Models of present and future distribution
B. konradini is the bumblebee with the narrowest suitable range of only 767 km2, while B. mucidus and B. mendax are estimated to occur in the study areas over 80445 and 56716 km2, respectively, and B. alpinus in 21875 km2 (Figure 1). Regionally, the current suitable areas of the studied species are largest in the Alps, while Pyrenees and Apennines harbour much smaller suitable extents: B. mucidus has ca 20 times smaller range in the Apennines and 2.6 times on the Pyrenees compared to that of the Alps, while B. mendax has ca 8 times smaller range in the Pyrenees compared to that of the Alps.
These bumblebees occur in areas with specific climates and habitats, as all species demonstrate a narrow or very narrow thermal range; average temperature stands out as the most important driver of distribution in all species but B. konradini (for which average temperature is anyway important). In addition, the distance from glacier edge and forests were important predictors for some of the studied species. The importance of variables included in the model for each species is reported in Table 1, and the main effects of the most relevant ones are shown in Supplementary Figure S1.
In future, all studied species are expected to shrink their distribution (Figure 1), confirming our first hypothesis (H1), although with different magnitude depending on the climatic model and on the mountain chain (Table 2); the future suitable areas will correspond to,6-80% of the current suitable extent according to the different combinations of species / area / climate model (Supplementary table S1). Under models predicting severe warming, future areas resulted in higher contraction and therefore less overlap between present and future distribution, compared to those predicting moderate warming (Table 2). Specifically, on average across models, the Apennines part of B. mucidus andB. konradini range will contract dramatically (ca 80%, but up to ca 94% in specific models), and Alps and Pyrenees will host severe contractions by about 40-56% in most cases, while only the Alpine range of B. mendax will shrink by 24%.
Most of the future areas will be within the current suitable range: on average 60-100 % of future areas will be included in present ones (Supplementary table S2). However, only a small fraction of future areas will constitute in-situ or ex-situ refugia (Table 3): only 2-6% of the present suitable areas in the Apennines, 23 and 35% ca of currently suitable areas in Alps and Pyrenees in most cases, and about 60% in the Alps for B. mendax . On the other side, while only about one third fall within current protected areas in the Alps, those from the Apennines and Pyrenees are almost entirely included. Ex-situ refugia are extremely small compared to in-situ refugia, and only B. konradini will have broader ex-situ refugia; their percentage of protection follows what is observed for in-situ refugia. Therefore, our second hypothesis H2 was largely confirmed, with the partial exception of B. konradini .