Community Structure in an Isolated Tropical Forest Biome: One Year of Fruit-Feeding Butterfly Trapping in Four Habitats in the Western Ghats, India

Insect communities in tropical forests tend to be structured vertically and with respect to tree fall gaps and edges. Furthermore, insect communities vary over time. Insight into such habitat specificity and temporal variation is needed to design and interpret biodiversity surveys and to compare conservation value among habitats. Some aspects of tropical insect community structure, such as the proportion of canopy specialists and temporal variation, vary among biogeographical regions and climatic zones. To date, few regions have been sampled systematically, so generalization remains difficult. We compared fruit-feeding butterfly communities among understory, canopy, natural treefalls, and forest edge, in a tropical forest of the Western Ghats, a strip of rainforest that is isolated from Sundaland, the large rainforest block of South-East Asia. During a yearlong study, we captured 3018 individuals belonging to 32 species and representing 14 genera. While some butterflies were captured in the canopy, no species was significantly more abundant in the canopy than in the understory. This observation was contrary to studies elsewhere in the tropics where 14–55% of the species could be classified as canopy specialists. Even though the largest number of species was captured at forest edges, species diversity was highest in the gaps. The communities at the forest edge differed importantly from those in treefall gaps: at the forest edge, we caught grassland species in addition to the forest species. Larger treefall gaps had higher butterfly abundance than smaller gaps. Both abundance and diversity peaked during the late monsoon season, and all common species in our sample also peaked during this period. The spatiotemporal community structure appears to depend on biogeography (less vertical stratification further from large forest blocks) and climate (more synchrony among species in seasonal abundance when there is a more severe dry season).