Shift in size of bumblebee queens over the last century

Abstract

Species can respond differently when facing environmental changes, such as by shifting their geographical ranges or through plastic or adaptive modifications to new environmental conditions. Phenotypic modifications related to environmental factors have been mainly explored along latitudinal gradients, but they are relatively understudied through time despite their importance for key ecological interactions. Here we hypothesize that the average bumblebee queen body size has changed in Belgium during the last century. Based on historical and contemporary databases, we first tested if queen body sizes changed during the last century at the intraspecific level among four common bumblebee species and if it could be linked to global warming and/or habitat fragmentation as well as by the replacement by individuals from new populations. Then, we assessed body size changes at the community level, among 22 species, taking into account species population trends (i.e. increasing, stable or decreasing relative abundance). Our results show that the average queen body size of all four bumblebee species increased over the last century. This size increase was significantly correlated to global warming and habitat fragmentation, but not explained by changes in the population genetic structure (i.e. colonization). At the community level, species with stable or increasing relative abundance tend to be larger than declining species. Contrary to theoretical expectations from Bergmann's rule (i.e. increasing body size in colder climates), temperature does not seem to be the main driver of bumblebee body size during the last century as we observed the opposite body size trend. However, agricultural intensification and habitat fragmentation could be alternative mechanisms that shape body size clines. This study stresses the importance of considering alternative global change factors when assessing body size change.

Global warming and plant–pollinator mismatches

“…The asynchrony resulting from spatial and/or temporal mismatches will be subject to selection and the ability of plants and pollinators to adapt remain unclear. To address this current knowledge gap, we need ambitious strategies for monitoring plants and their pollinators, not only to detect shifts in their temporal and spatial distribution ( Table 1), but also in their phenotypic distribution [105]. So far only a limited pollinator diversity has been studied in these contexts; in detail, evidence for phenological mismatches as a result of microevolutionary responses has already been observed [106], thus a critical challenge is now to assess if the pace of adaptive evolutionary changes will be fast enough the track climate warming and prevent species extinctions [107].…”

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

“…(2019) was not carried out along an urbanization gradient, but across a temporal increase in land‐use intensification of rural sites, which can be much more inhospitable for bees than cities (Hall et al., 2017; Samuelson, Gill, Brown, & Leadbeater, 2018; Theodorou et al., 2016, 2020). This could also be the reason why positive shifts in dispersal‐related traits have been found mainly in relation with land‐use intensification in bees (Gérard et al., 2019; Warzecha, Diekötter, Wolters, & Jauker, 2016), as in other taxa (Taylor & Merriam, 1995), rather than with increased urbanization. City life seems to elicit responses in body size that are taxon, context and scale dependent (Eggenberger et al., 2019; Merckx, Souffreau, et al, 2018; Piano et al., 2017).…”

Wildlife conservation strategies should incorporate both taxon identity and geographical context ‐ further evidence with bumblebees

“…Although these ideas can constitute adequate starting points for further strategic thinking, it is now acknowledged that global changes can show very distinct impacts on living organisms within a single monophyletic group, from highly beneficial (increasing a distribution range) to highly deleterious (leading to extinction) depending on the species (Kerr et al., 2015; Marshall et al., 2018; Prŷs‐Jones, 2019). In addition to global warming, the increased agricultural intensification and habitat fragmentation have been shown to be involved in species‐specific phenotypic changes in bumblebee queens over the last century (Gérard, Martinet, et al, 2020). Such species‐specific responses to habitat changes are also dietary, for instance, following the drastic drift of pollen resources that occurred in the second half of the 20th century (Roger et al., 2017).…”