Density of Microhodotermes viator (Hodotermitidae) mounds in southern Africa in relation to rainfall and vegetative productivity gradients

Giant earth mounds (heuweltjies) formed by the southern harvester termite Microhodotermes viator occupy a large proportion of the land surface of the Succulent karoo, Nama-karoo and Fynbos biomes. Microhodotermes viator is associated with specific vegetation types and low winter rainfall, prompting the hypothesis that rainfall and vegetative productivity govern the distribution and population densities of M. viator. Using an analysis of aerial photographs, the density of termite mounds at 34 sites in 10 vegetation types was determined across the full range of their distribution. The number of mounds ranged from 143 to 704 km(-2) with a mean of 297. When all sites were considered together, mound density was not related to temperature, altitude, rainfall and vegetative productivity [as measured by the normalized difference vegetation index (NDVI)]. However, if the single Fynbos and five Coastal Renosterveld sites (with rainfall > 350 mm year(-1)) were excluded from the analysis, then mound density was significantly related to both rainfall and NDVI values. A separate analysis of the relationship between mound density, rainfall and soil fertility was undertaken. There was a significant relationship between rainfall and mound density for sites occurring on soils of high-fertility status but not for sites occurring on soils of low-fertility status. These results suggest that soil fertility and vegetation composition are parameters that influence the effect of rainfall and vegetative productivity on termite mound distribution and density. Finally, the positive relationship between rainfall and mound density in areas with < 350 mm year(-1) suggests that M. viator populations are in equilibrium with climatic conditions. This provides further evidence that heuweltjies are constructed by their current occupants, M. viator, and are not the product of an extinct termite species as has been suggested previously.