Matrix effects on bird assemblages in fragmented Afromontane forests in South Africa

Abstract

We examine the effects of matrix type on forest bird species richness in historically fragmented forests of the KwaZulu-Natal midlands, South Africa. Bird species–area relationships for forest fragments within natural grassland were compared with those surrounded by plantation forestry (Pinus spp.). While fragments in grasslands displayed a species–area effect, no such effect (slope ∼0) was detected for fragments in plantations. The critical fragment size to avoid an island effect on species richness was 302 ha for fragments in grasslands, and contained 51 of the 61 forest bird species encountered. Small forest fragments (<50 ha) within grasslands were less species-rich than those surrounded by plantation, while the reverse was true for forests larger than 50 ha. Bird density was significantly lower in larger forest patches within plantation when compared to those in grassland. However, a slight decline in species density with increasing species richness in a patch (i.e. weak density compensation) characterised only those bird assemblages surrounded by a grassland matrix, suggesting these are approaching species saturation. Commercial plantations may increase the likelihood of colonisation of, and immigration from, small forest fragments by birds, reducing the incidence of area-dependent extinction in small fragments, but may also result in lower species richness in larger fragments. The latter is likely a consequence of the promotion by plantations of a wider distribution of generalist species in, and the loss of some specialised or rare species from, large patches. Management options are to avoid planting plantations near large forest patches and to increase the size of small patches where possible. Where afforestation is unavoidable, placing plantations in the vicinity of small forest patches rather than large forest patches may be preferable.