Herbivore-Driven Land Degradation: Consequences for Plant Diversity and Soil in Arid Subtropical Thicket in South-Eastern Africa

Abstract

Investigations were made of plant and soil responses to severe degradation through heavy grazing and browsing in arid, succulent, subtropical thicket. Severe degradation of thicket is of major concern in terms of threatened biodiversity, unsustainable utilization and collapse of other ecosystem services. We used a natural, field contrast, case‐study approach, sampling within plots under lightly and heavily stocked conditions. Mean plant species diversity and richness did not change significantly at sample plot level although there was a 27 per cent decline in richness with degradation at the scale of the study site. On degraded plots, there was a high species turnover and high beta diversity, which created a replacement zone rather than an impoverished zone. Replacement species were confirmed as mainly, but not exclusively, weedy annual grasses and alien forbs. The few persisting perennials were small trees that survived above the browse line, and hardy shrub species. Perennial persisting and replacement species below the browse line may suggest potential candidates for restoration. Nitrogen was the only measured nutrient that showed a significant decrease with degradation. Phosphorous, potassium and magnesium increased significantly, with the first two mentioned reaching potentially excessive levels. Soil salinization occurred with an order of magnitude increase in sodium. Thus, the increased cation exchange capacity occurred together with development of a nutrient imbalance. The elevation in some nutrients and soluble salts is ascribed to wind‐determined directional grazing that concentrate livestock in the degraded area. The implications of the above altered soil conditions for thicket restoration need to be further explored. Copyright © 2012 John Wiley & Sons, Ltd.

Management of Grazing Intensity in the Semi‐Arid Rangelands of Southern Australia: Effects on Soil and Biodiversity

“…Some reports of short duration grazing followed by long periods of pasture rest have shown increases in SOC, reduced soil compaction and the risk of erosion even within relatively short periods (<5 years; Sanjari et al, 2008;Teague et al, 2011;McSherry & Ritchie, 2013). While there is general agreement in the literature which shows a negative impact of increasing agricultural intensity on biodiversity (Lunt et al, 2007;Díaz et al, 2007;Rutherford et al, 2012), the impact of rangeland grazing intensity on diversity is less clear.…”

Belowground Bud Bank Responses to Grazing Intensity in the Inner‐Mongolia Steppe, China

“…In grasslands throughout the world, grazing by domestic livestock is often associated with a certain degree of changes in species composition and soil degradation (Milton et al, 1994;Busso, 1997;Cerdà & Lavée, 1999;Angassa, 2012;Rutherford et al, 2012;Mekuria & Aynekulu, 2013;Romina et al, 2014). Selective herbivory of the more desirable species appears to be the dominant mechanism in the processes of species replacement (Anderson & Briske, 1995).…”

“…Land degradation, caused by overgrazing and other inappropriate land uses, has been recognized as one of the major threats to ecosystem services and functioning on perennial grasslands worldwide, especially in arid or semi-arid ecosystems (Bai et al, 2007;Ford et al, 2012;Liu et al, 2012;Rutherford et al, 2012). Subjected to a considerable population pressure, grasslands in China are much susceptible to land degradation.…”

Ecosystem‐scale impacts of non‐timber forest product harvesting: effects on soil nutrients

“…Reduced cation binding causes less cation uptake and use by the plant which causes increased cation availability in the soil due to cations not being used (Oertli 2008). Elevated cations were also reported by Rutherford, Powrie & Husted (2014) in comparing heavily browsed sites with low above-ground vegetative biomass to less browsed sites with higher biomass. They suggested that it might have been a result of nutrient enrichment from livestock urine and dung.…”