Anthracotheriidae

Abstract

Historical increases in the density of unpalatable shrubs and trees have reduced the carrying capacity and threaten the sustainability of livestock production in arid and semi-arid grasslands and savannas around the world. Biodiversity, wildlife habitat and nutrient cycling (rates, magnitude, seasonality and spatial patterns) are also affected by these changes in vegetation. While historical increases in woody plane abundance have been widely noted, the rates, dynamics, spatial patterns and proximate causes have not been well documented. Here, I briefly review potential explanations for increased abundance of woody plants in dryland ecosystems and discuss how activities of domestic and native herbivores might influence the balance between grasses and woody plants. Conceptual models of woody plant seedling establishment in grazed ecosystems are presented and evaluated with case studies of an invasive North American arborescent, honey mesquite (Prosopis glandulosa). Reduction in grass interference, achieved experimentally or by grazing, may have a positive effect on woody plants but is not a prerequisite for successful seedling establishment in many systems. It is argued that thresholds of herbaceous utilisation required for woody plants to establish successfully may be readily exceeded, even at light levels of grazing. Thus, grazing management schemes muse aggressively incorporate the use of fire as well as emphasise the degree of grass utilisation and maintenance of species composition, if dominance of woody plants is to be regulated successfully.