Effective pollination is essential for both wildflowers and crops, but insect pollinators are increasingly threatened. Bumble bees represent major insect pollinators in temperate ecosystems. Some species are declining, and much remains to be established regarding the ecology of even the common and widespread species. A key aspect of foraging ecology is bees' use of space, since this determines how pollination services are distributed across landscapes. In this project, which was funded by the Insect Pollinators Initiative, we used a combination of field ecology, remote sensing, microsatellite genotyping and landscape modelling to understand the relationship between space use by bumble bee queens and workers and fine-scale features of their habitat, especially the quantity and quality of local foraging resources. The study species were Bombus terrestris, B. lapidarius, B. pascuorum, B. hortorum and B. ruderatus, and the study site was the Hillesden Estate, Buckinghamshire, UK. This is an area of arable farmland experimentally planted with sown field margins providing floral resources (from wildflower mixes) for pollinators as specified by the UK government's agri-environment scheme for England. We had three main findings:
1. Queens tend not to nest near where their mothers nested: It was previously unknown, even for common bumble bee species, whether queens tend to found nests near their natal nest or not. We captured workers of the five study species and sampled them non-lethally for DNA (before releasing them again in the study landscape). We genotyped the DNA samples at a panel of marker genes (microsatellite loci). Aided by sib-reconstruction software (COLONY), we used the genetic data to group workers into full sisterhoods, which represent workers all produced by one queen. Such workers must have come from the same nest. We estimated the position of each nest from the positions of the workers within the sisterhood, which we had mapped using GPS devices when the workers were collected. Using COLONY, we also reconstructed the genotype of each colony queen from the workers' genotypes.
We were then able to calculate, across all pairs of colony queens, the association between the genetic relatedness of queens and the physical distance between their nests in the landscape. We found that, for all five species, there was either no association between queen-queen relatedness and physical distance, or only a very weak one. This showed that queens do not tend to found nests near their natal nest (which would have led to a strong negative association of queen-queen relatedness and physical distance). Instead, on average, young queens from the same nest must disperse quite far, in different directions, before founding their nests, leading to populations that are genetically well mixed at fine spatial scales. These findings have been published in Dreier et al. (2014) and are also described in press releases from CEH and UEA.
2. Workers fly less far to forage when there are more floral resources in their immediate surroundings: We next sought to understand the movements of foraging workers. Using the data on workers' positions in the landscape, we estimated workers' colony-specific foraging distances, i.e. the average distance flown by workers from a given colony to forage at flowers. We then estimated the availability of floral resources around each nest from remote sensing data and field surveys. Combining these datasets showed that workers forage more closely to their nests in areas with greater coverage of semi-natural vegetation, including agri-environmental field margins. This suggests that sowing these margins will potentially help bee populations by reducing the energy workers spend on foraging longer distances (Redhead et al. 2016).
3. Colonies are more likely to produce queens surviving to the following year in areas with more floral resources: Focusing on Bombus terrestris, B. lapidarius and B. pascuorum, we used our genetic data to estimate, for a given colony, the chances of its producing a queen that survived to emerge the following year. We did this by looking for genetic matches indicating a sister relationship between workers sampled in the summer of one year and queens caught after emerging from hibernation in spring in the following year. We then looked for associations between such family lineage survival and the nature of the floral resources around each nest. This way, we showed that bumble bee colonies nesting within 250 - 1,000 metres of high-quality floral resources within the landscape are significantly more likely to have daughter queens surviving to the following spring. These findings both confirm the value of agri-environment schemes for pollinators and indicate the spatial and temporal scale of the floral resources required for such interventions to be effective. We also showed that, on average, queens emerging in the spring have dispersed 1.0 to 1.5 kilometres from their natal nest, allowing ample opportunity for the fine-scale spatial mixing of queens from different colonies detected earlier in the project. Our paper reporting this work is Carvell et al. (2017); see also the BBSRC press release.
The project was collaborative with Claire Carvell and Matt Heard (Centre for Ecology and Hydrology, Wallingford), Seirian Sumner and Stephanie Dreier (University of Bristol) and Jinliang Wang (Institute of Zoology, Zoological Society of London) and was led by CEH. It was one of the nine projects in the UK Insect Pollinators Initiative. The Insect Pollinators Initiative was funded jointly by the Biotechnology and Biological Sciences Research Council, the Department for Environment, Food and Rural Affairs, the Natural Environment Research Council, The Scottish Government and The Wellcome Trust, under the Living with Environmental Change Partnership.
Carvell C, Bourke AFG, Dreier S, Freeman SN, Hulmes S, Jordan WC, Redhead JW, Sumner S, Wang J, Heard MS (2017) Bumblebee family lineage survival is enhanced in high quality landscapes. Nature 543: 547-549.
Dreier S, Redhead JW, Warren IA, Bourke AFG, Heard MS, Jordan WC, Sumner S, Wang J, Carvell C (2014) Fine-scale spatial genetic structure of common and declining bumble bees across an agricultural landscape. Molecular Ecology 23: 3384-3395.
Redhead JW, Dreier S, Bourke AFG, Heard MS, Jordan WC, Sumner S, Wang J, Carvell C (2016) Effects of habitat composition and landscape structure on worker foraging distances of five bumblebee species. Ecological Applications 26: 726-739.