DOI: 10.5061/dryad.v41ns1s5j
title: Gardens reduce seasonal hunger gaps for farmland pollinators

description: Gardens can benefit pollinators living in surrounding farmland landscapes,
 but the reason for their value is not clear. Gardens are no different from
 many semi-natural farmland habitats in terms of the quantity of floral
 resources (pollen and nectar) they produce, but the timing of their
 resource supply is very different, which may explain their value. We show
 that gardens provide 15% of overall annual nectar in farmland landscapes
 in Southwest UK, but between 50% and 95% during early-spring and
 late-summer when farmland supplies are low. Gardens can therefore reduce
 seasonal nectar gaps experienced by farmland bumblebees. Consistent with
 this pattern, bumblebee activity increased in gardens relative to farmland
 during early spring and late summer. An agent-based model reinforces this
 point, showing that timing, not quantity, of garden nectar supply enhances
 bumblebee colony growth and survival in farmland. We show that over 90% of
 farmland in Great Britain is within one kilometre of a garden and
 therefore positive actions by gardeners could have widespread spillover
 benefits for pollinators across the country. Given the widespread
 distribution of gardens around the world, we highlight their important
 interplay with surrounding landscapes for pollinator ecology and
 conservation.
description: There are four datasets in this Dryad submission which are listed
 below along with information on how they were collected and
 processed: <strong>1)
 Garden_nectar_production</strong> How it was
 collected: Garden floral abundance was
 recorded during 2019 in 59 urban gardens in the city of Bristol, South
 West UK. Although gardens were sampled in a different year to farmland,
 the mean annual temperature, rainfall and growing season of these two
 years were similar and previous data from this region show a similar
 pattern of nectar phenology between years (Timberlake <em>et
 al</em>. 2019). Residential gardens included the land adjacent to
 and associated with each domestic property (both front and back gardens),
 irrespective of whether it was paved, vegetated, or used as a driveway. In
 total, gardens ranged in size from 31.3 m<sup>2</sup> to 407.7
 m<sup>2</sup> (mean 156.4 m<sup>2</sup> ±12.7 SE)
 and their selection was stratified by both geographical location and
 neighbourhood income (see Tew<em> et al. </em>2022 for further
 details). From March to October 2019, each garden was visited once per
 calendar month to record floral abundance by counting the number of open
 floral units of each flowering plant species within or directly above each
 garden. In contrast to farmland habitats, sampling along a 50 m transect
 is not practical in residential gardens, which exist as small discrete
 patches of a single land use type. Instead, floral units were either
 counted individually using a handheld tally counter or estimated by
 sub-sampling (e.g. for flowering shrubs and trees and flower-rich lawns).
 The floral unit values were then multiplied by the mean floral sugar
 production of each species to obtain an estimate of the grams of sugar per
 unit area per 24-hour period for each garden. Values for the nectar sugar
 production of garden species were from Baude<em> et al.</em>
 (2016), Hicks et al. (2016) and Tew<em> </em>et al. (2021,
 2022) who quantified the sugar production of all 636 flowering plant
 species present in these gardens using the methods described in the
 previous section. Variation in garden nectar supply stabilises at the
 point of around 20 gardens (Tew <em>et al. </em>2022),
 suggesting that 59 gardens is a sufficient sample from which to
 characterise the average nectar profile for Bristol gardens. Furthermore,
 values of average garden nectar production show no significant differences
 between UK cities (Tew et al. 2021) suggesting the patterns recorded in
 our study are likely to be broadly representative of gardens at a national
 scale. How it was
 processed: For each study garden on each
 sampling occasion (monthly), values for the nectar sugar production of all
 species are summed and divided by the area of garden (in m2) to give the
 total nectar sugar production of the garden in grams per m2 per day. A
 separate column shows the number of square metres of garden that are
 typically present in a 1km2 area of farmland landscape so that values for
 each garden can be scaled up to predict how much sugar they would produce
 in a typical farmland landscape (see paper methods for more
 information). <strong>2)
 Farm_garden_nectar_production</strong> How it was
 collected: Raw values of nectar sugar
 production per m2 of farmland (from Timberlake et al. 2019) and garden
 habitats (from Tew et al. 2022; see dataset above) in units of grams of
 nectar sugar/m<sup>2</sup>/day allowed us to characterise the
 phenology of nectar supply in high resolution for each landcover and to
 compare between them. A generalised additive model (GAM) was used to model
 this smooth, non-linear trend in nectar availability over time for each
 landcover (gardens and farmland). A thin-plate regression spline was used
 to model day of the year, with the degree of smoothing selected using the
 default generalised cross-validation method (Wood 2011). 
