Responses to future climates and conservation
The four cold-adapted bumblebees will dramatically shrink their ranges as the distribution models unanimously predicted negative responses to future climate change, despite their different geography and different subgenera, thus fulfilling our hypothesis H1. The severe range contractions predicted for the 2070s are particularly alarming as these bumblebees are already enlisted in categories of extinction risk from what had been observed in recent times (Nieto et al. 2014, Quaranta et al. 2018) and our study shows that future climate dynamics will add up to the current negative trends and threats. Responses as these are expected and observed in a range of cold-adapted insects (Koot et al. 2022) and, to some extent, also on endotherms organisms in the same contexts (de Gabriel Hernando et al. 2021, Brambilla et al. 2022), making it very likely that H1 is a general rule for animals of cold areas.
The severe future range shrinkage will likely be “concentric”, because most future areas are predicted to occur within the current suitable ones, while large surfaces will turn into inhospitable land. This result supports the hypothesis H2 and adds evidence to what was observed in previous studies in North America that clarified the determinant role of changing temperature in causing range loss in bumblebees (Jackson et al. 2022). Similar results were obtained in alpine grasshoppers and are concordant with other bumblebee species (Martínez-López et al. 2021, Koot et al. 2022). Therefore, it is clear that the strong future alteration of the current distribution pattern will further imperil the lasting of these pollinators. Furthermore, our study demonstrated that refugia areas estimated suitable by all scenarios will be of small size, especially in the Apennines. Thus, the remaining patches will be important refuges for the four bumblebee species studies, and conservation measures should be strengthened there.
Some differences in shrinkage amount were detected among mountain ranges, with the conditions in the Apennines being particularly harsh in the future for the two studied bumblebees occurring there. For instance,B. mucidus , which occurs in all the three mountain ranges, will decrease on the Apennines by a percentage almost twice those of the other ranges. Likewise, the Apennines endemic B. konradini will largely shrink its distribution. A similar scenario is expected for other local elements of fauna and flora of this mountain range. The Apennines range of two high-elevation butterflies will be considerably small and even possibly disappear in future years (Sistri et al. 2022, Bonifacino et al. 2022). Likewise, the area occupied by some Apennines rare plants will considerably shrink (Di Musciano et al. 2020). We suspect that differences in orientation, elevation range, general climate and in diversity and extent of microhabitats among the three massifs investigated here could explain these differential responses, although without affecting the validity of H2. However, future research efforts on understanding why the Apennines seems to be so subjected to distribution loss in cold adapted species are clearly a priority, given that this region will also pose important conservation challenges for preserving its cold adapted fauna and flora.
Unfortunately, considering the unprecedented and fast rate of climate change, it is difficult to estimate the efficacy of conservation measures. Nevertheless, in a previous study, Biella et al. (2017) suggested applying conservation measures for decreasing the non-climatic stresses often associated with mountain environments, such as the impacting tourist activities and the competition by domestic grazers. Future, additional, efforts and studies could integrate physiological aspects into conservation by valorizing potentially different response abilities to heating in different populations, as previously proposed for reptiles (Besson and Cree 2011). A critical consideration regards refugia: it is surely a promising measure to protect and apply mitigation strategies within areas that represent in-situ refugia (shared by present and future scenarios) and also ex-situ ones (only in future scenario) under a dynamic view of area conservation. Given that climate refugia are key areas for the endurance of species facing challenges by global change as these are likely to preserve suitable environments (Brambilla et al. 2022), they must be considered as priority areas for targeted conservation efforts. The fact that in certain parts of the ranges a high proportion of refugia lies within the Protected Area network could promote the implementation of conservation measures.