New research: What limits bumblebee populations on farmland?

Authors: Dr Tom Timberlake and Prof Jane Memmott 

A new study by Tom Timberlake and colleagues at the University of Bristol shows how important late summer flowers and rural gardens can be for supporting bumblebees on UK farmland.

Does this offer an opportunity to devise more targeted agri-environment schemes for pollinators?

Pollen and nectar are the main source of food for bees, including the charismatic and agriculturally important bumblebee. It is no surprise then that declining flower densities – particularly on farmland – are considered one of the most important drivers of pollinator decline. Agri-environment schemes which incentivise the planting of wildflower strips and the expansion of semi-natural flowering habitats such as hedgerows and field margins are an important tool for reversing pollinator declines, but are they really the most efficient way of supporting pollinators?

Image 1: For bumblebees like this one (Bombus terrestris), it’s not just about how much food is available, it’s also when that food is available through the year. A dandelion flowering in early spring for example, would be more valuable to a bumblebee than an equivalent flower in mid-summer.
Image: T. Timberlake

Whilst agri-environment schemes have successfully increased the overall numbers of flowers on farmland, they tend to overlook the timing of when these flowers are available to pollinators. Different plants flower at different times and most of the plants in agri-environment schemes flower in late spring and early summer which often isn’t the period of greatest need for pollinators. Pollinators need a continuous supply of food throughout their flight season, and for species with long flight seasons such as bumblebees, this means from late February, right through until October. ‘Hunger gaps’ of even one week could limit the number of pollinators surviving through the year.

Hunger gaps

To support pollinators in the most effective and cost-efficient way, it makes sense to find out when these hunger gaps occur and then devise targeted management or planting schemes to plug these gaps. A previous study by our team did just this and showed that nectar supplies on farmland were most limited in early spring (March) and late summer.

To check what effect these ‘hunger gaps’ were having on bumblebee populations, we carried out a study on 12 farms around the west of England. We captured, recorded and released hundreds of bumblebees and then measured all sorts of features of the farms to find out which aspects of the farm were most important in determining bumblebee density.

To our surprise, the supply of nectar in late summer (September) was by far the most important factor driving bumblebee density on these farms – more so even than the amount of natural habitat. Late summer is a very important stage in the lifecycle of bumblebees – it is when new queens are produced and must pile on the pounds before their winter hibernation. A rich supply of flowers is therefore crucial, but with fewer and fewer hay meadows, cover crops and weedy areas to provide this late summer nectar on farmland, bumblebees are struggling.

Image 2: Low nectar supplies in late summer coincide with an important stage in the colony lifecycle, limiting colony density the following year. How might we change the shape of this curve to reduce the September bottleneck?

How to plug the gap

So what can we do to plug this late-summer hunger gap on farmland and support bumblebees? On our farms at least, ivy was the most important plant for providing nectar during this sparse time, so managing your hedgerows and woodland edges to promote this amazing plant is a good first step. Leaving some rough weedy corners for late flowering species like thistles, knapweed, scabious and dandelions can also contribute to plugging the gap.

We found that Environmental Stewardship Scheme pollen and nectar mixes were really effective at increasing overall nectar supplies but were far more effective when mown early, or in multiple phases, to extend flowering into the late summer.

Finally, if you want to give those hungry queen bumblebees a real treat in the run-up to autumn, a tasty cover crop of late-flowering red clover would do just the trick. A single hectare of this crop could provide around 1 kilogram of raw nectar sugar each day and completely close the late summer gap.

There was one final surprise in our results… Small patches of garden were having a significant influence on the density of bumblebees. Farms with more gardens had more bumblebee colonies.

Gardens have a far denser and more diverse supply of flowers than farmland and are often managed to keep things flowering throughout the whole year. These little floral oases seem to be throwing bumblebees a lifeline during periods when farmland offers very little.

Image 3: Species which flower in September such as dandelions, red clover, ivy and thistles (clockwise from top left) are likely to be disproportionately important to bumblebees and other pollinators. Including these and other late-flowering species in conservation schemes will help fill the late summer hunger gap.
Images: T. Timberlake & Wikimedia Commons

How much and when

The take home message is that it’s not just about how much food we provide for pollinators, it’s also about when that food is available through the year.

Providing more flowers in the early spring and late summer when bumblebees are at their hungriest is a great way to support these important creatures.

Image 4: Some examples of low quality (left) and high quality (right) farmland habitats for pollinators. The more diverse and flower-rich high quality habitats tend to provide a more consistent supply of nectar through the year, helping to fill those important hunger gaps.
Image: Nick Tew

This article is the summary of a University of Bristol-led research paper published by Thomas Timberlake, Jane Memmott, Ian Vaughan and Mathilde Baude in the Journal of Applied Ecology. The work was funded by the Natural Environment Research Council (NERC) with additional support from the NERC Biomolecular Analysis Facility.

