A perfect storm: unprecedented expansion of the Namib Desert and cascading desertification processes in the northernmost Succulent Karoo

Arid regions are characterized by high unpredictability of rainfall. Consequently, ecosystems along their margins are naturally oscillating but usually resilient. Here, we report the severe and potentially irreversible degradation of vegetation, ecosystems, and biodiversity in the northernmost more than 1 million ha of the Succulent Karoo, a global biodiversity hotspot. In our study, we use monitoring data spanning 45 years to disentangle different processes of change which started decades ago. The regionally important, vulnerable ecosystem “Gariep silty plains” is inhabited by the species-rich vegetation alliance Brownanthion pseudoschlichtiani. The cushion-like dwarf shrub scorpionstail (Brownanthus pseudoschlichtianus, Aizoaceae) is the dominant plant species protecting the soil and facilitating silt sedimentation. Following disturbances, this vegetation type is thinning and losing perennial plant species. This allows aeolian erosion, which – as a tipping point - turns the silty topsoil into sandy soil across extensive areas. Increased mobilisation of aeolian sand causes abrasion and sedimentation, which buries vast landscapes. The newly developed sandy topsoils are invaded by species-poor grassland communities partly typical for the Namib Desert biome. We present a novel S&T model and discuss cascading effects which threaten nature, farmland and infrastructure. Farming, mining, road construction and climate change may be interacting drivers of degradation. • Most of the northernmost more than 100 km of Succulent Karoo in South Africa are severely degraded by desertification. • Farming, mining and climate change (precipitation, temperature, wind speed) may be interacting drivers of degradation. • A novel conceptual model of the degradation states and the processes which cause transitions is proposed. • Rich Succulent Karoo flora is partly replaced by Desert flora, indicating a southward expansion of the Namib Desert. • Ecosystem engineers control dangerous aeolian erosion and deposition of sand and dust, loss of silt is a tipping point.