Bird alpha, beta and functional diversities across three peri-urban woodland stands along an anthropogenic disturbance gradient: is formal protection a guarantee for ecological integrity?

Abstract

Urban woodlands are significant as refugia for species of wild birds which contribute to provisioning a wide range of ecological services beneficial to nature and to humans. Due to the increasingly small and fragmental structure of these habitats, arising from constant expansion of city infrastructure, these birds are in increasing conservation peril with serious ramifications to the ecological and touristic values they represent. We assessed the alpha, beta and functional diversities of birds in three small woodland stands within the metropolis of Nairobi, Kenya and compared their variations along an anthropogenic disturbance gradient (Low, Moderate; High) based on relative openness, vegetation cover and rate of human visitation. Birds were surveyed using timed species counts during six sampling periods and grouped into three diet-guilds and three vertical foraging strata to assess compositional and assemblage turnover responses to disturbance. Functional overlaps in birds’ ecological roles were also evaluated, and potential candidate species for monitoring disturbance trends were identified. Alpha diversities were determined using Chao1 species estimator, and beta diversity evaluated through the PERMDISP function and main-test Permanova in PERMANOVA+ add-on of Primer-E, to assess species turnovers. Parameters were compared cross-site using general linear models while indicator species were identified using Indval function in R. Significant responses to impacts of anthropogenic disturbance were observed in abundance of birds overall, abundance of upper-stratum species, and in terms of alpha diversity of carnivores but not for other guilds. On the other hand, species turnovers occurred along the disturbance gradient for birds overall, in frugivores and also for both upper- and lower-stratum species. Species functional redundancies, depicting diminished habitat complexities, were detected in the most disturbed site for birds overall, amongst carnivores, all herbivores combined, and among the lower and middle-stratum species, but were not evident for birds overall in the low- or moderately-disturbed sites, nor among upper-stratum bird species. Only two species, both carnivorous, qualified as potential candidates for monitoring disturbance trends across the three woodland sites. These results demonstrate negative effects of anthropogenic disturbance on peri-urban woodland bird assemblage and composition, and greater resilience of taxonomic over functional diversity in response to such disturbance, but also that while formal protection may enhance bird’s community-level functional roles, not all guilds may benefit. Mitigating effects of disturbance requires measures for improving habitat structural heterogeneity to broaden birds’ assemblage composition, diversify their ecosystem roles at all levels and promote their site-based avi-tourism potential.