DISTURBANCE BY THE MOUND-BUILDING TERMITE, TRINERVITERMES TRINERVOIDES, AND VEGETATION PATCH DYNAMICS IN A SEMIARID, SOUTHERN AFRICAN GRASSLAND

Changes in the composition and abundance of grasses and shrubs, soil f ertility, and the productivity and nutrition of the grass, Themeda tri andra, were examined along an age gradient of Trinervitermes trinervoi des mounds occurring in a semi-arid grassland of the Free State, South Africa. The composition and abundance of grasses and shrubs change al ongside mounds as they become inactive and then erode away. The pionee r grass, Tragus koelerioides, and the climax grass, Themeda triandra, dominate around active mounds. As the mounds become inactive and erode away, these two grass species are replaced by the subclimax grass, Er agrostis lehmanniana, along with an increase in the cover of the unpal atable shrub, Walafrida saxatilis. Mound soils, in contrast, are spars ely vegetated and only change in composition, and the population abund ance of T. triandra, on old active mounds compared to earlier or older mound age states. Soils on eroded mounds are more acidic, and contain higher concentrations of Mg, Ca, N, P, and total exchangeable cations (T.E.C.) than soils occurring 0.5 m from the margins of eroded, inact ive and active mounds. A plant bioassay, using Lolium perenne, confirm s the higher soil fertility on eroded mounds but also shows significan t increases in soil fertility alongside inactive and eroded mounds. Po t experiments show an increase in the production of T. triandra plants grown on soils from eroded mounds, and those occurring alongside inac tive and eroded mounds. Foliar protein and nitrogen increase when thes e plants are grown on soils from eroded mounds. Mounds of T. trinervoi des are foci of biotic disturbance because they alter soil resources, and the population abundance and composition of grasses and shrubs in the first metre around their margins. Increases in soil fertility alon gside inactive and eroded mounds, and the accompanying increase in the productivity of T. triandra, along with signs of its foliar nutrient enrichment, suggest the removal of this species through preferential g razing by animals as the mounds become inactive and erode away.