Endemic species maintain avian diversity and ecosystem services on the island of New Britain

Abstract Endemic species exemplify both the exceptional evolutionary value and extraordinary vulnerability of tropical island biodiversity. Conservationists often face the difficult task of balancing ecosystem stability with species-level protection. To reconcile these objectives of biodiversity management, it is essential to understand how endemic species contribute to ecosystem functioning. In this study, we assessed the taxonomic and functional diversity of birds along an elevational gradient on New Britain, the largest island of the Bismarck archipelago, and the role of endemic species in its rainforest ecosystems. We combined field surveys with abundance-weighed trait data to assess avian functional distinctiveness from the large neighbouring landmass of New Guinea. We found a non-linear decrease of bird diversity with elevation, measured by Shannon index. The proportion of endemic species increased with elevation, from 42 to 55% of total species richness. Compared to mainland New Guinea, New Britain assemblages shifted from invertebrate prey to other food sources, such as nectar or fruits. This shift was more prevalent in higher elevations than in the lowlands, and was facilitated equally by endemic and non-endemic species. Most omnivores and nectarivores were endemic, but frugivores and insectivores were more represented among non-endemics. Endemic birds dominated in the provision of pollination, but often adopted more generalist foraging strategies due to seasonal availability of fruits and nectar. In contrast, seed dispersal and insect control were more represented by non-endemics. Functional distinctiveness of New Britain birds may facilitate a different response to human-induced land use change compared to the New Guinean mainland. As only some ecosystem services are chiefly dependent on endemic species, conservation strategies should focus on preserving avian diversity regardless of endemic status.