Distribution of bumblebees across Europe

Bumblebee thermoregulation at increasing temperatures is affected by behavioral state

Over the past decades, increasing environmental temperatures have been identified as one of the causes of major insect population declines and biodiversity loss. However, it is unclear how these rising temperatures affect endoheterothermic insects, like bumblebees, that have evolved thermoregulatory capacities to exploit cold and temperate habitats. To investigate this, we measured head, thoracic, and abdominal temperature of bumblebee (Bombus terrestris) workers across a range of temperatures (24 °C-32 °C) during three distinct behaviors. In resting bumblebees, the head, abdomen, and thorax conformed to the environmental temperature. In pre-flight bumblebees, the head and abdominal temperatures were elevated with respect to the environmental temperature, while the thoracic temperature was maintained, indicating a pre-flight muscle warming stage. In post-flight bumblebees, abdominal temperature increased at the same rate as environmental temperature, but the head and the thoracic temperature did not. By calculating the excess temperature ratio, we show that thermoregulation in bumblebees during flight is partially achieved by the active transfer of heat produced in the thorax to the abdomen, where it can more easily be dissipated. These results provide the first indication that the thermoregulatory abilities of bumblebees are plastic and behavior dependent. We also show that the flight speed and number of workers foraging increase with increasing temperature, suggesting that bees do not avoid flying at these temperatures despite its impact on behavioral performance.

Urban habitat fragmentation and floral resources shape the occurrence of gut parasites in two bumblebee species

Urbanization and the expansion of human activities foster radical ecosystem changes with cascading effects also involving host-pathogen interactions. Urban pollinator insects face several stressors related to landscape and local scale features such as green habitat loss, fragmentation and availability reduction of floral resources with unpredictable effects on parasite transmission. Furthermore, beekeeping may contribute to the spread of parasites to wild pollinators by increasing the number of parasite hosts. Here we used DNA-based diagnostics tools to evaluate how the occurrence of parasites, namely microsporidians (Nosema spp.), trypanosomatids (Crithidia spp.) and neogregarines (Apicystis bombi), is shaped by the above-mentioned stressors in two bumblebee species (i.e. Bombus terrestris and Bombus pascuorum). Infection rates of the two species were different and generally higher in B. terrestris. Moreover, they showed different responses towards the same ecological variables, possibly due to differences in body size and foraging habits supposed to affect their susceptibility to parasite infection. The probability of infection was found to be reduced in B. pascuorum by green habitat fragmentation, while increased along with floral resource availability. Unexpectedly, B. terrestris had a lower parasite richness nearby apiaries maybe due to the fact that parasites are prone to be transmitted among the most abundant species. Our finding supports the need to design proper conservation measures based on species-specific knowledge, as suggested by the variation in the parasite occurrence of the two species. Moreover, conservation policies aiming at safeguarding pollinators through flower planting should consider the indirect effects of these measures for parasite transmission together with pollinator biodiversity issues.

Effect of urbanization and its environmental stressors on the intraspecific variation of flight functional traits in two bumblebee species

The way urbanization shapes the intraspecific variation of pollinator functional traits is little understood. However, this topic is relevant for investigating ecosystem services and pollinator health. Here, we studied how urbanization affects the functional traits of workers in two bumblebee species (Bombus terrestris and B. pascuorum) sampled in 37 sites along a gradient of urbanization in North Italy (an area of 1800 km² including the metropolitan context of Milan and other surrounding capital districts). Namely, we investigated the effect of land use composition, configuration, air temperature, flower resource abundance, and air pollutants on the variation of traits related to flight performance and of stress during insect development (i.e., wing size, wing shape and size fluctuating asymmetry). The functional traits of the two bumblebees responded idiosyncratically to urbanization. Urban temperatures were associated with smaller wing sizes in B. pascuorum and with more accentuated fluctuating asymmetry of wing size in B. terrestris. Moreover, flower abundance correlated with bigger wings in B. terrestris and with less asymmetric wing size in B. pascuorum. Other traits did not vary significantly, and other urban variables played minor effects. These species-specific variation patterns highlight that environmental stressor linked to urbanization negatively impact the traits related to flight performance and development stability of these syntopic bumblebees, with possible consequences on the pollination service they provide.

Large variability in response to projected climate and land-use changes among European bumblebee species

Bumblebees (Bombus ssp.) are among the most important wild pollinators, but many species have suffered from range declines. Land-use change, agricultural intensification, and the associated loss of habitat have been identified as drivers of the observed dynamics, amplifying pressures from a changing climate. However, these drivers are still underrepresented in continental-scale species distribution modeling. Here, we project the potential distribution of 47 European bumblebee species in 2050 and 2080 from existing European-scale distribution maps, based on a set of climate and land-use futures simulated through a regional integrated assessment model and consistent with the RCP-SSP scenario framework. We compare projections including (1) dynamic climate and constant land use (CLIM); (2) constant climate and dynamic land use (LU); and (3) dynamic climate and dynamic land use (COMB) to disentangle the effects of land use and climate change on future habitat suitability, providing the first rigorous continental-scale assessment of linked climate-land-use futures for bumblebees. We find that direct climate impacts, although variable across species, dominate responses for most species, especially under high-end climate change scenarios (up to 99% range loss). Land-use impacts are highly variable across species and scenarios, ranging from severe losses (up to 75% loss) to considerable gains (up to 68% gain) of suitable habitat extent. Rare species thereby tend to be disproportionally affected by both climate and land-use change. COMB projections reveal that land use may amplify, attenuate, or offset changes to suitable habitat extent expected from climate impact depending on species and scenario. Especially in low-end climate change scenarios, land use has the potential to become a game changer in determining the direction and magnitude of range changes, indicating substantial potential for targeted conservation management.

Urbanization is associated with shifts in bumblebee body size, with cascading effects on pollination

Urbanization is a global phenomenon with major effects on species, the structure of community functional traits and ecological interactions. Body size is a key species trait linked to metabolism, life-history and dispersal as well as a major determinant of ecological networks. Here, using a well-replicated urban-rural sampling design in Central Europe, we investigate the direction of change of body size in response to urbanization in three common bumblebee species, Bombus lapidarius, Bombus pascuorum and Bombus terrestris, and potential knock-on effects on pollination service provision. We found foragers of B. terrestris to be larger in cities and the body size of all species to be positively correlated with road density (albeit at different, species-specific scales); these are expected consequences of habitat fragmentation resulting from urbanization. High ambient temperature at sampling was associated with both a small body size and an increase in variation of body size in all three species. At the community level, the community-weighted mean body size and its variation increased with urbanization. Urbanization had an indirect positive effect on pollination services through its effects not only on flower visitation rate but also on community-weighted mean body size and its variation. We discuss the eco-evolutionary implications of the effect of urbanization on body size, and the relevance of these findings for the key ecosystem service of pollination.

Climate change contributes to widespread declines among bumble bees across continents

Climate change could increase species’ extinction risk as temperatures and precipitation begin to exceed species’ historically observed tolerances. Using long-term data for 66 bumble bee species across North America and Europe, we tested whether this mechanism altered likelihoods of bumble bee species’ extinction or colonization. Increasing frequency of hotter temperatures predicts species’ local extinction risk, chances of colonizing a new area, and changing species richness. Effects are independent of changing land uses. The method developed in this study permits spatially explicit predictions of climate change–related population extinction-colonization dynamics within species that explains observed patterns of geographical range loss and expansion across continents. Increasing frequencies of temperatures that exceed historically observed tolerances help explain widespread bumble bee species decline. This mechanism may also contribute to biodiversity loss more generally.