Case Study: Managing Farmland for Grey Long-eared Bats

Author: Craig Dunton, Grey long-eared bat Project Officer, Bat Conservation Trust

Species: Grey Long-eared bat:

© Craig Dunton/www.bats.org.uk

 

Why is farmland important for this species?

With as few as 1000 individuals In the UK, the grey long-eared bat is one of our rarest mammals. Their distribution is restricted to the southern coast (Devon, Dorset, Somerset, Hampshire, Isle of Wight and Sussex) and there are only 9 known maternity colonies; meaning that this species is on the brink of extinction in this country. Currently, work is being carried out to conserve this species as part of 'Back from the Brink' – a Heritage Lottery funded conservation project aiming to save 20 species from extinction. https://naturebftb.co.uk/

Farmland is vitally important to all bat species. 70% of land in the UK is used for agriculture, so for very mobile species that utilise the whole landscape, farming practices can have significant impacts on bat populations. Some bat species are more resilient than others, but for those that have very low populations, are particularly sensitive to light and disturbance, and have very specific habitat requirements, there are significant threats to their survival.

Like all bats, the grey long-eared bat needs some key elements throughout its range in order to survive and thrive.

Roosts: Bats utilise a wide variety of structures to roost in, as their requirements change throughout the year. Roosts are needed for different activities – hibernation roosts, maternity roosts, night feeding roosts (for some bats) – these are all needed at different times of the year, and different bat species choose different structures for different activities.

Many bats hibernate underground, using caves, mines, cellars and tunnels that have a cool and constant temperature that bats need to hibernate. Many farms have these types of features, so it is important to be aware of their importance for bats. Some bats roost in a variety of different tree features; cracks, splits, woodpecker holes, loose bark – so it is important to retain trees that have ‘interesting’ features.

Grey long-eared bats are mostly associated with man-made roosts – usually the roof spaces of large, traditional, stone buildings with slate roofs. There are some records of them using caves and rock crevices, but no records of them using bat boxes in England.

Many buildings on farms can provide important roosts for grey long-eared bats, particularly large stone buildings with slate roofs and large, open roof voids. These can provide vital maternity roosts, temporary day and night roosts and opportunities for hibernation. As all bat species are protected, if you are planning to carry out any work that might impact bats or their roosts, the National Bat Helpline will point you in the right direction, please email enquiries@bats.org.uk

Foraging sites: As with roosts, different species of bats have different types of habitats that they favour for foraging. All UK bat species are insectivores, and depending on their size, flight habits and other physical adaptations, different species need different insect prey and therefore different habitats for foraging in. Greater horseshoe bats are large bats and so favour large prey, particularly around the maternity period. Cockchafers and dung beetles are their food of choice. Daubenton’s bats are often seen foraging over water, trawling insects from the surface of the water with their specially adapted, large hairy feet.

© Craig Dunton/www.bats.org.uk

Grey long-eared bats have a close association with unimproved lowland grassland and riparian (wet) habitats such as meadows and marshes.

Managing permanent pasture more extensively with very low or no inputs will benefit grey long-eared bat populations, offering more opportunities for invertebrates including the moths and craneflies that grey long-eared bats favour. Managing land to maintain or increase species diversity (more flowering plants) as well as structural diversity (a range in sward height), will give insects food and shelter resources. If possible, consider transitioning land back to hay meadow management as opposed to silage production, as this allows insects and plants to complete life cycles and thrive.

In terms of cultivated land, increase pollen and nectar plots and wildflower margins, and manage by cutting or grazing to maintain plant diversity. These areas alongside trees and tall hedgerows have proven benefits for some bat species. Minimise pesticide use and consider crops that contain lots of flowering plants such as herbal leys.  Within arable land, conservation headlands or low input cereals can be beneficial if management results in increased invertebrate numbers. These are all ways of increasing foraging opportunities on less productive areas of land for grey long-eared (and other) bats.

Farming organically has many advantages, including huge benefits to biodiversity, natural resource protection and reducing greenhouse gas emissions. Following organic principles should be considered if aiming to encourage bats and other biodiversity.

Connectivity: For so much of our wildlife, good habitat connectivity is critically important. Linear features in the landscape such as hedgerows, watercourses and ditches, enable different species to flow freely through the landscape, enabling opportunities for feeding, interacting and breeding. For bats, good connectivity is vital, as many bats use linear features to navigate through the landscape, using their echolocation to move from roosting sites to foraging sites. When these features are well managed, they can also provide foraging opportunities as well as providing obvious features to allow free movement. Extensive hedgerow management (2-3 year cutting cycle) allows hedgerow plants to flower and fruit, providing vital resources for insects. Buffer strips along streams and rivers not only provides increased opportunities for foraging, but can also protect watercourses from nutrient and pesticide run off, protecting aquatic flora and fauna.

For more information on land management for grey long-eared bats contact Craig Dunton (Grey long-eared bat project officer) on cdunton@bats.org.uk