Large trees, fertile islands, and birds in arid savanna

LargeAcacia eriolobatrees 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 2.055 times greater under canopies ofA. eriolobatrees than in surrounding grassy shrubland. Plant species with fleshy fruits (Boscia, Grewia, LyciumandSolanumspp.) occurred in 8% of treeless plots and beneath 17% ofA. eriolobasaplings, but their frequency increased to 90% beneath large trees. DeadA. eriolobatrees 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 asAcacia erioloba. The shade beneath the canopies of large spreading trees was used by birds (particularly Kori BustardsArdeotis kori) and mammals (mainly SpringbokAntidorcas marsupialis, GemsbokOryx gazella, Blue WildebeestConnochaetes taurinusand Bat-eared FoxOtocyon megalotis) as a resting place during the heat of the day. Large raptors (> -031.055 kg) and vultures (> -035 kg) seldom perched on saplings, and more frugivorous bird species and individuals were seen on mature than sapling or dead trees. Nests of raptors, and the large communal nests of Sociable WeaversPhiletarius sociuswere found mainly in large trees, and the nests of the Tree RatThallomys paedulcuswere found in cavities in the stems of large trees.