Encroaching woody plant species and boreholes: is cattle density the main driving factor in the Olifants Drift communal grazing lands, south-eastern Botswana?

Abstract

Fifteen environmental variables, among them cattle density (hereafter ‘pressure index’), were measured along grazing gradients of eight boreholes at Olifants Drift in south-eastern Botswana. The aim was to investigate whether cattle density, among other variables, had any direct relationship with the occurrence of bush-encroaching woody plant species. This is important because high cattle densities in the communal rangelands of Botswana have been held responsible for the increasing dominance of woody encroachers. On the basis of thet-values and the variance inflation factors (VIF) obtained from CANOCO (canonical community ordination) programme, four environmental variables were retained as significantly influencing woody plant species composition along the grazing gradients: cattle density, soil nitrogen, distance from borehole and tree cover. The retained variables were forward-selected in a regression and cattle density explained most (33%) of the variance out of the total 60% explained by the four variables. Woody plant species associated with high cattle density were also found on high nitrogen sites, and these were thorny plant species that have been previously classified as encroachers. Therefore, high cattle density around boreholes is associated with bush-encroaching woody plant species. High cattle density is also likely to be found around natural water points (pans and wells), rivers and kraals, hence encroaching woody plant species are likely to dominate in such places. Shifting of the high cattle density foci (boreholes, wells, kraals, etc.) is likely to result in most of the grazing pasture being composed of the encroaching woody plant species at the expense of the non-encroaching woody plant species.