Extinction debt of high-mountain plants under twenty-first-century climate change

Quantitative estimates of the range loss of mountain plants under climate change have so far mostly relied on static geographical projections of species’ habitat shifts1–3. Here, we use a hybrid model4 that combines such projections with simulations of demography and seed dispersal to forecast the climate-driven spatio-temporal dynamics of 150 highmountain plant species across the European Alps. This model predicts average range size reductions of 44–50% by the end of the twenty-first century, which is similar to projections from the most ‘optimistic’ static model (49%). However, the hybrid model also indicates that population dynamics will lag behind climatic trends and that an average of 40% of the range still occupied at the end of the twenty-first century will have become climatically unsuitable for the respective species, creating an extinction debt5,6. Alarmingly, species endemic to the Alps seem to face the highest range losses. These results caution against optimistic conclusions from moderate range size reductions observed during the twenty-first century as they are likely to belie more severe longer-term effects of climate warming on mountain plants. Many plant and animal species have already been shifting their ranges in response to the past century’s climatic trends7–9. In mountains, owing to the altitudinal temperature gradient, species should primarily move upslope under warming, as has indeed been frequently documented during the recent decades10,11 as well as in the palaeorecord12,13. As mountains usually have conical shapes, upslope movement inevitably results in range loss and may even lead to ‘mountain-top extinctions’14 in extreme cases. However, previous predictions of the magnitude of such range and biodiversity losses during the twenty-first century have been criticized4,15 for relying on static ‘niche-based’ modelling approaches16, which disregard several processes crucial to range shifts, notably: propagule dispersal; the establishment and growth