Evaluation of Themeda triandra as an indicator for monitoring the effects of grazing and fire in the Bontebok National Park

Abstract

Up until 2004, the burning regime applied in the Bontebok National Park was aimed at maintaining grazing conditions suitable for bontebok (Damaliscus pygargus pygargus). It was, however, not suitable for maintaining plant species diversity, an increasingly urgent conservation priority for the park. Accordingly, the burning regime was changed in 2004 to increase the interval between fires. A possible unintended outcome of the new burning regime is the spread of grazing lawns which is likely to be deleterious for maintaining the diversity of rare plants. Red grass, Themeda triandra, a species often locally abundant in areas preferred by bontebok, is potentially a good indicator of the anticipated change because, although it persists with moderate grazing, it is sensitive to localised intense grazing and will decrease on grazing lawns. To gauge the potential of this indicator, the canopy spread cover, degree of defoliation and inflorescence production of T. triandrawas determined at 13 permanently marked sites in November 2005. The results are compared with a survey conducted 20 years previously (October–November 1984 and October–November 1985) using similar methodology. The results suggest that T. triandra remained abundant over the previous 20 years’ application of the prior burning regime. In 1984–1985, defoliation of T. triandra was high within 1 year after a fire but declined quickly thereafter. In 2005, the tendency for the defoliation level to decline with increasing time after a fire was still apparent, but it was much less marked than in the previous survey period. A likely cause of this was the fact that Cape mountain zebra (Equus zebra zebra) and red hartebeest (Alcelaphus buselaphus caama) were present in 2005 but absent in 1984–1985 and these taller-grass grazers would have contributed to the use of the older veld. Provided it is interpreted together with other monitoring programmes, the use of T. triandra cover and defoliation intensity appears promising as an efficient indicator of some of the potentially deleterious outcomes of the interactions between herbivory and the new burning regime.

Conservation implications: The conservation objectives of maintaining (1) large mammal herbivory as an ecological process and (2) plant species diversity may be difficult to reconcile with each other in the highly fragmented renosterveld and lowland fynbos ecosystems. This paper explores a rapid approach to monitoring impacts of bontebok and other grazing ungulates.