Genomic divergence and a lack of recent introgression between commercial and wild bumblebees (Bombus terrestris)

The global movement of bees for agricultural pollination services can affect local pollinator populations via hybridization. When commercial bumblebees are of the same species but of different geographic origin, intraspecific hybridization may result in beneficial integration of new genetic variation, or alternatively may disrupt locally adapted gene complexes. However, neither the existence nor the extent of genomic introgression and evolutionary divergence between wild and commercial bumblebees is fully understood. We obtained whole-genome sequencing data from wild and commercial Bombus terrestris collected from sites in Southern Sweden with and without long-term use of commercially imported B. terrestris. We search for evidence of introgression, dispersal and genome-wide differentiation in a comparative genomic analysis of wild and commercial bumblebees. Commercial B. terrestris were found in natural environments near sites where commercial bumblebees were used, as well as drifting wild B. terrestris in commercial bumblebee colonies. However, we found no evidence for widespread, recent genomic introgression of commercial B. terrestris into local wild conspecific populations. We found that wild B. terrestris had significantly higher nucleotide diversity (Nei's pi, pi), while the number of segregating sites (Watterson's theta, theta w) was higher in commercial B. terrestris. A highly divergent region on chromosome 11 was identified in commercial B. terrestris and found to be enriched with structural variants. The genes present in this region are involved in flight muscle contraction and structure and pathogen immune response, providing evidence for differing evolutionary processes operating in wild and commercial B. terrestris. We did not find evidence for recent introgression, suggesting that co-occurring commercial B. terrestris have not disrupted evolutionary processes in wild B. terrestris populations

Evaluating competition for forage plants between honey bees and wild bees in Denmark

A recurrent concern in nature conservation is the potential competition for forage plants between wild bees and managed honey bees. Specifically, that the highly sophisticated system of recruitment and large perennial colonies of honey bees quickly exhaust forage resources leading to the local extirpation of wild bees. However, different species of bees show different preferences for forage plants. We here summarize known forage plants for honey bees and wild bee species at national scale in Denmark. Our focus is on floral resources shared by honey bees and wild bees, with an emphasis on both threatened wild bee species and foraging specialist species. Across all 292 known bee species from Denmark, a total of 410 plant genera were recorded as forage plants. These included 294 plant genera visited by honey bees and 292 plant genera visited by different species of wild bees. Honey bees and wild bees share 176 plant genera in Denmark. Comparing the pairwise niche overlap for individual bee species, no significant relationship was found between their overlap and forage specialization or conservation status. Network analysis of the bee-plant interactions placed honey bees aside from most other bee species, specifically the module containing the honey bee had fewer links to any other modules, while the remaining modules were more highly inter-connected. Despite the lack of predictive relationship from the pairwise niche overlap, data for individual species could be summarized. Consequently, we have identified a set of operational parameters that, based on a high foraging overlap (>70%) and unfavorable conservation status (Vulnerable+Endangered+Critically Endangered), can guide both conservation actions and land management decisions in proximity to known or suspected populations of these species

Bees increase seed set of wild plants while the proportion of arable land has a variable effect on pollination in European agricultural landscapes

Background and aims - Agricultural intensification and loss of farmland heterogeneity have contributed to population declines of wild bees and other pollinators, which may have caused subsequent declines in insect-pollinated wild plants. Material and methods - Using data from 37 studies on 22 pollinator-dependent wild plant species across Europe, we investigated whether flower visitation and seed set of insect-pollinated plants decline with an increasing proportion of arable land within 1 km. Key results - Seed set increased with increasing flower visitation by bees, most of which were wild bees, but not with increasing flower visitation by other insects. Increasing proportion of arable land had a strongly variable effect on seed set and flower visitation by bees across studies. Conclusion - Factors such as landscape configuration, local habitat quality, and temporally changing resource availability (e.g. due to mass-flowering crops or honey bee hives) could have modified the effect of arable land on pollination. While our results highlight that the persistence of wild bees is crucial to maintain plant diversity, we also show that pollen limitation due to declining bee populations in homogenized agricultural landscapes is not a universal driver causing parallel losses of bees and insect-pollinated plants.Peer reviewe

Non-bee insects are important contributors to global crop pollination

Wild and managed bees are well documented as effective pollinators of global crops of economic importance. However, the contributions by pollinators other than bees have been little explored despite their potential to contribute to crop production and stability in the face of environmental change. Non-bee pollinators include flies, beetles, moths, butterflies, wasps, ants, birds, and bats, among others. Here we focus on non-bee insects and synthesize 39 field studies from five continents that directly measured the crop pollination services provided by non-bees, honey bees, and other bees to compare the relative contributions of these taxa. Non-bees performed 25–50% of the total number of flower visits. Although non-bees were less effective pollinators than bees per flower visit, they made more visits; thus these two factors compensated for each other, resulting in pollination services rendered by non-bees that were similar to those provided by bees. In the subset of studies that measured fruit set, fruit set increased with non-bee insect visits independently of bee visitation rates, indicating that non-bee insects provide a unique benefit that is not provided by bees. We also show that non-bee insects are not as reliant as bees on the presence of remnant natural or seminatural habitat in the surrounding landscape. These results strongly suggest that non-bee insect pollinators play a significant role in global crop production and respond differently than bees to landscape structure, probably making their crop pollination services more robust to changes in land use. Non-bee insects provide a valuable service and provide potential insurance against bee population declines

