Sand Termite Herbivory, Not Plant Water Stress, Causes Namibia's Fairy Circles – a Response to Getzin Et Al. (2022)

Abstract

In parts of Angola, Namibia and South Africa the sparse vegetation at the margin of the Namib Desert is often dotted with roughly circular bare patches. The origin of these “fairy circles” is subject of an ongoing debate. In a recent article in PPEES, Getzin et al. (2022) provided assessments of grasses and termites combined with soil moisture measurements,  in and near to fairy circles in several areas in Namibia. In their interpretation they state that termite herbivory is not causing this grass death as the plants had undamaged roots. Instead they propose that the matrix grasses severely depleted the water in FCs.Here, we use a far more comprehensive, longer and more detailed body of measurements and assessments collated by us during the last 14 years to propose an alternative interpretation. We structure our interpretation by proposing five hypotheses: (1) Long-term soil moisture measurements confirm that the soil beneath the dry topsoil of the bare patch of fairy circles contains an equal or, especially during the biologically active season, higher amount of moisture than the surrounding matrix at any given time. (2) Within the sandy soils of fairy circle landscapes, there is no sufficiently strong “uptake –diffusion feedback” that could cause a horizontal movement of soil moisture over several meters within a few days. (3) The grasses of the fairy circles bare patch die during the moist phase of the first weeks after a rain, before the soil beneath the matrix vegetation gets depleted by transpiration. (4) The grasses of the fairy circles bare patch first die at the centre of the bare patch and later towards the margin. (5) The grass in the bare patch of fairy circles dies because of damage to roots due to herbivory by sand termites.

Keywords: Namib Desert, Sand termite, Psammotermes, Fairy circles, Soil hydrology