The long-term dynamics of avifaunal communities at Nylsvley Nature Reserve, South Africa

Abstract

Long-term datasets are critical for understanding how ecological communities respond to non-stationary environmental conditions, such as those experienced in South African savannas. Using a 20-year citizen science survey of bird communities across three distinct savanna woodland types (Acacia, Burkea, Combretum) in the Nylsvley Nature Reserve, we examined temporal trends in taxonomic, functional and phylogenetic diversity. Annual species richness remained stable and did not differ between woodland types (mean 91–94 species yr-1). However, community composition was highly dynamic, with turnover driving beta diversity (∼90% of dissimilarity) rather than nestedness. Functional diversity exhibited significant clustering (85% of samples Z < –1.96), indicating strong environmental filtering, while phylogenetic structure was consistently random, suggesting that filtered traits are evolutionarily labile. The hyper-abundant Red-billed Quelea Quelea quelea temporarily homogenised functional and phylogenetic diversity in Acacia woodlands during outbreak years. Despite overall stability, three species declined consistently across all woodlands (Fork-tailed Drongo Dicrurus adsimilis, Rufous-naped Lark Corypha africana, Crested Barbet Trachyphonus vaillantii) while twelve increased (e.g. Ring-necked Dove Streptopelia capicola, Rattling Cisticola Cisticola chiniana), revealing a ‘winners and losers’ dynamic. Taxonomic composition differed among all three woodland types, but functional composition differed only between Acacia and Burkea, indicating that Acacia and Combretum support similar ecological functions despite different species. Total beta diversity remained stable over time, with no directional trend towards homogenisation or divergence. Our results demonstrate that savanna bird communities can remain functionally and taxonomically stable under non-stationary conditions, yet exhibit dynamic composition driven by turnover, species-specific trends, and occasional nomadic irruptions. Protecting heterogeneous woodland mosaics, not single habitat types, is essential for conserving avian diversity.