Avian assemblages in native Acacia and alien Prosopis drainage line woodland in the Kalahari, South Africa

Abstract

Plant and bird species richness and abundance in two patches of native Acacia karroo and two patches of alien Prosopis species drainage line woodland in the southern Kalahari, South Africa, were compared using 37 plot (100 m2) samples for the plants and 12 line transects (ca. 1 km long) for the birds. Vertical distribution of canopy cover differed among the four sites in all four strata. The densest Prosopis woodland had less herbaceous understorey cover, and more cover between 1 and 5 m above-ground than Acacia woodlands or the relatively openProsopis woodland. The tallest Acacia woodland differed from other sites in having 40% canopy above 5 m. The densities of all trees >50 mm basal diameter (mainly A. karroo and Prosopis, but including a few A. hebeclada and Ziziphus mucronata) did not differ among sites, but abundance of A. karroo andProsopis species differed greatly among sites. Mean plant species density per 100 m2 plot differed among sites, being greater in the Acacia woodlands than in Prosopis -dominated vegetation. The densest Prosopis woodland was the least diverse site botanically, and was also least similar to the tallest Acacia woodland, sharing only 11 plant species. The ratio of fleshy fruited plant species to all others encountered differed between woodland types being lower in Prosopis than in Acacia woodland. Individual A. karroo trees differed from size-matchedProsopis trees in being less multi-stemmed and having fewer branches in contact with the soil when mature. The numbers of species of sub-canopy plants and fleshy fruited plant species increased with the size of the trees, but tended to be lower for large Prosopis than for large A. karroo.Sorenson's similarity index for bird species assemblages forAcacia sites compared with both Prosopis sites was 0·61. All species that occurred in Prosopis, with the exception of Red-eyed Bulbul Pycnonotus nigricans, were a subset of species that occurred in Acacia woodland. Bird species assemblages were least similar between the densest Acacia site and the most open Prosopis site. The Shannon indices for the Acacia and Prosopis communities showed some variation between patches and between counts. Indices for the Acacia sites were significantly higher than indices for the Prosopis sites, indicating that the bird communities in the native woodland sites are consistently more species-rich and more diverse. In terms of guilds, raptors were absent from Prosopis woodlands, in general frugivores were sparse in Prosopis woodlands, and insectivores were about half as abundant, but there were no large differences between nectarivores, mixed feeders (seeds and insects) and granivores in both types of woodland.