Ecological impacts of Highveld gerbils (Tatera Brantsii) on a rehabilitated ash disposal site
Wright, Nevil Ian
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Tatera brantsii was numerically dominant in the small mammal community on the plateaux of the rehabilitated ash disposal sites of ESKOM's Hendrina Power Station in 1998 and 1999 (Vermaak 2000). The species seemed well adapted to exploit this environment and, through biopedturbation, had altered the topsoil structure and chemistry. The consequences of this and other activities also affected the rehabilitated plant community of the PFA-dam habitat. Burrowing appeared limited to just under the topsoil layer, and seemed more extensive than burrows of this species in natural ecosystems. The burrow system architecture was mapped and quantified, and localised increases in nitrates, phosphorous and organic carbon in immediately associated substrate were noted. However, this substrate enrichment was transient, and disappeared following the abandonment, and subsequent collapse of burrow systems, when gerbil colonies migrated away from the area. The mixing of soil horizons also resulted in a more homogeneous substrate, which was more friable, and thus drier. The high pH and salinity of the topsoil layer in areas undisturbed by gerbil burrowing, and concentrations of particular elements associated with either the topsoil covering or the ash, were reduced as a consequence of substrate mixing in disturbed areas. Gerbil impacts on the substrate of this habitat seemed to promote pedogenesis, eliminating the sharp distinction between the topsoil covering and the ash below, but the re-exposed ash of the burrow mounds would become subject to erosion, and reduce the effectiveness of the rehabilitation effort. Gerbil activities increased the number of plant species, especially ruderal forbs, comprising the plant community of the PFA-dam habitat, but plant community diversity was not significantly increased. However, numerical dominance by few tussock grass species was diminished , possibly reflecting burial under mounds of excavated substrate. The biomass and cover of some grass species were reduced in areas of gerbil impacts, and plant lifecycles appeared to be completed sooner in areas affected by gerbil activities. These effects may be as a result of the drier substrate produced following the collapse of the extensive network of abandoned burrows. The succession of this plant community towards an underutilised grassland state, the expected outcome of the rehabilitation effort, was minimally affected by gerbil activities. The effects of T.brantsii activities in this PFA-dam habitat were not as distinct as the effects noted by other authors studying fossorial rodent impacts in less disturbed habitats. This could be because further disturbances in this habitat would merely add to the currently disturbed state, whereas disturbance in more natural habitats, would show more of a change from the initial state.