THE TREE BUMBLE BEE (BOMBUS HYPNORUM): A RANGE-EXPANDING POLLINATOR

The Tree Bumblebee (Bombus hypnorum) is of special interest in the context of bumble bee ecology and conservation. Since 2001, when it was first recorded in the UK (in southern England), having apparently arrived as a natural colonist from continental Europe, it has spread extremely rapidly and has now become one of the UK's commonest bumble bee species. Meanwhile, the ranges of other UK bumble bee species have either remained broadly stable or shrunk as their populations have declined. So investigating the ecology, genetics and social biology of B. hypnorum allows investigators to explore the factors underlying ecological success in bumble bees. In our group at UEA, we have carried out a number of projects on B. hypnorum with this aim. Here are some of our findings:

1. B. hypnorum shows a distinctive set of habitat and forage preferences relative to other UK bumble bee species: To try to understand the ecology of the UK B. hypnorum population, we investigated landscape-scale habitat use and foraging preferences of B. hypnorum in its English range (Crowther et al. 2014). Counts of B. hypnorum and five other bumble bee species or species groups were made at 42 sites within a mixed landscape in south-eastern Norfolk. The extent of different landscape elements around each transect was quantified at three scales (250 m, 500 m and 1500 m).

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We found that B. hypnorum numbers were significantly positively associated with extent of both urban and woodland cover. Uniquely in comparison with the other bumble bee species present at the sites, this was the case at all three scales for urban cover and two of the three scales for woodland cover. Relative to the other bumble bee species, B. hypnorum also exhibited a significantly higher foraging preference for two flowering trees, Hawthorn (Crataegus monogyna) and Blackthorn (Prunus spinosa), and, among crops and wildflowers in the study, showed significantly lower preferences for Oilseed Rape (Brassica napus), Ground Ivy (Glechoma hederacea) and White Dead-nettle (Lamium album).

These findings confirmed a previously suspected association of B. hypnorum with urban and woodland habitats, but at a strength not previously demonstrated. They also suggested that in the UK the species preferentially visits a set of plants different from (though overlapping with) those preferred by other bumble bee species, with a bias towards visiting trees and shrubs. Combined, these findings suggest that range expansion in B. hypnorum depends, at least partly, on exploitation of widespread habitats and forage sources underutilised by native bumble bee species (Crowther et al. 2014).

2. B. hypnorum workers forage over unusually short distances and colonies can occur at high densities: We have been using genetic tools to investigate whether specific features of the spatial ecology of B. hypnorum are associated with its rapid range expansion (Crowther et al. 2019). In particular, we used a panel of 14 microsatellite DNA markers to estimate worker foraging distance and nest density in a representative UK population in suburban Norwich.

Genotyping stored sperm taken from sampled queens confirmed that, in this population as in some populations in continental Europe, queens mate with multiple males (the mean minimum mating frequency was 1.7 males per queen). After assigning workers to colonies based on full or half sibship, we estimated the average worker foraging distance as 104 metres, which is considerably less than values reported from most other bumble bee populations. For example, a study using the same methods estimated that workers of the widespread native UK species, the Buff-tailed Bumble Bee B. terrestris, foraged over an average distance of 551 metres (Redhead et al. 2016).

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In B. hypnorum, we also found that estimated nest density was considerably higher than estimates from other bumble bee species in the UK, at least in our first year of sampling (estimated B. hypnorum nest densities were 256 and 72 nests per square kilometre in 2014 and 2015, respectively). High nesting densities could be linked to the well known trait of B. hypnorum of nesting in above-ground cavities, including artificial ones such as holes in walls and bird nest-boxes.

Overall, this study established some critical spatial ecological parameters and the mating system of the UK B. hypnorum population. In addition, the findings suggest that this population's rapid range expansion is associated with its being able: (a) to meet its foraging needs over short foraging distances (as would be predicted in a growing population), so reducing the workers' energetic costs of foraging; and (b) achieve high nest densities; with both these traits potentially allowing it to export many queens to new areas (Crowther et al. 2019).

3. The UK population of B. hypnorum is unlikely to have undergone a severe genetic bottleneck on colonising its new range, but instead experiences ongoing gene flow from source populations in continental Europe: In ants, bees and wasps, because of their unusual system of sex determination, low genetic variation at the sex-determination locus leads to the production of diploid males (whereas males are usually haploid in these groups, i.e. have a single set of chromosomes per cell).  So high levels of diploid male production represent a signature of low genetic variation.  We reared colonies of B. hypnorum from queens collected in the field and genotyped the males they produced (at microsatellite loci) to measure the level of diploid male production.  We found that diploid males were produced by 15.4% of reared colonies, from which we could estimate that there are 21.5 alleles at the sex-determining locus in the UK B. hypnorum population, which is not an especially low number.  In addition, we found that the population exhibits levels of overall genetic diversity at microsatellite loci falling between those of widespread and range-restricted UK bumblebee species.  We therefore concluded that, contrary to what is seen in some introduced species, the post-arrival population of B. hypnorum has not experienced a severe genetic bottleneck.  Instead, matching evidence that the species has been spreading westwards across Europe over many decades, the UK population appears still to be in genetic contact (via continued immigration of queens and/or males) with its continental source populations, which helps maintain its levels of genetic variation via gene flow.  In turn this means that the spectacular ecological success of the UK B. hypnorum population has not occurred against a background of low genetic variation stemming from a genetic bottleneck (Brock et al. 2021).

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These projects were carried out by Ryan Brock and Liam Crowther in Andrew Bourke's group at UEA, in collaboration with Claire Carvell (Centre for Ecology and Hydrology, Wallingford), Pierre-Louis Hein (IUT Nancy-Brabois, Villers-les-Nancy, France), David S. Richardson and Martin Taylor (both in the School of Biological Sciences, UEA) and David Wright (Earlham Institute, Norwich). They were supported by the Applied Ecology & Conservation MSc at UEA and by funding from a NERC PhD studentship held by Liam Crowther in a CASE partnership with CEH and a NERC EnvEast Doctoral Training Partnership PhD studentship held by Ryan Brock.

Brock RE*, Crowther LP*, Wright DJ, Richardson DS, Carvell C, Taylor MI, Bourke AFG (2021) No severe genetic bottleneck in a rapidly range-expanding bumblebee pollinator. Proceedings of the Royal Society B 288: 20202639. *Equal contribution.

Crowther LP, Hein P-L, Bourke AFG (2014) Habitat and forage associations of a naturally colonising insect pollinator, the Tree Bumblebee Bombus hypnorum. PLoS One 9: e107568.

 

Crowther LP, Wright DJ, Richardson DS, Carvell C, Bourke AFG (2019) Spatial ecology of a range-expanding bumble bee pollinator. Ecology and Evolution 9: 986-997.

 

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.