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.

[1]  P. Roos,et al.  Soil nutrient ecology associated with Acacia sieberana at different tree densities in a South African savanna , 1997 .

[2]  B. W. Wilgen,et al.  Fire and Plants , 1995, Population and Community Biology Series.

[3]  J. I. Muoghalu,et al.  Litter decomposition and inorganic element dynamics in a secondary rainforest at Ile‐Ife, Nigeria , 1994 .

[4]  C. Canham,et al.  Forest Gaps and Isolated Savanna Trees , 1994 .

[5]  J. Skinner,et al.  Some costs of maintaining a perennial territory in the springbok, Antidorcas marsupialis , 1993 .

[6]  S. Milton,et al.  Preferential utilization of pans by springbok (Antidorcas marsupialis) , 1992 .

[7]  D. Richardson,et al.  Colonization of Cape fynbos communities by forest species , 1992 .

[8]  C. Skarpe.  Spatial patterns and dynamics of woody vegetation in an arid savanna , 1991 .

[9]  S. Milton,et al.  Patch disturbances in arid grassy dunes : antelope, rodents and annual plants , 1991 .

[10]  Robert J. Scholes,et al.  The influence of soil fertility on the ecology of southern African dry savannas , 1990 .

[11]  S. Riha,et al.  The Effects of Trees on Their Physical, Chemical and Biological Environments in a Semi-Arid Savanna in Kenya , 1989 .

[12]  R. Cowling,et al.  Seed predation and germination of Acacia erioloba in the Kuiseb River Valley, Namib Desert , 1989 .

[13]  T. M. Smith,et al.  The effects of shading on the establishment and growth of Acacia tortilis seedlings , 1988 .

[14]  S. Milton,et al.  Notes and Records.: Dispersal of seeds by raptors , 1988 .

[15]  Robert C. Maggio,et al.  Autogenic succession in a subtropical savanna: conversion of grassland to thorn woodland , 1988 .

[16]  T. M. Smith,et al.  The effect of competition on the structure and dynamics of Acacia savannas in Southern Africa , 1986 .

[17]  R. Yeaton,et al.  Organization of Vegetation Mosaics in the Acacia Schaffneri-Opuntia Streptacantha Association, Southern Chihuahuan Desert, Mexico , 1986 .

[18]  D. Janzen.  Dispersal of Small Seeds by Big Herbivores: Foliage is the Fruit , 1984, The American Naturalist.

[19]  E.A.N. le Riche,et al.  The influence of veld fire on an Acacia erioloba community in the Kalahari Gemsbok National Park , 1984 .

[20]  T. H. V. Rooyen.  The soils of the Kalahari Gemsbok National Park , 1984 .

[21]  A. Milewski.  The Occurrence of Seeds and Fruits Taken by Ants Versus Birds in Mediterranean Australia and Southern Africa, in Relation to the Availability of Soil Potassium , 1982 .

[22]  M. Kellman.  Soil Enrichment by Neotropical Savanna Trees , 1979 .

[23]  F. Meulen,et al.  A phytosociological classification of the Nylsvley Nature Reserve , 1976 .

[24]  G. Maclean.  THE SOCIABLE WEAVER, PART 1: DESCRIPTION, DISTRIBUTION, DISPERSION AND POPULATIONS , 1973 .

[25]  B. Walker,et al.  Relationships between tree canopy cover and Panicum maximum in the vicinity of Fort Victoria , 1973 .

[26]  R. Huey,et al.  Bird species density in the Kalahari and the Australian deserts , 1971 .

[27]  J. D. P. Bothma.  Notes on the stomach contents of certain Carnivora (mammalia) from the Kalahari Gemsbok Park , 1966 .

[28]  O. A. Leistner.  On the dispersal of Acacia Giraffae by game , 1961 .

[29]  F. Eloff.  Observations on the migration and habits of the Antelopes of the Kalahari Gemsbok Park - Part II , 1959 .

[30]  C. K. Brain,et al.  A short account of the reptiles of the Kalahari Gemsbok National Park , 1958 .

[31]  S. Milton,et al.  Acacia Erioloba: Monograph and Annotated Bibliography , 1997 .

[32]  O. A. Leistner.  The subcanopy flora in the dynamics of the Kalahari Thornveld , 1996 .

[33]  R. Amundson,et al.  Comparative effects of isolated trees on their undercanopy environments in high- and low-rainfall savannas. , 1993 .

[34]  M. Westoby,et al.  Nutrient costs of vertebrate- and ant-dispersed fruits , 1993 .

[35]  T. H. Arnold,et al.  Plants of southern Africa: names and distribution. , 1993 .

[36]  Nancy Huntly,et al.  Herbivores and the dynamics of communities and ecosystems , 1991 .

[37]  E. Veenendaal,et al.  Plant responses to human activities in the tropical savanna ecosystem of Botswana , 1991 .

[38]  A. Moore,et al.  The effect of tebuthiuron on the vegetation of the thorn Bushveld of the Northern Cape – a preliminary report , 1985 .

[39]  R. Smithers,et al.  The mammals of the southern African subregion , 1983 .

[40]  W. Dean.  Tyto Alba Prey in South West Africa and the Northern Cape , 1975 .

[41]  G. Maclean.  An analysis of the avifauna of the southern kalahari Gemsbok National Park , 1970 .

[42]  K. Tinley.  An ecological reconnaissance of the Moremi wildlife reserve, northern Okovango swamps, Botswana. , 1966 .

[43]  R. Story.  A botanical survey of the Keiskammahoek District. , 1952 .

[44]  O. A. Leistner.  The plant ecology of the Southern Kalahari , 1900 .