Effects of habitat-patch size and patch isolation on the diversity of forest birds in the urban-forest mosaic of Durban, South Africa

Loss of habitat area and structural heterogeneity through anthropogenic fragmentation poses a threat to the survival of wildlife, which may be exacerbated by urban pressures. Understanding the underlying ecological processes that influence species' persistence in fragmented landscapes is vital for conservation. We described the effects of forest fragmentation via measures of patch size and isolation on the taxonomic richness and functional richness of forest bird communities in five Protected Areas within the urban-forest mosaic of the Durban Metropolitan Area, South Africa. We conducted 137 fixed-radius point-count surveys across 41 distinct forest patches during the austral breeding season. We quantified the avian taxonomic, guild and functional richness and measured at each survey patch. We measured the influence of patch size, Euclidean isolation distance, patch shape and habitat configuration (i.e. habitat amount) on each diversity measure. We then conducted a series of General Linear Models to determine how fragmentation and patch configuration influenced the diversity of forest birds. The explanatory variables in the top models had a significant effect on all avian diversity measures but habitat amount did not. The amount of habitat surrounding a patch was not significant for measures of bird diversity but based on the Akaike's weight it was important for specialist species. Isolation distance did not have a significant effect on the measures of bird diversity. An increase in patch shape index increased species richness. These results show the importance of large forest fragments/patches for the conservation of forest birds and for maintaining ecosystem functioning and services of forests in increasingly urbanising landscapes, to the benefit of the environment and its human population.