The distribution and spatial patterning of mima-like mounds in South Africa suggests genesis through vegetation induced aeolian sediment deposition

Abstract

Large earthen mima-like mounds, occur over vast areas globally, including south-western Africa, where they are called heuweltjies. Heuweltjies are commonly believed to be produced by fossorial fauna (termites and mammals), although it is also possible that they are the consequence of vegetation stabilising soil against erosion and/or trapping aeolian sediment. We sampled mound presence/absence in South Africa and analysed environmental correlates with mound distribution. Mounds mostly occur in sparse (0.1 < NDVI < 0.5) semi-arid Mediterranean-climate Succulent Karoo vegetation at modest elevations (<1000 m) with relatively high average annual wind speeds. These correlates are consistent with aeolian deposition of sediment in vegetation patches, but not with a termite-induced pattern. We analysed the spatial characteristics of mounds at 23 sites and found that over-dispersion is strongest and the size of mounds smallest closest to neighbouring mounds, indicating competitive interactions between mounds. Large inter-mound distances (ca. 40 m) make it unlikely that spacing is a direct consequence of root-based inter-plant competition. Rather, spacing is likely to be related to the influences of neighbouring mounds on aeolian sediment accretion. We suggest that the mounds arose in the Mediterranean climate zone at a time of greater aridity than currently.