Environmental barriers to the distribution of the genus Lithops: present, past and future

Summary: The genus Lithops (Aizoaceae; Ruschioideae) is widely distributed across South Africa, Namibia and into Botswana. Using the environmental niche modelling software Maxent, 416 Lithops localities were examined against environmental variables of climate, dominant soil, geology, ecoregion and altitude. Predictions obtained suggested that a combination of suitable soil or geology and climatic conditions determine the limits of the range of distribution of the genus. Predictions for paleoclimatic conditions at the last glacial maximum suggested the area of distribution of the genus Lithops was significantly contracted at this time but that all current species had at least one locality suitable to sustain them. Predictions using future climate simulations suggest a significant reduction in the range of the genus and potential extinction of two species due to habitats becoming unsuitable. A prediction using suitability of different ecoregions supports the hypothesis for an origin of the genus near to the mouth of the Orange River.