| Summary: | Forest fragmentation is caused by the clearing of patches of indigenous vegetation for
agriculture, urban development, and other human land uses. Such action results in
patches of remnant natural vegetation being surrounded by altered vegetation. I
investigate the effects of forest fragmentation and matrix type on avian diversity and
assemblage structure in forest patches of the historically fragmented Karkloof / Balgowan
forest range, KwaZulu-Natal, South Africa. This study compares the bird assemblage
diversity and composition of indigenous forest patches surrounded by commercial
forestry (Gilboa complex) with that surrounded by natural grassland matrix (Balgowan complex). Insularisation of Afromontane Mistbelt forest in KwaZulu-Natal has led to loss of
species where forest fragments support fewer bird species than comparably sized patches
of mainland forest. Small fragments within natural grassland have fewer bird species per
unit area than larger fragments. Forest patch area-dependent density compensation is
evident and bird assemblages appear saturated. Bird assemblages are characterised by a
non-random species distribution pattern where area-dependent processes are dominant,
and the loss of species from fragments follows a deterministic sequence. In forests in the
plantation-dominated matrix no island-effect is detectable and it appears that forest
patches are converging on the same bird species richness, regardless of forest size. No
density compensation is evident and bird assemblages are not saturated. The sequence of
species loss from forest patches is not as predictable, where a random yet prominent
colonisation process exists. As commercial plantations provide suitable habitat cover for
movement of forest birds, colonisation of both distant and small indigenous forest
patches has been possible, reducing the effects of area-dependent extinction in the forest
patches but also resulting in lower species richness in larger patches.
Bird species of the Karkloof / Balgowan forest range appear to be fragmentation
adapted, and most species are resilient to further landscape change. Certain species are
however more prone to local extinction than others. The major predictors of extinction
risk are body size, abundance status, and feeding guild. Patch area is the dominant force
governing traits in the natural Balgowan complex where larger species with low natural
abundance and an insectivorous diet are most prone to local extinction. In the Gilboa
complex the nature of the plantation matrix appears to be masking the species natural
responses to fragmentation making it difficult to predict which species are most at risk.
In order to preserve maximum bird diversity, including high-risk species, the largest
intact forest units (≥302ha) must be conserved. Evidently, the nature of the matrix affects
avifaunal diversity and distribution in forest patches, and plantations have the capacity to
significantly alter bird assemblage structure and composition in indigenous forest
patches. Forest fragments must be considered as integrated parts of a complex landscape
mosaic, and this study emphasises the importance of understanding landscape-scale
processes. Knowledge of ecological and life history traits proves valuable for predicting community level response to landscape change. === Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2001.
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