Pollinators and Insect Pollination in Changing Agricultural Landscapes

Agricultural intensification and associated loss of semi-natural grasslands, has resulted in reduced landscape heterogeneity. A concomitant loss of forage and nest sites for pollinating insects has driven declines across several pollinator taxa. Because 75% of the cultivated crops and more than 85% of the flowering plant species are insectpollinated, concerns have been raised that the pollinator declines will have consequences for human food supply and plant conservation. Several instances (e.g. the UN and EU) have reacted and initiatives are taken at different levels to increase floral resources for pollinators and also to benefit beekeeping. In order to restore populations of wild pollinators and protect their valuable functions, it is important to understand the complexity of plant-pollinator interactions. In this thesis, I assessed how some factors related to agriculture affect pollinators and pollination, mainly focusing on wild plant andpollinator species. One study was performed by compiling data from several native plant species across Europe. For three additional studies, I collected field data. I found that a lack of landscape heterogeneity can reduce the number of flowervisiting pollinators in European semi-natural habitats (including semi-natural grasslands and semi-open bushlands) and result in competition for floral resources between managed honey bees and wild bumble bees. There was no general effect of landscape heterogeneity on the reproductive success of insect-pollinated plants in European agricultural landscapes. Instead, the field studies suggested that landscape heterogeneity modifies the impact of other factors. For example, the cultivation of oilseed rape benefitted the reproductive success of one later-flowering study species, but not another, and only in landscapes with low heterogeneity. Also, sown flower strips, which are used to supply pollinators with floral resources, benefitted the reproductive success of adjacent plants in landscapes with low heterogeneity, but had a negative impact on the reproductive success of adjacent plants in more heterogeneous landscapes. As many previous studies, this thesis highlights the importance of preserving landscape heterogeneity and semi-natural grasslands in order to protect viable pollinators populations. Although I found no general impact of landscape heterogeneity on plant reproductive success, the persistence of insect-pollinated plants will largely depend on pollinator availability, and thereby on maintained semi-natural grasslands. The maintenance of semi-natural grasslands and landscape heterogeneity in general are important for wild pollinators and can reduce the negative consequences on their populations from competition with managed honey bees. The restoration and maintenance of these habitats may be particularly important in heterogeneous landscapes, where they are particularly species rich and the opportunity cost for the farmers is lower than in homogeneous landscapes. In homogeneous landscapes, where semi-natural grasslands are scarce and more difficult to maintain because of a lack of animal husbandry and a high opportunity cost, sown flower strips can be used to benefit common ecosystem service providing organisms and to some extent the pollination of wild plants

Red_clover

This file contains data used for the main analysis on the proportional seed set of red clover (per inflorescence), including landscape data. The proportions of oilseed rape and semi-natural grassland refer to landscape sectors with 1 km radius, surrounding the pots where reproductive success was measured

Strawberry weight

This file contains data used for the main analysis on strawberry weight, including landscape data. It also contains the data that were used in Appendix D. The proportions of oilseed rape and semi-natural grassland refer to landscape sectors with 1 km radius, surrounding the pots where reproductive success was measured

Strawberry_achenes

This file contains data used for the main analysis on the proportion of developed strawberry achenes, including landscape data. The proportions of oilseed rape and semi-natural grassland refer to landscape sectors with 1 km radius, surrounding the pots where reproductive success was measured

Data from: The relation between oilseed rape and pollination of later flowering plants varies across plant species and landscape contexts

Increasing cultivation of oilseed rape may have consequences for pollinators and wild plant pollination. By providing pollinating insects with pollen and nectar, oilseed rape benefits short-tongued, generalist insect species. Long-tongued bumble bee species, specialized to other flower types, may instead be negatively affected by increased competition from the generalists (e.g. due to nectar-robbing of long-tubed flowers) after oilseed rape has ceased flowering. We expected that the increased abundance of short-tongued pollinators and reduced abundance of long-tongued bumble bees in landscapes with a high proportion of oilseed rape would impact the pollination of later flowering wild plant species. In addition, we expected contrasting effects on plants pollinated by short-tongued pollinators and those pollinated by long-tongued bumble bees. We predicted that semi-natural grasslands, which provide insects with alternative floral resources, would reduce both negative and positive effects on pollination by mitigating competition between pollinators. In 16 semi-natural grasslands, surrounded by agricultural landscapes, with a variation in both the proportion of oilseed rape and the proportion of semi-natural grassland within 1 km, we studied reproductive output in two species of potted plants with different pollination strategies: the woodland strawberry (Fragaria vesca) and red clover (Trifolium pratense). The first species is mainly pollinated by short-tongued pollinators, e.g. hoverflies and solitary bees, and the latter by long-tongued bumble bees. Both species flowered after oilseed rape. Strawberry weight was higher in landscapes with a high proportion of oilseed rape, but only in landscapes with a low proportion of semi-natural grassland. The proportion of developed achenes was also positively related to the proportion of oilseed rape, but only during the latest flowering period. In contrast, red clover seed set was unrelated to the proportion of oilseed rape. Whereas the discrepancy between the two strawberry measurements calls for further research, this study suggests that oilseed rape can affect later flowering plants and that the impact differs among species