Introduction
Climate shapes biodiversity in natural and human-dominated landscapes (Peters et al. 2019, Wieczynski et al. 2019, Biella et al. 2022). The growing concern towards climate change is increasing the research efforts aimed at understanding its effects on global biodiversity, to apply conservation efforts and a strong focus has been given to ecosystem indicators like insects (Harvey et al. 2023). Climate change effects have been observed in any kind of habitat and species, from deserts to high elevations (Kimball et al. 2010, Koot et al. 2022). Among the most evident and readily detectable consequences, the species distribution is rearranged by the new Earth climate, showing negative, positive or neutral range trends, with expansion or contraction patterns depending mostly on species thermal requirements (Hill et al. 2011, Williams and Blois 2018). Iconic cases are the thermophilic species, that have been reported to generally expand northwards or uphill thanks to warmer winters (Zait et al. 2020, Biella et al. 2021a, Stiels et al. 2021), but also the cold-adapted species that are retreating ranges due to climate warming (Marshall et al. 2020, McCain et al. 2021). While patterns of this kind have been widely detected, it remains urgent to better understand what are the underlying environmental features contributing the most to species trends and how this global change interplays with habitat features and species ecology (Stuhldreher et al. 2014). Under this view, it is crucial to detect, protect and improve areas that will constitute refugia against climate change and disturbance (Brambilla et al. 2022).
Climate change is especially stressing cold-adapted life forms (McCain et al. 2021, Seaborn et al. 2021) and the negative impacts are particularly acute in the arctic and alpine biomes (Pearson et al. 2013). The “cold biodiversity” is threatened by temperature warming and changes in precipitation regimes especially because of the melting of ice and snow surfaces, the expansion of forest at the expense of grasslands and the encroachment of species from lower elevational belts and latitudes, altering competition, causing changes in trophic interactions, reducing available resources (Brambilla et al. 2020, Körner and Hiltbrunner 2021, Kuo et al. 2021). Furthermore, in high-mountain areas, climate is changing faster than the global average (Nogués-Bravo et al. 2007). This warming is harmful because it accelerates the metabolisms of ectothermic organisms and it also increases the activity of harmful fungi and parasites (Scharsack et al. 2021, Bertini et al. 2021) or impacts survival and fecundity in different taxa (Irwin and Lee 2000, Williams et al. 2003), including cold-adapted bumblebees (Martinet et al. 2021, Ghisbain et al. 2023). These phenomena are often reflected by large population declines occurring in many species and, even more strikingly, by retreats towards the highest elevation, as for the case of orophylic bumblebees in the Alps and Pyrenees (Ornosa et al. 2017, Biella et al. 2017). Therefore, the spatial patterns of distribution changes due to climatic variations have the potential to diagnose the climatic sensitivity of biodiversity and warn towards a biodiversity-friendly management of cold areas (Brambilla et al. 2016, 2017).
Bumblebees are crucial high-altitude pollinators (Biella et al. 2021b). However, many bumblebee species are facing negative population trends, range contraction and altitude shifts with climate change considered one main cause among others (Kerr et al. 2015, Biella et al. 2017, Marshall et al. 2018). Moreover, laboratory tests indicated a high sensitivity to high and extreme temperatures (Oyen et al. 2016) and field observations detected body alterations due to heat islands in urban areas (Tommasi et al. 2022). In fact, bumblebees are mostly linked to fresh and cold habitats (Condamine and Hines 2015) and their diversity thrive in many mountain regions. Their high sensitivity and key role for ecosystem functioning make these organisms an ideal model to investigate the effects of climate change on mountain biodiversity and ecosystems.
Based on their sensitivity to climate and especially to temperature (Ghisbain et al. 2023), bumblebees should possess a high ability of tracking thermocline variation over time. Therefore, in face of the past and future climate change, we expect bumblebee species of cold areas to suffer range contraction: considering the realised and predicted magnitude of climate change, such variations should be evident when comparing the current patterns with both the past and future ones (Hypothesis 1, “H1”). Moreover, in the case of range variation, we hypothesise that such changes could happen mainly by concentric retreats (i.e., abandoning peripheral areas) rather than by displacement (i.e., by colonising new areas in the future) (Hypothesis 2, “H2”). This pattern can be expected because mountain orography and uneven distribution of cold microhabitat could limit dispersal (Ceresa et al. 2023), in particular when the species are habitat specialists (Alessandrini et al. 2022). Furthermore, by tracking their optimal thermal niche under a changing climate (Harvey et al. 2023), cold-adapted bumblebees should also undergo a strong upward shift in the average elevation of their occurrence sites and range (Hypothesis 3, “H3”). In this concerning scenarios, the critical study of the distribution of climate refugia and their spatial relation with protected areas will also inform area prioritisation for conservation, under adaptive conservation strategies (Rannow et al. 2014).
To test the three hypotheses formulated above and critically investigate the conservation challenges posed by ongoing and future distribution patterns of cold adapted species, this study focuses on four bumblebee species occurring on mountain areas in the southern and central part of Europe, differing in mountain chain orientation, mean elevation and extent (Kapos et al. 2000). Given these differences, we expected to see idiosyncratic responses to climate change by area at the regional level, i.e. the bumblebee ranges in different chains showing responses of different magnitude to climate change. Acknowledging these regional patterns is crucial for effective and ‘realistic’ conservation strategies, tailoring area-specific actions.
We focus on four cold-adapted bumblebee species and consider (i) the changes in occurrence elevation over past decades and in the future, (ii) the predicted distribution of suitable areas at present and under future climate, to highlight patterns of distribution change at the regional level, (iii) the distribution of climate refugia in relation to the Protected Area Network. Because the four species are often difficult to detect, either because of their rarity or because they occur in areas of difficult accessibility (i.e. high mountains), we combined approaches based on known occurrences and on Species Distribution Modelling integrating environmental variables of habitat and climate. In this way, we obviated the lack of complete knowledge on their distribution and retrieved clear ecological patterns that will aid conservation efforts of these species.