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 .