Indian Ocean coastal thicket is of high conservation value for preserving taxonomic and functional diversity of forest-dependent bird communities in a landscape of restricted forest availability

•Indigenous coastal forest is diminished, fragmented and endangered in South Africa.•Functional diversity of forest birds was relatively very high in secondary forest.•Its protection would support avian diversity, dispersal and forest functioning.

The Indian Ocean Coastal Belt Forest is extremely biodiverse but is threatened by anthropogenic land-use change. In South Africa, remnant forest patches are divided into highly restricted but protected indigenous forest (IF) and abundant but unprotected coastal thicket/dense bush (DB), which likely represents secondary/regenerating IF. We tested the hypothesis that DB had value for conserving forest-dependent avian taxonomic and functional diversity, as (a) DB is situated with in the forest-anthropogenic land-use mosaic and (b) birds maintain diverse ecosystem functions and are sensitive to environmental change given their varied biological traits. We conducted 149 and 112 fixed-radius point-count surveys of avian communities in 99, and 24, patches of DB and IF, respectively, at each location recording microhabitat vegetation structure. We compared vegetation characteristics and taxonomic species richness between vegetation classes, and tested the breadth of functional trait-space and for significant associations with vegetation structures at each survey location, using RLQ analysis. Vegetation classes differed significantly, but floral species overlap and vegetation structures in DB closely resembled regenerating IF habitat. Specialist ground-nesting and insect-gleaning species corresponded to taller, more diverse IF structures. Other biological traits and total species richness overlapped broadly across both classes, but IF had significantly higher mean species richness. Given the relatively high taxonomic and functional diversity of the avian community in the abundant DB, the scarcity of IF, and the proximity of DB to the anthropogenic land-use mosaic, we recommend allocating suitable DB patches to the Protected Area Network, which may simultaneously help preserve avian taxonomic and functional diversity, facilitate colonisation throughout the fragmented landscape and provide ecosystem services to anthropogenic land uses.