 How it
 was processed: The outputs from these
 models (grams of nectar sugar/m<sup>2</sup>/day) were
 multiplied by the area of each habitat in a 1 km<sup>2</sup>
 area of farmland landscape (including gardens which comprised 1.9% ±0.01
 SE of our study landscapes), to compare their contributions to
 landscape-level nectar supply.  <strong>3)
 BeeWalk_data_South-West</strong> How it was
 collected: To establish seasonal patterns
 of bumblebee activity in farmland and gardens, we analysed occurrence
 records of four common farmland bumblebee species (<em>Bombus
 terrestris, B. pascuorum, B. lapidarius</em> & <em>B.
 pratorum</em>) from BeeWalk transect data collected between 2008 and
 2019 (Comont 2020). BeeWalk is a standardised monitoring scheme which
 collects data on the abundance and distribution of bumblebees in Britain.
 Fixed transects with clearly defined habitat composition are
 systematically surveyed by trained volunteer recorders once per month from
 March-October. How it was
 processed: We included only records from
 the South West region of the UK (counties of:  Bristol, Cornwall, Dorset,
 Devon, Gloucestershire, Somerset and Wiltshire) to minimise any
 confounding effects of latitudinal differences in phenology. Records were
 filtered to include transect sections whose primary habitat designation
 was strictly related to either farmland or garden. Any transect sections
 classified as primarily agricultural but secondarily garden (99 records in
 total) were removed to avoid ambiguity; this left us with a total of 493
 km of purely farmland transect sections and 168 km of garden-associated
 transect sections. For each of the two habitat types (i.e., farmland and
 garden), we calculated the total number of worker and queen bumblebees
 recorded in each month of the year for each of the four bumblebee species
 (hereafter referred to as bumblebee activity). Male bumblebees were
 excluded from the analysis as their foraging patterns and phenology of
 activity are different from those of females. Total observations for each
 species were divided by the total length of transect recorded in each
 habitat type so that results were scaled by sampling effort. Activity
 values were therefore expressed as the mean number of bees recorded per
 kilometre of transect. For each month of the year, we divided bumblebee
 activity values in gardens by activity values in farmland (i.e. the ratio
 of garden: farmland bumblebee activity) to identify which periods of the
 year bumblebees were relatively more active in gardens, and vice
 versa. <strong>4)
 BEE-STEWARD_RESULTS</strong> How it was
 collected: To test the influence of garden
 nectar supply on farmland bumblebee population dynamics and disentangle
 the effects of resource quantity (how <em>much</em> nectar)
 from the effects of resource phenology (<em>when</em> the
 nectar is available) we ran an in-silico experiment using the agent-based
 model ‘BEE-STEWARD’ developed by Twiston‐Davies<em> et
 al.</em> (2021) implemented in <em>Netlogo</em>. The
 model simulates the foraging behaviour, life history and colony growth of
 bumblebees foraging for nectar and pollen in a temporally-dynamic,
 spatially explicit, user-defined landscape. Bee foraging activity is
 modelled based on the detection probability and attractiveness of
 individual forage patches (influenced by their size and distance from nest
 sites), flower handling time (affected by corolla depth and resource
 depletion) and sugar concentration (this can be defined by the user).
 Colony growth is an emergent property of floral resource acquisition (as
 well as other environmental factors) and is based on empirical data (see
 Fig. S6 for an overview of model structure). BEE-STEWARD, and related
 models, have been widely used in pollination ecology research as they
 enable users to alter the spatial, temporal and nutritional features of
 floral resources (as well as other aspects of the environment) and
 investigate the resulting impacts on bee colony dynamics (see
 https://beehave-model.net/publications/). Our model
 landscapes were based on 12 real circular farmland landscapes with a 1 km
 radius (3.14 km<sup>2 </sup>area) in Southwest UK which were
 characterised and mapped by Timberlake<em> et al.</em> (2021)
 (Fig. S2 & S7). Each of these study landscapes had at least some
 garden present, ranging from 0.2 – 5.9% coverage, with a mean of 1.9%
 ±0.01 SE (Table S1). The quantity and phenology of nectar supply in each
 habitat was based on empirical data from our study (see methods
 above). In each of the 12 model landscapes we ran four
 alternative simulations to test the importance of gardens. These were: 1)
 gardens present as normal; 2) gardens removed completely; 3) gardens
 replaced with pasture (the most common land use in this region, which
 could also be considered an analogue of a simple lawn garden); and 4)
 gardens replaced with an alternative landcover of identical total floral
 resource value but with the phenological pattern of pasture (i.e., the
 same total resources but distributed differently through the year), to
 test the role of phenology <em>per se</em>. All other features
 of the landscape, including pollen availability and nesting site
 availability, were kept identical amongst treatments. In real landscapes,
 there will inevitably be other population-limiting factors such as pollen
 availability, nesting sites, predation and weather. However, we focus only
 on the impact of nectar availability in otherwise benign conditions.
 Experiments were run with <em>Bombus terrestris</em> only and
 began with 500 queens, as is the default for BEE-STEWARD. Mean emergence
 date and standard deviation were kept consistent between all sites, using
 default values from BEE-STEWARD (01 April ±28 days SD).
 We selected five different population response variables to
 provide us with a broad view of the mechanisms driving population change.
