Food for pollinators: quantifying the nectar and pollen resources of urban flower meadows

Planted meadows are increasingly used to improve the biodiversity and aesthetic amenity value of urban areas. Although many ‘pollinator-friendly’ seed mixes are available, the floral resources these provide to flower-visiting insects, and how these change through time, are largely unknown. Such data are necessary to compare the resources provided by alternative meadow seed mixes to each other and to other flowering habitats. We used quantitative surveys of over 2 million flowers to estimate the nectar and pollen resources offered by two exemplar commercial seed mixes (one annual, one perennial) and associated weeds grown as 300m2 meadows across four UK cities, sampled at six time points between May and September 2013. Nectar sugar and pollen rewards per flower varied widely across 65 species surveyed, with native British weed species (including dandelion, Taraxacum agg.) contributing the top five nectar producers and two of the top ten pollen producers. Seed mix species yielding the highest rewards per flower included Leontodon hispidus, Centaurea cyanus and C. nigra for nectar, and Papaver rhoeas, Eschscholzia californica and Malva moschata for pollen. Perennial meadows produced up to 20x more nectar and up to 6x more pollen than annual meadows, which in turn produced far more than amenity grassland controls. Perennial meadows produced resources earlier in the year than annual meadows, but both seed mixes delivered very low resource levels early in the year and these were provided almost entirely by native weeds. Pollen volume per flower is well predicted statistically by floral morphology, and nectar sugar mass and pollen volume per unit area are correlated with flower counts, raising the possibility that resource levels can be estimated for species or habitats where they cannot be measured directly. Our approach does not incorporate resource quality information (for example, pollen protein or essential amino acid content), but can easily do so when suitable data exist. Our approach should inform the design of new seed mixes to ensure continuity in floral resource availability throughout the year, and to identify suitable species to fill resource gaps in established mixes

Non-metric multidimensional scaling (NMDS) analyses of changes in the composition of floral meadows through time and among cities.

<p>A. Perennial meadows across all cities, separated by survey round. B. Annual meadows across all cities, separated by survey round. In A and B, letters and ellipses show the centroid and 95% confidence limits for each survey round. C. Perennial meadows across all survey rounds, separated by city. D. Annual meadows across all survey rounds, separated by city. In C and D, letters and ellipses show the centroid and 95% confidence limits for each city. In D. the centroid for Bristol is hidden by, and almost identical to, the centroid for Leeds.</p

Seasonal patterns in nectar sugar availability across four UK cities for meadows sown with A. perennial, B. annual A1, and C. annual A2 treatments, assessed at three-week intervals in 2013.

<p>Values at each time point are means based on 7x 1m² quadrats, with dark shading showing 95% confidence limits. Plots for individual sites are shown for Bristol, Reading and Leeds in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158117#pone.0158117.s003" target="_blank">S3 Fig</a>.</p

Mean pollen volume per 24h per floral unit for species in A. the annual seed mix, B. the perennial seed mix, and C. native weeds in either mix.

<p>Values shown are ranked means in each group. Mean values and standard errors, longevity and pollen volume/floral unit are provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158117#pone.0158117.s011" target="_blank">S1 Table</a>). Images of the top ranked species in each group are shown, with the highest-ranked at right. Images are provided by the project team with the exceptions of <i>Chamerion angustifolium</i> (author: Ewan Cole) provided in 2016 under a (CCAL) CC BY 4.0 license by the Urban Flora of Scotland project.</p

Mean nectar sugar mass per 24h per floral unit for species in A. the annual seed mix, B. the perennial seed mix, and B. native weeds in either mix.

<p>Values shown are ranked means in each group (mean values and standard errors are provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158117#pone.0158117.s011" target="_blank">S1 Table</a>). Images of the top ranked species in each group are shown, with the highest-ranked at right. Images are provided by the project team with the exceptions of <i>Echium vulgare</i> (author: <a href="https://commons.wikimedia.org/wiki/User:Wsiegmund" target="_blank">Ewan</a> Cole) provided in 2016 under a (CCAL) CC BY 4.0 license from the Urban Flora of Scotland project.</p

Mean, median and quartile values for nectar sugar mass/m²/day for each meadow treatment type across all cities and surveys, showing significant difference groups (p < 0.05) by the post-hoc Tukey-Kramer-Nemenyi test.

<p>Mean, median and quartile values for nectar sugar mass/m²/day for each meadow treatment type across all cities and surveys, showing significant difference groups (p < 0.05) by the post-hoc Tukey-Kramer-Nemenyi test.</p

Seasonal patterns in pollen availability across four UK cities for meadows sown with A. perennial, B. annual A1, and C. annual A2 treatments, assessed at three-week intervals in 2013.

<p>Values at each time point are means based on 7x 1m² quadrats, with dark shading showing 95% confidence limits.</p

Seasonal patterns in daily nectar sugar availability for individual Edinburgh meadows sown with A. perennial, B. annual A1, and C. annual A2 treatments.

<p>Data were sampled at three-week intervals through 2013. Values at each time point are means based on 20x 1m² quadrats, with dark shading showing 95% confidence limits. The poor performance of the Saughton perennial meadow was due to accidental mowing, and this replicate was excluded from statistical analyses.</p

Seasonal patterns in daily pollen availability for individual Edinburgh meadows sown with A. perennial, B. annual A1, and C. annual A2 treatments.

<p>Data were sampled at three-week intervals through 2013. Values each time point are means based on 20x 1m² quadrats, with dark shading showing 95% confidence limits.</p