title: A fence line in time demonstrates grazing-induced vegetation shifts and dynamics in the semiarid Succulent Karoo.
abstract: The mediterranean ecosystems of the world harbor exceptional biodiversity, and factors such as livestock grazing which may negatively affect biodiversity are of global concern. The mediterranean ecosystems of southern Africa have only sporadically been exposed to high densities of indigenous herbivores and may not be as typically resilient to livestock grazing pressure as the savannah and grassland ecosystems of semiarid Africa. We investigate this issue by assessing the impacts of two contrasting livestock grazing intensities associated with communal and commercial ranching systems on the dwarf succulent shrublands of the Succulent Karoo in Namaqualand, South Africa. We interpret the results in terms of vegetation dynamics and the implications for the current debate on nonequilibrium rangelands. The results demonstrate that the relative extent of plant community divergence between the communal and commercial rangeland has been maintained over the past 10 years, overall and within different plant growth forms. Recruitment of certain palatable shrub species occurred on the heavily grazed communal rangeland, indicating that a further decline in their populations is not inevitable. Recruitment of the unpalatable shrub Galenia africana on the communal rangeland was disproportionately greater than that of the palatable species, reenforcing the dominance of this species. Due to the longevity of woody shrubs, short-term vegetation shifts are small, limiting the potential for the recovery of overgrazed shrublands during periods of low grazing pressure. The shift on the communal rangeland toward annuals and geophytes has increased the variability of forage production and contrasts with the dominance of dwarf shrubs on the commercial rangeland. Shrub-dominated ecosystems create the potential for forage storage and inter-annual transfer of excess production, buffering livestock numbers in the face of a fluctuating environment. Despite the semiarid nature of the area and contrary to the predictions of the nonequilibrium theory, livestock and vegetation in Namaqualand interact strongly, and a close relationship between livestock numbers and primary production persists over the long-term on the communal rangeland.
authors.affiliation: Plant Conservation Unit, Botany Department, University of Cape Town, Rondebosch 7701, South Africa. Simon.Todd@uct.ac.za
authors.forename: Simon W
authors.initials: SW
authors.lastname: Todd
authors.forename: M Timm
authors.initials: MT
authors.lastname: Hoffman
mesh_terms: D000383:Agriculture; D000818:Animals; D044822:Biodiversity; D002417:Cattle / Q000502:physiology*; D005247:Feeding Behavior / Q000502:physiology*; D018521:Plant Physiological Phenomena; D011157:Population Dynamics; D013019:South Africa; D013997:Time Factors