 These were: 1) colony density throughout the flight season (recorded on
 the 15<sup>th</sup> day of each month); 2) maximum number of
 colonies produced per year; 3) total number of bees produced per year; 4)
 number of new queens produced per year; and 5) percentage spring queen
 survival.  How it was
 processed: The model was run 20 times for
 each treatment-farm combination (using a different random seed in each
 case), and for each simulation run we calculated the mean value for each
 response variable over the first five years of the simulation. The great
 majority of populations persisted longer than five years, so five years
 was taken as a cut-off for the period of relative population
 stability. 
description: # Gardens reduce seasonal hunger gaps for farmland pollinators
 [https://doi.org/10.5061/dryad.v41ns1s5j](https://doi.org/10.5061/dryad.v41ns1s5j) ## Description of the data and file structure The data included in this Dryad submission were used to answer the following questions: 1) Do gardens fill seasonal gaps in the resource supply of farmland landscapes?; 2) Do pollinators increase their use of gardens during gaps in farmland resource availability?; and 3) Do bumblebees respond more strongly to changes in the timing than the total quantity of garden resources? Using bumblebees as a model group, and Southwest UK as a study region, we test these three hypotheses with a combination of empirical field data and agent-based-modelling. **Question 1: Do gardens fill seasonal gaps in the resource supply of farmland landscapes?** To determine whether floral resources provided by gardens have the potential to fill seasonal gaps in the food supply of farmland pollinators, we quantified the nectar production of three farmland landscapes in Southwest UK throughout an entire flowering season and calculated the additional nectar provided by small clusters of rural gardens within these landscapes. **Question 2: Do pollinators increase their use of gardens during gaps in farmland resource availability?** The purpose of this analysis was to establish whether there is a seasonal pattern of bumblebee activity in garden versus farmland habitats and to assess whether abundance in gardens increases during periods of the year when garden resources are relatively more abundant. To establish seasonal patterns of bumblebee activity in farmland and gardens, we analysed occurrence records of four common farmland bumblebee species (*Bombus terrestris, B. pascuorum, B. lapidarius* & *B. pratorum*) from BeeWalk transect data collected between 2008 and 2019 (Comont 2020). BeeWalk is a standardised monitoring scheme which collects data on the abundance and distribution of bumblebees in Britain. Fixed transects with clearly defined habitat composition are systematically surveyed by trained volunteer recorders once per month from March-October. **Question 3: Do bumblebees respond more strongly to changes in the timing than the total quantity of garden resources?** To test the influence of garden nectar supply on farmland bumblebee population dynamics and disentangle the effects of resource quantity (how *much* nectar) from the effects of resource phenology (*when* the nectar is available) we ran an in-silico experiment using the agent-based model ‘BEE-STEWARD’ developed by Twiston‐Davies* et al.* (2021) implemented in *Netlogo*. ### Files and variables #### File: BeeWalk\_data\_South-West.xlsx **Description:** Dataset showing the summarised values of all worker and queen bumblebees recorded in farmland and garden transects of the Beewalk Scheme in Southwest UK. ##### Variables * The file contains a metadata sheet called 'Info' which lists each of the variables and explains them in detail #### File: BEE-STEWARD\_RESULTS.xlsx **Description:** Dataset showing the results of the in-silico experiment conducted using the using the agent-based model ‘BEE-STEWARD’ developed by Twiston‐Davies et al. (2021) implemented in Netlogo (see methods for more information). ##### Variables * The file contains a metadata sheet called 'Info' which lists each of the variables and explains them in detail #### File: Farm\_garden\_nectar\_production.xlsx **Description:** Estimated values of daily nectar production in farmland habitats and gardens for each day of the year. Values are predictions from the generalised additive model which modelled the smooth non-linear trend in the nectar production of each habitat over time, based on raw values. ##### Variables * The file contains a metadata sheet called 'Info' which lists each of the variables and explains them in detail #### File: Garden\_nectar\_production.xlsx **Description:** Measured values of daily nectar production in each study garden during each month of the year, expressed as grams of sugar per m2 per day ##### Variables * The file contains a metadata sheet called 'Info' which lists each of the variables and explains them in detail ## Code/software Microsoft Excel (or equivalent software) is required to view these .XLS files. No code or scripts are included in this submission ## Access information Other publicly accessible locations of the data: * NA Data was derived from the following sources: * Timberlake, T.P., Vaughan, I.P. and Memmott, J., 2019. Phenology of farmland floral resources reveals seasonal gaps in nectar availability for bumblebees. *Journal of Applied Ecology*, 56(7), pp.1585-1596. [https://doi.org/10.1111/1365-2664.13403](https://doi.org/10.1111/1365-2664.13403) * Tew, N.E., Baldock, K.C., Vaughan, I.P., Bird, S. and Memmott, J., 2022. Turnover in floral composition explains species diversity and temporal stability in the nectar supply of urban residential gardens. Journal of Applied Ecology, 59(3), pp.801-811. [https://doi.org/10.1111/1365-2664.14094](https://doi.org/10.1111/1365-2664.14094)