Unraveling the habitat preferences of two closely related bumble bee species in Eastern Europe

Abstract

Co‐occurrence of closely related species is often explained through resource partitioning, where key morphological or life‐history traits evolve under strong divergent selection. In bumble bees (genus Bombus), differences in tongue lengths, nest sites, and several life‐history traits are the principal factors in resource partitioning. However, the buff‐tailed and white‐tailed bumble bee (Bombus terrestris and B. lucorum respectively) are very similar in morphology and life history, but their ranges nevertheless partly overlap, raising the question how they are ecologically divergent. What little is known about the environmental factors determining their distributions stems from studies in Central and Western Europe, but even less information is available about their distributions in Eastern Europe, where different subspecies occur. Here, we aimed to disentangle the broad habitat requirements and associated distributions of these species in Romania and Bulgaria. First, we genetically identified sampled individuals from many sites across the study area. We then not only computed species distributions based on presence‐only data, but also expanded on these models using relative abundance data. We found that B. terrestris is a more generalist species than previously thought, but that B. lucorum is restricted to forested areas with colder and wetter climates, which in our study area are primarily found at higher elevations. Both vegetation parameters such as annual mean Leaf Area Index and canopy height, as well as climatic conditions, were important in explaining their distributions. Although our models based on presence‐only data suggest a large overlap in their respective distributions, results on their relative abundance suggest that the two species replace one another across an environmental gradient correlated to elevation. The inclusion of abundance enhances our understanding of the distribution of these species, supporting the emerging recognition of the importance of abundance data in species distribution modeling.

Environmental differences explain subtle yet detectable genetic structure in a widespread pollinator

“…Individuals of B. terrestris, specifically workers (96), can be difficult to distinguish morphologically in the field from another closely related bumble bee species, the white-tailed bumble bee (Bombus lucorum) (97). We therefore confirmed species identity using a 1,043 bp long fragment of the cytochrome c oxidase subunit I (CO1) gene (22). We then amplified 12 previously developed (98) microsatellite loci in 3 multiplex reactions (PM1-PM3, Additional file 1: Table S1).…”

“…CC-BY-NC-ND 4.0 International license made available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprint this version posted July 10, 2021. ; https://doi.org/10.1101/2021.07.09.451741 doi: bioRxiv preprint across various environmental gradients (22,23), and morphological differences have prompted a division in nine subspecies (24). Studies on the population structure in this species have inferred little to no genetic divergence at spatial scales ranging from small-scale assemblages (25) over countries (26) to continents (27,28).…”

“…It is The copyright holder for this preprint this version posted July 10, 2021. ; https://doi.org/10.1101/2021.07.09.451741 doi: bioRxiv preprint Although quite subtle, the divergence we observed was best explained by environmental dissimilarities, in particular through the mean temperature of the coldest quarter, Leaf Area Index (LAI) and slope. The importance of temperature did not come as a surprise, as it is an important predictor of the distribution of Bombus species (22,55) and known to govern the emergence time of queens from hibernation (56). Hence, queens hibernating in warmer areas might emerge earlier than those in colder areas.…”

“…To further investigate whether subtle population structure could be detected when habitat characteristics were included in our analyses, and to estimate the relative effects of the potential drivers of structure, we ran generalized dissimilarity models (GDMs, 32). Local habitat conditions were characterized using a set of topographical, climate and vegetation variables, and resistance distances were based on a species distribution model (SDM) generated previously (22). GDMs explained ~ 33.5 and 39.2% of the divergence observed for 'dpds' and 'mpds', respectively (Table 1).…”

“…Its native distribution covers much of the Palearctic realm, including Europe, North Africa, and the British and most Mediterranean and Atlantic islands (27,30). Even within regions, the species exhibits a remarkable niche breadth and is present across a wide range of habitats (22,83). Its polylectic foraging and high pollination efficiency (24,84) has rendered this species a prime candidate for pollination in the greenhouse industry where it is deployed in high numbers (85).…”