The effect of gerbils (Tatera brantsii) on primary production and plant species composition in a southern African savanna.

Due to burrowing activity and selective feeding, the highveld gerbil, Tatera brantsii A. Smith, has a great influnce on grassland vegetation. Within old colonies, in comparison to control sites, total biomass of plants was reduced by 47.6% from an average of 234.0 g/m to 122.6 g/m. Root biomass was reduced by 34% from 539 g/m to 355 g/m. Correspondingly maximum vegetation heights decreased by 33.5% from 106.4 cm to 70.8 cm. The number of plant species was not different between disturbed and undisturbed sites but species diversity and evenness of plants were significantly increased in gerbil colonies, mainly due to the reduction in dominant grass species. In disturbed sites dicotyledone plants, especially legumes, were favoured and increased in biomass. The gerbils are considered an integral part of the system and necessary for the maintepance of a high species diversity of plants. The regular "disturbance" of savanna vegetation followed by the "recovery" of the system is driven by the movement of gerbil colonies across the area. The proposed mechanism and the alternating vegetation pattern on similar soils due to disturbances which are an integral part of the system is in agreement with Remmert's mosaic cycle theory.