Evidence of climate change impacts on the iconic Welwitschia mirabilis in the Namib Desert

Climate change represents an important threat to global biodiversity and African ecosystems are particularly vulnerable. Recent studies predicted substantial variations of climatic suitability for Welwitschia mirabilis under future conditions. Latitudinal/altitudinal range shifts are well-known responses to climate change but not coherent patterns were documented. This study aims to verify whether welwitschia populations are responding to climate change and if the assumption of a latitudinal/altitudinal shift is applicable. We collected field data on welwitschia distribution, health condition, reproductive status, and plant size in northern Namibia. We used ecological niche models to predict the expected geographic shift of climatic suitability under future scenarios. For each variable, we compared the observed pattern with the expected responses. Finally, we tested the presence of simple geographical gradients in the observed patterns. The realized thermal niche of welwitschia will be almost completely unavailable in the next 30 years in northern Namibia. Expected reductions of climatic suitability in the stand sites are strongly associated with indicators of negative population conditions. The same population conditions does not fit any simple latitudinal or altitudinal gradient. The observed pattern of population conditions mirrors the expected pattern of climate change effect but no simple geographical gradient was relieved. Overall, we observed negative population conditions in areas with stronger reductions of suitability. This makes welwitschia a suitable sentinel for climate change effect in the Namib Desert ecosystems. Our approach to detect population responses to climate change could be extensively adopted for selecting sentinel species in other regions and ecosystems.