There is considerable concern over declines in insect pollinator communities and potential impacts on the pollination of crops and wildflowers1,2,3,4. Among the multiple pressures facing pollinators2,3,4, decreasing floral resources due to habitat loss and degradation has been suggested as a key contributing factor2,3,4,5,6,7,8. However, a lack of quantitative data has hampered testing for historical changes in floral resources. Here we show that overall floral rewards can be estimated at a national scale by combining vegetation surveys and direct nectar measurements. We find evidence for substantial losses in nectar resources in England and Wales between the 1930s and 1970s; however, total nectar provision in Great Britain as a whole had stabilized by 1978, and increased from 1998 to 2007. These findings concur with trends in pollinator diversity, which declined in the mid-twentieth century9 but stabilized more recently10. The diversity of nectar sources declined from 1978 to 1990 and thereafter in some habitats, with four plant species accounting for over 50% of national nectar provision in 2007. Calcareous grassland, broadleaved woodland and neutral grassland are the habitats that produce the greatest amount of nectar per unit area from the most diverse sources, whereas arable land is the poorest with respect to amount of nectar per unit area and diversity of nectar sources. Although agri-environment schemes add resources to arable landscapes, their national contribution is low. Owing to their large area, improved grasslands could add substantially to national nectar provision if they were managed to increase floral resource provision. This national-scale assessment of floral resource provision affords new insights into the links between plant and pollinator declines, and offers considerable opportunities for conservation.
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This research was supported by the UK Insect Pollinators Initiative (IPI) ‘AgriLand: Linking agriculture and land use change to pollinator populations’ project, funded by the Biotechnology and Biological Sciences Research Council (BBSRC), Wellcome Trust, Scottish Government, Department of Environment, Food and Rural Affairs (DEFRA) and Natural Environment Research Council (NERC) under the auspices of the Living with Environmental Change partnership: grant BB/H014934/1 (http://www.agriland.leeds.ac.uk). Land Cover and Countryside Survey data are owned by NERC – Centre for Ecology & Hydrology (http://www.countrysidesurvey.org.uk).
The authors declare no competing financial interests.
The floral resource database will be made available from the NERC Environmental Information Data Centre (http://dx.doi.org/10.5285/69402002-1676-4de9-a04e-d17e827db93c and http://dx.doi.org/10.5285/6c6d3844-e95a-4f84-a12e-65be4731e934).
Extended data figures and tables
a, Box plots of log10 (x + 1) nectar productivity according to the location of the vegetation surveyed (area (nonlinear) versus linear features) in each habitat. b, Box plots of species nectar diversity according to the location of the vegetation surveyed (area versus linear features) in each habitat. c, Box plots of functional nectar diversity according to the location of the vegetation surveyed (area versus linear features) in each habitat. Significant differences of locations (area versus linear features) in habitats are indicated by asterisks as follows: *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001. Statistical models were re-run without calcareous grassland habitat (to meet residuals homoscedasticity constraint) in order to check that significant effects remained. See Extended Data Table 1 for ANOVA results. See ‘Statistical analyses’ section of the Methods for detailed statistical methods and definition of box plot elements.
Extended Data Figure 2 Historical changes in nectar productivity and diversity per habitat over recent decades (1978 to 2007).
a, Box plots of log10 (x + 1) nectar productivity per habitat, based on vegetation data for 1978, 1990, 1998 and 2007. b, Box plots of species nectar diversity per habitat, based on vegetation data for 1978, 1990, 1998 and 2007. c, Box plots of functional nectar diversity per habitat, based on vegetation data for 1978, 1990, 1998 and 2007. Significant differences of time periods per habitats are indicated by asterisks (*P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001). See Extended Data Table 1 for ANOVA results and Supplementary Table 3 for sample sizes. See ‘Statistical analyses’ section of the Methods for detailed statistical methods and definition of box plot elements.
Extended Data Figure 3 Habitat contributions to the national nectar provision shifts and species contributions to habitats over recent decades (1978 to 2007).
a–c, Habitat contributions to the national nectar provision changes from a, 1978 to 1990, b, 1990 to 1998, and c, 1998 to 2007. All bar plots represent the absolute changes (in 1 × 106 kg of sugars) for each habitat during the time period considered. Numbers in brackets indicate the relative changes (in %). d–n, Species contributions to nectar provision in 1978, 1990, 1998 and 2007 per habitat type. Only species that contribute to the first 90% are shown. See Supplementary Table 10 for main contributing species to the national changes from 1978 to 2007.
