The impact of rare and common species on the functional diversity of forest birds in a palm-dominated landscape in the Eastern Amazon

The advance of industrial agriculture in the Amazon is a major threat to biodiversity. It is causing intense deforestation to make way for cattle ranching and monocultures such as oil palm. Forest loss can alter the distribution and population size of rare species, which are often limited in number or range, affecting the functional structure of communities and the ecosystem services they provide. Here, we assessed the contribution of rare and common species to the functional diversity of forest bird communities in a landscape dominated by oil palm plantations. We sampled birds in plantations and forest fragments in the eastern Amazon, Brazil. A total of 232 bird species were recorded, 198 in the fragments and 53 in the plantations. We calculated a rarity index for each species occurring in forest fragments, combining local abundance, geographic range, and habitat specificity. We calculated functional richness (FRic), functional originality (FOri), functional specialization (FSpe), and community-weighted trait means (CWM), and then compared these metrics between forest and plantation communities across three scenarios: 1) total species pool recorded in forest versus plantation, 2) removal of the rarest species, and 3) removal of common species. Removal of rare species increased functional diversity but decreased functional redundancy. Conversely, removal of common species decreased FOri and FSpe values, demonstrating that common forest bird species make unique functional contributions. We conclude that the loss of both rare and common species affects forest bird communities. Therefore, conservation and protection of forested areas is essential to ensure ecosystem health and resilience.