Large trees, fertile islands, and birds in arid savanna

Large Acacia erioloba trees scattered through the sparse grassy vegetation of arid oligotrophic savanna are focal points for animal activity because they supply nest sites, shade and scarce food resources. Faeces, fallen nest material and carcass remains left below trees elevate levels of nutrients available to plants in the soil beneath large trees. Soil concentrations of N and K were two times greater, and P concentrations 2.5 times greater under canopies of A. erioloba trees than in surrounding grassy shrubland. Plant species with fleshy fruits (Boscia, Grewia, Lycium and Solanum spp.) occurred in 8% of treeless plots and beneath 17% of A. erioloba saplings, but their frequency increased to 90% beneath large trees. Dead A. erioloba trees were replaced by matrix vegetation (54%) and large shrubs with fleshy fruits (28%) rather than by conspecifics (17%). The distribution of fleshy-fruited plants in the Kalahari is thus dynamic and tied to the distribution of large trees such as Acacia erioloba. The shade beneath the canopies of large spreading trees was used by birds (particularly Kori Bustards Ardeotis kori) and mammals (mainly Springbok Antidorcas marsupialis, Gemsbok Oryx gazella, Blue Wildebeest Connochaetes taurinus and Bat-eared Fox Otocyon megalotis) as a resting place during the heat of the day. Large raptors (>1.5 kg) and vultures (>5 kg) seldom perched on saplings, and more frugivorous bird species and individuals were seen an mature than sapling or dead trees. Nests of raptors, and the large communal nests of Sociable Weavers Philetarius socius were found mainly in large trees, and the nests of the Tree Rat Thallomys paedulcus were found in cavities in the stems of large trees. Acacia erioloba and the only other large tree, Acacia haematoxylon, apparently structure plant and animal communities and determine pattern and patch dynamics in this arid savanna. We suggest that their role in maintaining biodiversity in the Kalahari cannot be served by the homogeneous thickets of stunted acacias that develop where the vegetation is overgrazed. (C) 1999 Academic Press.