Extended Data Figure 4 Sensitivity analyses of historical trends from 1978 to 2007 in nectar productivity and species diversity with alternative data sets.
a, b, Box plots of log10 (x + 1) nectar productivity (a) and box plots of species nectar diversity per habitat (b) based on vegetation data for 1978, 1990, 1998 and 2007 discounting the contribution of grazed white clover in improved grassland. c, d, Box plots of log10 (x + 1) nectar productivity (c) and box plots of species nectar diversity per habitat (d), based on vegetation data for 1978, 1990, 1998 and 2007 and computed with the alternative rectangular phenology function. e, f, Box plots of log10 (x + 1) nectar productivity (e) and box plots of species nectar diversity per habitat (f), based on vegetation data for 1978, 1990, 1998 and 2007 and computed considering only the species with empirical nectar values. Significant differences of time periods per habitat are indicated by asterisks (*P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001). See Supplementary Table 3 for sample sizes and Supplementary Information for details. See ‘Statistical analyses’ section of the Methods for detailed statistical methods and definition of box plot elements.
Extended Data Figure 5 Historical timeline in changes in nectar resources and flower-visiting insects in Great Britain.
Historical periods with the greatest negative changes in nectar resources (in percentage change in England and Wales (E&W) and Great Britain (GB)) and flower-visiting insects (in percentage change or in species extinction per decade (sp./dec.)) are indicated in red, those with intermediate changes are in orange, and those with the lowest (or even reversing) changes are in green. Main historical trends from this study (Baude et al.) are presented in regard to those described in Carvalheiro et al.10 and Ollerton et al.9 studies. The white chevron indicates a provisional extinction rate that needs to be confirmed on a 20 year period of time (see Supplementary Information from Ollerton et al.9).
a, Major axis linear regression of log10 (x + 1) nectar values per flower obtained in the second location against those obtained in the first one. b, Major axis linear regression of log10 (x + 1) flower density values obtained in the second location against those obtained in the first one. c, Major axis linear regression of log10 (x + 1) peak flower density values obtained in the second location against those obtained in the first one. d, Standardized major axis regression of the log10 (x + 1) length of the flowering period used for analyses with those derived from IPI AgriLand floral transects (unpublished data). e, Standardized major axis regression of peak date of flowering season used for analyses with those derived from IPI AgriLand floral transects (unpublished data). f, Major axis linear regression performed on the log10 (x + 1) empirical (empirical data set) and published nectar values (literature data set from Raine and Chittka40) at the flower scale. g, Standardized major axis linear regression performed on the log10 (x + 1) empirical (empirical data set) and published nectar values (literature data set, see Supplementary Table 13 for references) at the vegetative scale. h, Standardized major axis linear regression performed on the log10 (x + 1) empirical and modelled nectar values generated by a leave-one-out approach. Estimated values and 95% confidence intervals (solid and dashed lines, respectively) of all equations are derived from (standardized) major axis regression (ma and sma function from ‘smatr’ package in R36; see Supplementary Information for details).
Linear regressions between the number of open flowers counted in a quadrat of 0.5 m2 according to the vegetative cover of the focus species in the quadrat (in %). Data are extracted from IPI AgriLand floral transects survey in 2012 (unpublished data) for 23 out of the 35 main nectar contributing species (panels a–w). The number of flowers was analysed according to the vegetative cover (‘Cover’), the month of the survey (‘Month’) and the interaction between these two terms (‘Cover:Month’) using negative binomial generalized linear models (see Supplementary Information for details). Coloured lines represent the linear regression between flower abundance and vegetative cover for each month of the survey. Black lines represent the overall linear regression between flower abundance and vegetative cover when the ‘Month’ covariate cannot be included in the model. Line equations were derived from statistical intercept and slope estimates.
This file contains Supplementary Methods, Supplementary Results, a Supplementary Discussion, Supplementary Figure 1, Supplementary Tables 1-10 and Supplementary references - see contents page for further details. (PDF 3572 kb)
This file contains Supplementary Table 11, which shows plant traits, flowering phenology, flower density, nectar productivity at the flower scale and nectar sugar productivity at the vegetative scale for the list of 260 species. (XLSX 74 kb)
This table contains a reference list for flower-visiting insects of the four main nectar providers nationally. It lists sources and data used to investigate the visiting insects of the main nectar providing plant species (Extended Data Table 2). This combines published and unpublished plant-pollinator interactions data from Memmott’s group and a review of literature of insect species visiting flowers of Trifolium repens, Calluna vulgaris, Cirsium palutre and Erica cinerea. (XLSX 26 kb)
This table contains a reference list for published sugar potential values in kg/ha /year. It lists sources and data used to compare our nectar values (in kg/ha cover/year) to those found in literature (Extended Data Figure 6g). Published values of sugar potential are available for 128 species at the time of writing. Where values were available from more than one source, an average was calculated. Where values were given only as honey potential in the literature, these values were multiplied by 0.8 to give sugar potential. This ratio has been reported in the majority of the published sources (References 1, 5-17). (XLSX 16 kb)
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Baude, M., Kunin, W., Boatman, N. et al. Historical nectar assessment reveals the fall and rise of floral resources in Britain. Nature 530, 85–88 (2016). https://doi.org/10.1038/nature16532
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