Current distribution and environmental relationships
Our study demonstrates that the four cold-adapted bumblebees occupy a
rather narrow environmental niche as the higher environmental
suitability for each species is associated with a rather narrow range of
climatic and habitat variables, that could be the mechanism why H2 holds
true in the context of cold-adapted animals. Thus, by being specialists,
they are usually rare in the field and will suffer from severe range
contraction in future. In other words, environmental variations over
time, between years or due to weather extreme events are likely to
affect the populations and therefore the distribution, as observed for
the population density fluctuations in Bombus alpinus (Rasmont et
al. 2015). Moreover, previous studies suspected that heat waves could
exert a strong impact on bumblebees (Iserbyt and Rasmont 2012), which
seems realistic also for cold-adapted bumblebees considering their
ecological niche. Of particular concern are the temperature and glacier
dependencies of the environmental niches, given the temperature warming
and especially the dramatic ongoing glacier contraction (Zemp et al.
2015). Therefore, especially the warming and the glacier melting rate
seriously imperils the fate of the alpine ecosystem and the cold-adapted
bumblebees.