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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 âpollinator-friendlyâ 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
On the floral rewards and flower-visitor assemblages of annual urban flower meadow seed mixes
Flower seed mixes are increasingly used to enhance the biodiversity
and amenity values of urban green spaces. Urban or âpictorialâ flower seed
mixes are often used because they are designed using cultivars and non-native
species to provide more colourful and longer-lasting flower displays.
Although these seed mixes are effective in providing a high density of large
colourful flowers, over an extended season, their value for biodiversity, and
in particular the floral rewards they provide for flower-visitors, is largely
unknown. The overall aim of my thesis was to assess and improve the value
of these new urban habitats as forage resources for flower-visiting insects.
My approach was to quantify and compare floral reward provision
and insect visitation between meadows grown from three exemplar
commercial pictorial flower meadow seed mixes (called Marmalade Annual,
Short Annual and Cornfield Annual). I also compared these standard
commercial mixes with corresponding ânectar-enrichedâ formulations, which
were designed by increasing the proportional seed weight contribution of
selected species predicted to produce high quantities of nectar within each
mix. To compare floral rewards and visitation between meadows grown
from these seed mixes, I set up a field experiment in Sheffield, UK, using a
complete randomised block design with six replicate blocks, each with six 25
m2 plots sown with one of the six seed mix treatments.
My first objective was to quantify the floral nectar and pollen rewards
provided by each flowering species recorded in the meadows (on the scale of
a single flower or inflorescence). My second objective was to use these data
to quantify the floral rewards provided per unit area by replicate meadows
of different seed mix treatments, testing whether enrichment of seed mixes is an effective method of increasing floral nectar sugar rewards. My third
objective was to corroborate/correct my morphology-based flower-visitor
identifications using DNA barcoding to screen for misidentifications and
morphologically cryptic species. I then used these DNA barcode-based
identifications to assess whether there are systematic biases in the structure
of flower-visitor networks constructed using molecular taxon identifications
compared to traditional morphology-based taxon identifications. My fourth
objective was to quantify patterns of insect visitation to meadows, testing
whether meadows of different seed mix types attract different flower-visitor
assemblages.
Meadow floral composition surveys revealed that contamination by
unintended horticultural species was widespread across replicate seed mix
treatments, with contaminants likely germinating from a seed bank laid
down during a failed attempt at this experiment the previous year.
Contamination particularly affected Marmalade mixes, mainly because the
common contaminant species were often also components of the Short and
Cornfield mixes. For example, contaminants contributed on average about a
third of nectar sugar mass or pollen volume per unit area in Marmalade mix
meadows. Hence, contamination fundamentally undermined the internal
validity of seed mix treatments, reducing the ability to directly attribute
meadow level patterns in floral rewards or flower-visitors to seed mixes. As
result, examination of patterns of floral resource provision and insect
visitation were more informative at a species scale.
In terms of patterns of insect visitation, Centaurea cyanus received 91%
of bumblebee visits, 88% of honeybee visits and 29% of hoverfly visits, whilst
T. inodorum received 27% of hoverfly visits. Patterns of bumblebee and honeybee visitation indicated preferential visitation to floral units of
Centaurea cyanus. Although this species produced high quantities of nectar
sugar mass and pollen volume, this did not differentiate it from other
Asteraceae, such as Glebionis segetum, Rudbeckia hirta and Coreopsis tinctoria,
which all produced high quantities of both floral rewards. Hence, it is likely
that floral traits not measured in this study, such as nectar accessibility
(ânectar-holder depthâ) or concentration/volume characteristics (which can
affect accessibility due to constraints imposed by feeding morphology),
drove patterns of preferential visitation in bumblebees and honeybees to C.
cyanus. Given that in the absence of contamination there would have been
very few bumblebee or honeybee visitors to Marmalade mix meadows,
aesthetically designed pictorial meadows can fail to jointly provide benefits
for people and some important flower-visiting insect taxa.
DNA barcoding did not change specimen identifications for most
morphotaxa. However, splitting and/or lumping processes affected almost
one third of morphotaxa, with lumping of morphotaxa the most common
type of change. This was in part because males and females from sexually
dimorphic species were often separated by morphological identification.
These DNA barcode-based changes to visitor taxonomy resulted in
consistent minor changes in network size and structure across replicate
networks. Lumping of morphotaxa decreased taxon richness, reducing the
number of unique links and interaction diversity (the effective number of
links). Lumping also increased flower-visitor generality, reducing plant
vulnerability and increasing overall network connectance. However,
taxonomic changes had no effect on interaction evenness or network
specialisation. Thus, for this well-studied fauna, DNA barcode-based flower-visitor
networks were systematically biased toward fewer taxa and links, with more generalist visitors and specialist plants. Given that many tropical
faunas have more species and are less described than in Britain this pattern
may not be replicated in other studies. Further studies in contrasting plant-pollinator
communities are required before generalisations can be made
about systematic biases between networks constructed using morphological
versus molecular data.
Overall, meadows grown from annual pictorial flower meadow seed
mixes provide abundant floral units per unit area of meadow and are a
valuable alternative to traditional horticultural flower beds or amenity
grasslands in high profile urban contexts. Nevertheless, care must be taken
during design of seed mixes and selection of mixes for planting to ensure
that species in the mix provide suitable floral resources for an array of
flower-visitors, including bees. This would be aided by the integration of
informative measures for candidate species of floral rewards or visitor types
and visitation rates during seed mix design