Patterns of avian diversity across a decreasing patch-size gradient in a critically endangered subtropical forest system

Aim: We investigated habitat heterogeneity and patterns of avian taxonomic and functional diversity change across a decreasing patch-size gradient in a critically endangered, fragmented forest-system to elucidate: (1) habitat patch-size and structural drivers of avian diversity change, (2) potential patch-size thresholds at which avian diversity and habitat heterogeneity changes occurred and (3) avian species/communities within thresholds that indicated distinct patch-size categories. Location: Indian Ocean Coastal Belt, South Africa. Methods: We conduced habitat-structure and fixed-radius point-count surveys during the avian breeding season across 123 habitat patches (range 0.06–386.9 ha). We calculated taxonomic and functional diversity/patch (α) using a distance matrix of all traits present in the avian community (γ) and described community change across the patch-size gradient (β). We compared linear regressions of patch-size effects on avian functional α-diversity and habitat-structural heterogeneity to segmented regression at five bands, determined by mean patch sizes responsible for provisioning avian functional α-diversity at <50%, 50%–62.5%, 62.5%–75%, 75%–87.5% and 87.5%–100% of avian functional γ-diversity to explore patch-size thresholds, and tested for significant avian indicator species of categories identified. Results: Avian taxonomic and functional α-diversity significantly decreased with decreasing patch size and habitat heterogeneity; correlations between taxonomic and functional diversity declines and a positive significant influence of habitat heterogeneity on avian functional diversity indicated selective extinction pressure on niches and concomitant functional traits. Species turnover significantly increased in smaller patches. Segmented regressions of patch size-effect on avian functional diversity and habitat heterogeneity out-performed linear regressions at three patch-size thresholds (157, 96 and 9 ha). Forest-dependent specialists were significant indicators of high-diversity patches >90 ha. Species characterizing small patches <9 ha were generalists, but many were also present in larger patches, highlighting nested-community subsets of γ-diversity. Main conclusions: Specialized niches and concomitant species became locally extinct at patch-size thresholds. We present replicable methodology and recommendations for conservation of forest systems using a critically endangered biome containing disproportionately high avifauna.