DOI: 10.5061/dryad.931zcrjjb
title: Bumblebee colony density on farmland is influenced by late-summer nectar supply and garden cover

description: 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.
description: 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 m<sup>2</sup>
 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<i> et al.</i>
 2016). A floral unit was defined as one or multiple flowers that can be
 visited by insects without flying (Carvalheiro<i> et al.</i>
 2008). Values for the nectar sugar production of each species were from
 Baude<i> et al.</i> (2016) and Timberlake<i> et
 al.</i> (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 <em>Bombus
 terrestris </em>individuals collected at 14 farms in the west of
 England during June and July 2018. Individuals were genotyped at 14
 <i>B. terrestris</i> microsatellite loci (see Appendix S3 and
 Table S2) following Dreier<i> et al.</i> (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).  
  
description: 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.