Impact of Wildlife Grazing During Drought on Herbaceous Vegetation in a Semiarid Rangeland and Postdrought Recovery

Abstract

Rainfall is regarded as the primary determinant of vegetation dynamics in non-equilibrium semi-arid rangelands and mitigates the potential impacts of heavy grazing. In reality, the impact of grazing and rainfall on vegetation dynamics in these systems are far more complex. It is well documented that the combined effect of heavy grazing and drought conditions in semi-arid rangelands can be particularly impactful on herbaceous vegetation. In Witsand Nature Reserve, located in the southern Kalahari, vegetation was subjected to heavy grazing by wild herbivores (game species) for prolonged periods. The region also experienced the most severe drought recorded in the region in the last 60 years. A study was conducted to determine the extent that heavy grazing impacted the response of vegetation to rainfall variability. Furthermore, the applicability of non-equilibrium based models in this semi-arid rangeland was tested. The study also investigated the effects of heavy grazing by wildlife and compared it to the reported detrimental effects of overgrazing caused by livestock. The results found that rainfall was the main determinant of cover abundance and biomass changes, but did not affect species composition. Heavy grazing resulted in areas of low species diversity and the dominance of undesired annuals. Heavily grazed areas were also far more susceptible to drought conditions compared to lightly grazed areas. In lightly grazed areas, vegetation responses post-drought conformed to the description of non-equilibrium systems while heavily grazed sites did not. This was indicative of vegetation responses being closely coupled to the inherent resilience of the vegetation. Heavy grazing by wildlife resulted in the degraded state of the rangelands, as has also been found with livestock farming. The key difference of overgrazing caused by livestock and wildlife was concluded to be in the spatial extend of the impact. This is due to the management and behavioural differences between livestock and wildlife.