volcanic activity (130-160 years ago) to examine the relationship between bird densities and fragment size, connectivity, food resources, and forest structure. Point transect sampling was used to determine the mean annual density of birds in 32 forest fragments (ranging in size from 0.1-10 ha) along with an adjacent intact forest site. Bimonthly fruit and flower counts combined with density of trees in each fragment were used to create indices of food resources. The maximum size (DBH) of the dominant tree species occurring across all fragments was used as an indicator of forest structure. Measures of connectivity included forest cover around each fragment and distance to nearest neighboring patch. 

Results/Conclusions

Fragment size was the best predictor of two bird species: ʻIʻiwi (Vestiaria coccinea) density increased with fragment size and Hawaiʻi ʻElepaio (Chasiempis sandwichensis), absent from smaller fragments. Fruit resource density was the most important factor in predicting the density of the resident frugivore (ʻŌmaʻo- Myadestes obscurus), with no preference observed for fruit diversity. Overall this work demonstrates the importance of incorporating food resource measures into studies of fragmentation and the differential impacts of fragmentation on territorial species. Model ecological systems, such as the one used in this study, provide a unique opportunity to examine the natural effects of fragmentation on species providing a baseline model to help future studies differentiate between the impacts of human disturbance and fragmentation.