Thanks to the EU FP7 project STEP (Potts et al. 2011), over one million bumblebee records from all over Europe have been collated. Based on data from 1970 to 2000 we modelled the current climatic niche for almost all European species (56 out of 69) and projected future climatically suitable conditions based on three climate change scenarios (SEDG, BAMBU and GRAS) for the years 2050 and 2100. Due to limited knowledge of actual bumblebee dispersal, we made two extreme assumptions: (i) the species has full dispersal abilities (meaning that the species is able to spread all over its suitable area) or (ii) the species is unable to disperse at all (i.e. that changes in climatic conditions can only lead to projected range retractions). However, to aid the assessment as to which of these two extreme assumptions are more likely to meet reality, we also provide a rough indication of the species’ potential dispersal ability based on the ecology of the different bumblebees. The book includes up-to-date pin-pointed maps and photographs of all West-Palaearctic bumblebee species, including Russian and Near-Orient ones.
Since bumblebees are mainly adapted to colder conditions, they appear as highly vulnerable to climate change. In 2100, depending on the scenario of climate change, up to 36% of the European bumblebees are projected to be at an high climatic risk (i.e. losing more than 80% of their current range), 41% will be at risk (loss between 50% and 80%). Non-modelled species are all very rare and localised and their ranges are most likely to be shrinking considerably under all of the scenarios. Only a few species are projected to benefit from climate change and can potentially enlarge their current distributions in Europe, such as B. argillaceus and B. haematurus.
As expected, the three scenarios considerably differed in their projections for 2100. While under the moderate change scenario (SEDG) only five species are projected to be at the verge of extinction by 2100, twenty species are at particularly high risk under the intermediate change scenario (BAMBU). Under the most severe change scenario (GRAS) as many as 34 species are projected to lose almost all of their climatically suitable area.
Also the ability to keep track with climate change has a considerable impact of the projected changes. For instance, under the most severe climate change scenario (GRAS) nine species are at an extremely high climatic risk when full dispersal is assumed for all of them. However, under the assumption of no dispersal within the next 100 years 34 species would fall into this category. When potential dispersal abilities, inferred from species traits and their auto-ecologies, are considered to decide for which species no or full dispersal assumptions are more realistic, it seems that up to 3 species might expand their ranges by 2100, no species is likely to remain at the status quo, and 25 species would be at an extremely high climatic risk.
We also found that for many species (about 30%), especially the cold-adapted ones in Alpine and Arctic regions (e.g. B. alpinus, B. balteatus, B. hyperboreus and B. polaris) their dispersal abilities are actually irrelevant for the assessment of their future fate because climate change will only lead to reductions of areas with suitable climatic conditions while no extra suitable regions will emerge.
Given the great sensitivity of bumblebees to climate change and further considering the severe projected changes in the light of the great relevance of bumblebees as pollinators, designing management plans to sustain the highest level of pollination services on the one hand and to ensure the survival of as many bumblebee species on the other hand is of utmost importance. Given the different mechanisms leading to change, especially at the leading versus the trailing edge of species distributions and the geographical differences in the severity of climate change, management actions must be well and target-specific designed. One important issue would be to prioritise management actions across different geographic regions in Europe. We have seen that the expected species loss due to climate change increases with decreasing latitudes, i.e. that regions in the south of Europe will be most affected by pollinator loss.
Important means to support European bumblebees would be to facilitate the movement of species trying to keep track with changing climates at the trailing edge and to prolong the persistence in regions where climatic conditions are deteriorating. Landscape management can be of particular help in this context. Increased connectivity and quality of bumblebee habitats can help colonising species, while habitat heterogeneity will generate heterogeneity in the microclimate and can thus increase population persistence at the trailing edge as a kind of “Noah’s Ark”. Areas with naturally high levels of microclimatic heterogeneity (such as mountainous areas) can be of particular importance and deserve special attention. Finally, the idea of assisted migration, i.e. purposeful anthropogenic translocations, seems appealing at first sight for species whose original distributional areas are projected to shrink tremendously and cannot move to suitable areas because of natural or anthropogenic barriers. However, the feasibility of such actions is still questionable.
To conclude, climatic risks for bumblebees can be extremely high, depending on the future development of human society, and the corresponding effects on the climate, strong mitigation strategies are needed to preserve this important species group and to ensure the sustainable provision of pollination services, to which they considerably contribute.