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.