Bumblebee colony density on farmland is influenced by late-summer nectar supply and garden cover



1. Floral resources are important in limiting pollinator populations, but they are often highly variable across time and space and the effect of this variation on pollinator population dynamics is not well understood. The phenology (timing) of floral resources is thought to be important in structuring pollinator populations, but few studies have directly investigated this. 2. Our study quantifies the landscape composition, seasonal nectar and pollen supply, and Bombus terrestris colony density of 12 farms in southwest UK to investigate how landscape composition influences the phenology of floral resources and how both these factors affect colony density. We use this information in a spatially explicit predictive model to estimate the effect of different farmland management scenarios on seasonal resource supplies and colony density. 3. We find that farmland nectar supply during September is a strong predictor of B. terrestris colony density in the following year, explaining over half of all the variation in colony density; no other period of resource availability showed a significant association. Semi-natural habitat cover was not a good proxy for nectar or pollen supply and showed no significant association with colony density. However, the proportional cover of gardens in the landscape was significantly associated with colony density. 4. The predictive model results suggest that increasing the area of semi-natural flowering habitat has limited effect on bumblebee populations. However, improving the quality of these habitats through Environmental Stewardship and other management options is predicted to reduce the late-summer resource bottleneck and increase colony density. 5. Synthesis and Applications: Our results demonstrate the importance of considering the phenology of resources, rather than just total resource availability, when designing measures to support pollinators. Late-summer appears to be a resource bottleneck for bumblebees in UK farmland, and consequently management strategies which increase late-summer nectar availability may be the most effective. These include mowing regimes to delay flowering of field margins until September, planting late-flowering cover crops such as red clover, and supporting late-flowering wild plant species such as Hedera helix. Our results also suggest that rural gardens may play an important role in supporting farmland bumblebee populations.,The dataset was collected during 2017 and 2018 on 12 farms in the west of England. Floral abundance data was collected during four sampling periods (March, May, July and September). On each of the four sampling occasions, six 50 m transects were randomly placed in each of the sampled habitat types on each farm (i.e. 24 transects for a farm with all four habitat types). On each transect, the number of floral units of each forb species was recorded in 1 m2 quadrats at 5 m intervals along its entire length (i.e. 10 quadrats per transect). For trees and shrubs, all floral units in a 5 m vertical column above the quadrat were counted. Above this, the tree’s height within the vertical column was estimated with a clinometer and the floral abundance values were multiplied up accordingly (Baude et al. 2016). A floral unit was defined as one or multiple flowers that can be visited by insects without flying (Carvalheiro et al. 2008). Values for the nectar sugar production of each species were from Baude et al. (2016) and Timberlake et al. (2019); these were multiplied by the number of open flowers per square metre, per habitat, to provide an estimate of the grams of sugar per unit area per 24-hour period for each habitat. The same was done for the pollen volume of each species, though these values are not expressed as a daily rate as pollen is not replenished each day. Genotype data is from a total of 886 Bombus terrestris individuals collected at 14 farms in the west of England during June and July 2018. Individuals were genotyped at 14 B. terrestris microsatellite loci (see Appendix S3 and Table S2) following Dreier et al. (2014). PCR products were diluted and visualised on an ABI3730 Automated Capillary Sequencer (Applied Biosystems) using a ROX 500 size standard. Allele sizes were assigned using GeneMapper v4.1 and the genotype of each individual recorded (Appendix S4).,These datasets contain all the information required to use them including an information sheet in each file which explains each column of data. There are some missing values denoted by NA. Please contact the author if there are any further questions about the data.,
Date made available16 Dec 2020

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