Deciduous forests hold conservation value for birds within South Andaman Island, India

Abstract

Greater diversity of habitats on islands is often correlated with higher species richness (including endemic and threatened taxa), implying the need to understand species-habitat associations. Such habitat associations would also point towards the role of abiotic filtering and competition in structuring species communities, necessitating the examination of the role of species traits and phylogenetic relationships in intra-island community organisation, an aspect poorly examined in literature. We investigated the composition and structuring of forest bird communities in closely co-occurring evergreen and deciduous forests within South Andaman Island (Indian Ocean), wherein the importance of deciduous forests for birds is undervalued. We sampled 27 transects over two years and compared bird species composition and diversity across the two habitats. We examined species-specific associations with habitat (forest) type, basal area, and distance from human settlements, and tested if these responses were explained by species functional traits and tested for phylogenetic niche conservatism after factoring the effects of environmental predictors. Bird species compositions were markedly distinct across the two habitat types, with deciduous forests having greater taxonomic, but not functional or phylogenetic, diversity of forest birds. The distribution of forest birds, including several endemic and threatened species within the island was largely explained by habitat type (with 39% of the bird species analysed showing higher occurrence probabilities in deciduous forests), followed by distance from human settlements and basal area. We did not find evidence of species traits or phylogenetic relationships mediating these habitat preferences, perhaps due to a relatively impoverished species pool, as is typical on islands. Nevertheless, our results underscore the value of deciduous forests in harbouring high islandic species diversity, and being the preferred habitat of several endemic and threatened bird species. Given the historic focus on evergreen forests and the increasing anthropogenic pressure on the forests of the Andamans, we highlight the critical need to include the rapidly diminishing deciduous forests in existing conservation plans.