Ecological correlates of small mammal community structure in the semi-arid Karoo, South Africa

The understanding of the determinants of small mammal community structure in arid and semiarid ecosystems is of importance, both in the light of the role that small mammals play, and the impact of livestock grazing on the flora of these systems. In a study aimed at identifying these determinants, small mammal assemblages and environmental features were quantified at six localities (a gradient of floristic structure, with constant annual rainfall) across the southern Karoo, South Africa. Stepwise variable regression indicated that small mammal diversity was correlated with plant and rock cover, as well as plant cover and horizontal foliage diversity at intermediate heights (40–60 cm). Initially, small mammal diversity increased with increasing plant cover, but decreased at cover levels greater than 30%. This relationship is similar to that found in other desert systems, although the peak in diversity found here is at higher levels of plant cover than found previously. I suggest that this higher peak may be owing to the lack of reliance on granivory by these animals, which are relatively omnivorous. This model may therefore explain the conflicting reports on the impact of livestock grazing on desert small mammals, with small mammal diversity decreasing with grazing below the peak, and increasing with grazing above the peak.