The effects of fragmentation and forest structural components on the diversity of forest bird species in Durban, South Africa.

Abstract

The pressure to meet the demands of the growing human population has resulted in the conversion of a large proportion of the Earth’s natural habitats into modified landscapes. With increasing urbanisation, generalist species may persist and thrive within the anthropogenically-modified landscapes, whereas specialist species are likely to decline in numbers and possibly become extinct. Consequently, this affects biodiversity and threatens the long-term functioning of the ecosystem as some species’ functional traits are lost. Therefore, understanding the ecological requirements of species with various functional traits to persist within human-modified landscapes is crucial for biodiversity conservation. The present research was conducted in the urban mosaic of Durban (eThekwini Municipality), KwaZulu-Natal, South Africa. The landscape of the study region comprised of extensive patches of Indigenous Forest and Thicket/Dense Bush. Previous research has highlighted the dramatic loss of natural forests within Durban as a result of anthropogenic activities, such as tourism-orientated development and human settlements. However, only a few studies have documented the threats that losing these coastal forests pose to wildlife, particularly birds. With the loss of forests and increasing urbanisation in Durban, the present study aimed to investigate the (i) forest fragmentation effects via patch size and isolation distance of habitat patches on the diversity of forest birds (ii) differences in vegetation structures of Indigenous Forest (hereafter IF) and Thicket/Dense Bush (hereafter TDB; a secondary forest habitat representing regenerating IF) with the aim of showing their importance in the provisioning of habitat and a diversity of niches and resources for avian species in Protected Areas (PAs) within an urban mosaic. During the southern Africa breeding season between October 2016 and March 2017, we conducted bird point-count surveys in IF and TDB patches within five PAs within eThekwini Municipality. We further recorded microhabitat vegetation structure at each survey point. We recorded a total of 75 bird species. Furthermore, we recorded the species richness per patch as overall species richness, and then calculated the functional diversity of the community recorded as a difference matrix of each species’ functional traits. Principally, we found that taxonomic richness was not affected by increasing isolation distance between forest patches, and that habitat patch size positively influenced taxonomic richness and functional diversity. Furthermore, the number of avian forest specialist species increased with patch size, probably because of the diverse environmental niches and resources present in larger patches. Secondly, the overall vegetation structures and species richness of IF and TDB did not differ significantly. However, the presence of avian specialist species in TDB survey sites was of interest because TDB vegetation structure was not predicted to be ideal habitat for forest specialist species, as TBD was expected to represent secondary forested habitat. The lack of significant differences in vegetation structures between IF and TBD, and the provisioning of forest specialist species in TBD led us to conclude that the TDB in our study region is at an advanced stage of regeneration into IF. Overall, the present study highlighted the diverse avian species that may exist within the urban mosaic forests, provided that availability of specialised niches persist. Therefore, the findings of this study highlight the conservation importance of natural landscapes in human modified landscapes. Furthermore, they emphasize the necessity of legally protecting both forest and thicket dense bush.