Bird diversity and density in south african forests

Summary

Afromontane woodlands are fairly continuous in eastern Cape Province of southern Africa, but occur as increasingly smaller and more isolated patches towards Cape Town in the southwest. These habitat patches are similar in both vegetation structure and floristic composition. Although bird species numbers in the patches decline three-fold from east to west to 15 species at Table Mountain, total bird density remains roughly constant, and bird density per species increases threefold as species diversity declines by the same factor.

A few species, especially frugivores, seedeators and nectarivores, may be restricted to the larger more easterly woodlands because their food resources are so restricted. Some species, perhaps the sallying flycatchers, may be more abundant in the western species-poor habitat patches because of the higher productivity in their food supplies, but evidence for this view is scant. Some guilds, most notably foliage insectivores and slow-searching omnivores, exhibit good density compensation, such that the elevated densities of species in impoversished woodlands are predictable from densities and overlaps in foraging ecology at the species-rich sites. The guild of foliage insectivores “warbler-types,” declines in size serially from east to west at five sites: 5 spp-4-3-2-1; in this guild food resources are best matched amongst sites, and the predictions allowed, of the structure and composition of the increasingly smaller species subsets, are most detailed.