Simulated long-term vegetation response to grazing heterogeneity in semi-arid rangelands.
The long-term effects of small-scale spatial heterogeneity of livestock (cattle) grazing on vegetation dynamics were studied with a grid-based model of major life forms of savanna vegetation. Based on southern Kalahari (Botswana and South Africa) ecology, the model includes stochastic life-history variables, resource competition for soil water, and biomass production for annuals, perennial grasses and shrubs. Grazed at individual intensities, the model's 25 m2 grid cells defined the spatial scale of heterogeneity. Different scenarios of grazing heterogeneity were generated by modifying distributional and behavioural features of the grazing model. Simulations were run over 50 years under moderate to high constant stocking rates. Results confirmed a previously reported threshold response of shrub cover increase: under moderate grazing pressure, little change in shrub cover occurred; when grazing pressure exceeded a threshold, shrub cover increased drastically. Under moderate or high stocking rates, grazing heterogeneity did not modify grazing effects. However, within an intermediate range of stocking rates, small-scale heterogeneity determined the long-term impact of grazing. In particular, utilization intensity at the threshold of shrub cover increase was 60% less under high compared with low local grazing heterogeneity. Sensitivity of vegetation dynamics to local grazing heterogeneity was also exemplified under a landscape-scale grazing gradient as observed at watering points: at a given utilization intensity, a wide zone of increased shrub cover occurred under large local grazing heterogeneity, while under the least heterogeneous grazing only a narrow zone of slightly increased shrub cover occurred. Because of the slow progress of shrub cover increase, a mismatch of management and ecological time scales was diagnosed and its implications for management are discussed. It is concluded that knowledge of local grazing heterogeneity is crucial for correct assessment of livestock impact on vegetation dynamics. Consequently, management aiming at sustainable land use should account for spatial grazing aspects.