Climate tracking by mountain bumblebees across a century: Distribution retreats, small refugia and elevational shifts

Cold-adapted species endangered by global change are crucial cases for understanding range dynamics and its interface with conservation and environmental decision-making. However, the study of their distribution change is either neglected or conducted at coarse resolutions at continental level, thus having little indicator value for regional and local conservation strategies. In view of climate change, it can be expected that cold-adapted insects should reduce distribution ranges by mainly concentric retreats and moving uphill. To test these hypotheses, we targeted four threatened, high-altitude bumblebees differing in subgenera and elevation ranges, and covering the main central and south European mountains. We performed species distribution models including climate and habitat at a 1 km-resolution, and we estimated elevation uphill and the year of elevation change with broken-line regressions. Results indicate that climate change will likely cause severe future range contractions across large areas, more in the Apennines (80 %-85 % ca) than the Alps and Pyrenees (24-56 % ca), with mostly concentric retreats as future extents will nearly entirely be included in the present ones. Remarkably, since the '80 s elevation uplift has started by about 325-535 m, a period coinciding with the beginning of the main warming, and will continue. The size and distribution of climate refugia will challenge conservation: they will be small (2-60 % of current areas) and even vary regionally, but while in the Apennines and Pyrenees they will be nearly entirely within Protected Areas, only a third will be so for the Alps. Such impressive distribution and elevation changes demonstrate that cold- adapted bumblebees can track climate change, reasons for it to be found in the specialist niches mainly requiring narrow temperature ranges and glacier presence. Overall, the distribution changes of cold specialist bumblebees driven by climate change demonstrate that conservation and policy makers should act upon the time dynamics and regional responses because future range contraction, the little availability of new areas and the movement uphill emerge as consistent patterns.