Plant diversity consequences of a herbivore-driven biome switch from Grassland to Nama-Karoo shrub steppe in South Africa
How does heavy grazing change plant community structure, composition and species richness and diversity in an ecotone between grassland and semi-arid shrub steppe-type vegetation? Does grazing favour plants with arid affinity over those with less arid affinity? Does the grazing-induced transformation constitute a switch to the equivalent of a shrub-dominated biome?
Central South Africa.
Using systematic scanning of SPOT 5 imagery and ground-truthing, a grazing treatment area was selected that met criteria of intensity of grazing, sampling requirements, and biogeographical position within a broad ecotonal zone. Differential vegetation responses to heavy grazing were tested for significant differences in plant traits, vegetation structure, and species diversity, richness and evenness. Gamma diversity was calculated for the whole study site, whereas, independent beta diversity was calculated across the treatments assuming the additive partitioning of diversity. In addition, the biogeographical association of grazing-induced species shifts was determined using a range of available databases.
Canopy cover and height of woody shrubs increased significantly with heavy grazing whereas that of graminoid plants declined. The resultant species turnover was modest, apparent extinctions of local species were minimal, species richness was maintained and species diversity was significantly enhanced. There was a significant increase in species evenness, through possible suppression of dominant species. Significant increases in species cover were those associated with mainly the Nama-Karoo biome indicating that species from more arid areas are more resistant to grazing as would be expected by the convergence model of aridity and grazing resistance.
The significant increase in shrub cover in heavily grazed semi-arid grassland followed general global expectations. The study confirmed that the supposed former large shift of grassland to shrubby Nama-Karoo in the eastern upper Karoo can indeed be readily affected by heavy grazing. The negative connotations for biodiversity that have often been associated with intense grazing seem, in terms of the positive responses of plant species diversity in this study, to perhaps be exaggerated. The elevated species diversity with grazing of vegetation with a long evolutionary grazing history in a low resource area may require a reappraisal of the application of certain grazing hypotheses.