Avian functional diversity retained in a tropical rainforest fragment after more than 120 years of turnover

Deforestation is linked to the increasing prevalence of small forest fragments worldwide and an associated loss in functional diversity. However, our long-term understanding on how biodiversity and functional roles respond in such isolated fragments is limited, especially in Southeast Asia. We compared the bird community in a small primary rainforest fragment in the Singapore Botanic Gardens, a UNESCO World Heritage Site, using historical records in 1898 and contemporary surveys conducted in 1998 and 2018–2021. Species composition was expectedly dissimilar between the 1898 and contemporary bird communities (50%), driven mostly by turnover (45.5%) rather than nestedness (4.5%). Despite the changes in community composition and species extirpations, both species richness and functional diversity were retained. Our results suggest that small forest fragments can form novel bird communities that are functionally similar to previous communities that inhabited the fragment, leading to the retention of functional diversity. Such functional redundancy may be encouraged through habitat restoration initiatives adjacent to fragmented forests.