Mind the gap: why flower timing matters to farmland pollinators

A new study by Thomas Timberlake et al. reveals seasonal ‘hunger gaps’ in farmland nectar supplies, which could be limiting pollinator populations. But does this offer an opportunity to devise more targeted and cost-effective conservation and agri-environment schemes for pollinators?

Nectar and pollen are crucial resources which give bees and other pollinators the energy and protein they need to fly around, reproduce and maintain their populations. It was concerning therefore when a recent study found that nectar resources in the UK have dropped by 32% since the 1930s. Larger fields with neater, more sterile edges, fewer hedgerows and the increasing use of pesticides mean there simply aren’t as many flowers around on farmland anymore. Agri-environment schemes have sought to reverse this decline by paying farmers to plant up margins with wildflowers and manage their hedgerows more carefully. These have gone some way towards recovering nectar levels across the country, yet many pollinators are still in decline. Could the timing of flowering be something to do with this? Our recent work endeavours to find out.

Timing is everything

Whilst agri-environment schemes have successfully increased the overall numbers of flowers on farmland, they tend to ignore the timing (or phenology) of these flowers. Most of what is planted flowers in late spring and early summer, which isn’t necessarily the period of greatest need. Pollinators need a continuous supply of nectar throughout their flight season, and for species with long flight seasons such as bumblebees, this means from late February, right through until late October. Gaps of even one week could create hungry bottlenecks which could drastically limit the number of pollinators surviving through the year. Identifying and filling these gaps is therefore a priority if we want to design more targeted and effective schemes for conserving pollinators.

Bombus terrestris_Credit TT (1)
A buff-tailed bumblebee queen forages from a dandelion, one of the few sources of pollen and nectar in the early spring. [Image: T. Timberlake]

Seasonal nectar supplies

We investigated the seasonality of nectar supply by painstakingly counting half a million flowers from February through to November on four farms in the south-west UK. We combined this data with the amount of nectar sugar in each individual flower (from a previous study) and found that nectar supplies were highly seasonal. The year was dominated by two big peaks of nectar: one in April/May and one in July, with a ‘June Gap’ in between and very little nectar before early April or after late July. As well as being seasonally limited, the nectar supply was fairly homogenous, with over half of all nectar produced by just three species (see image below). This could be a problem for pollinators, given what we know about the importance of a balanced diet for bees. As principal investigator, Professor Jane Memmott explains, ‘just as humans cannot live off tea and toast alone, bees need pollen and nectar from multiple different plant species to remain healthy and fight off disease’.

Wild garlic_thistle and clover_Credit Wiki Commons
(L-R) Wild garlic, creeping thistle and white clover made up over 50% of all farmland nectar in the study. [Images: Wikimedia Commons]

When we compared the available nectar supplies with the energetic requirements of common farmland bumblebees through the year, the ‘hunger gaps’ became clear. The first gap is in early spring (March). Nectar levels are pitifully low at this time (less than four teaspoons of sugar per square kilometre), but bumblebee queens are already starting to emerge from hibernation, searching for nests and establishing new colonies. During this time, fewer than 19 queen bumblebees could be sustained on every square kilometre of farmland. The second major gap identified in the study begins in mid-August, continuing through much of September. Bumblebee colonies are still large at this time, but the intense grazing of pasture and frequent cutting of grass for sileage production mean the number of nectar-producing flowers are drastically reduced, leaving a large gap between supply and demand. As Prof. Jane Memmott explains, ‘these hunger gaps are likely to be a feature of many human altered landscapes, especially ones which have been heavily simplified, such as intensive farmland’.

Filling the gaps

Although potentially very important, the early season gap isn’t that long and could potentially be filled by simply planting a small patch of willow which, with its nectar and pollen rich catkins, could more than double nectar production of farmland at this time. Dandelions and various other early-flowering trees and shrubs could also be important.

Filling the late season gap is more of a challenge however, requiring between 500 and 2000 extra grams of sugar each day per square kilometre. This would need something like a hectare of flowering red clover, or an extra 40 bramble flowers in every square metre of farmland hedgerow. Encouraging the later flowering of plants such as knapweed, scabious and thistles in the pasture, and ivy in the hedgerows, will also help to fill this gap. Successfully achieving this would provide food for all the newly produced queen and male bumblebees which are key to producing next year’s colonies.

By comparing the phenology of nectar supply with the phenology of pollinator demand, this study demonstrates that the timing of nectar supply may be just as important as total nectar production in limiting farmland pollinators. With a more detailed understanding of these timings, conservation schemes would be able to focus their efforts on the periods of the year when nectar is needed most. This could make for more targeted and cost-effective agri-environment and conservation schemes, helping to ensure the survival of pollinators and the many valuable services they provide.

Read the full open access article, Phenology of farmland floral resources reveals seasonal gaps in nectar availability for bumblebees, in Journal of Applied Ecology.

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