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 for wildlife in Caithness

Author: Katy Malone
Farm: Todholes Farm, Caithness

Todholes Farm is a 170ha lowland livestock farm in Caithness, where Ian Campbell raises prize winning beef cattle and sheep.

Ian Campbell took over the 170ha holding around 1990. Previously his father had managed the farm, having moved to Caithness in 1951. Since then, Ian has built up a herd of prize winning livestock, and won many rosettes for his Beef Shorthorn cattle as well as Lleyn and Texel sheep at a county level and further afield.

Aims:

Caithness holds nationally important populations of breeding waders such as lapwings, curlews, redshanks and snipe as well as twite, which have suffered from long-term declines in their breeding numbers across the UK. It is also a stronghold for the biodiversity action plan (BAP) priority species great yellow bumblebee which was previously widespread in the UK but is now one of our rarest native bumblebees. Populations of all these species have been maintained in Caithness thanks to high nature value farming systems. Todholes Farm is a typical example of these systems, which has been further enhanced through the tailoring of management for the benefit of key species.

The majority of the farm is grazed, with smaller areas of arable for livestock feed: spring barley for cattle feed, fodder rape for fattening lambs, and fodder turnips for wintering sheep.

Much of the farm is directly adjacent to the River Thurso, a Special Area of Conservation for Atlantic Salmon and a Site of Special Scientific Interest for a number of nationally rare or scarce plants.

Management:

Management advice was provided by the RSPB Scotland, specifically in relation to breeding waders and seed eating birds. Scottish Natural Heritage advised on management relating to designated sections of the site, and the Bumblebee Conservation Trust advised on the provision of pollen and nectar sources.

Of the options available for the non-designated parts of the farm, Ian opted for:

- Open Grazed Grassland for Wildlife (29ha)

- Management of Habitat mosaics (16ha)

- Management of Species Rich Grassland (4.5ha)

Open Grazed Grassland for Wildlife

Stock are excluded for six consecutive weeks between 15th March and 15th June, to restrict trampling of nests of waders such as lapwing and curlew. This could have had a high impact on the farm business, particularly in late spring, so the timing of the exclusions are calculated to allow two areas to be shut off while a third is opened up. This works well and has little detrimental impact on the economics of the business.

Management of Habitat Mosaics

This is an area of bog and heath with scattered rush and gorse bushes. Grazing is kept low between May to August to avoid trampling of wader nests and at the same time to avoid the vegetation becoming too rank.

Species Rich Grassland

Three areas of species rich grassland are managed on the farm. Wildflowers were relatively abundant right into early September, providing a much-needed late nectar source to bumblebees. In particular, the nationally rare great yellow bumblebee has been found on Todholes, and lapwings and curlews nest on the other areas of species rich grasslands.

There are some small areas of wetland within the habitat mosaics field, which were of concern to Ian in case they presented a problem with liver fluke. These areas were fenced off to address this. However, to prevent them from becoming rank, the wetlands still need to be grazed and cattle have access to them through the autumn/winter months. The cattle are dosed for fluke three times annually and Ian has not noticed any detrimental effect of fluke on his livestock as a result of grazing in these wetland areas.

Achievements:

It is still early days for the created species rich area and other taller more nutrient-loving species such as thistle currently dominate the field. Species that we hope will flourish here include vetches (e.g. tufted vetch and meadow vetchling), bird’s-foot trefoil and flag iris. Livestock are excluded between April and August but by the time the field is opened up again, the level of grazing is not sufficient to control the ‘weeds’. The grazing plan will be adjusted over time e.g. to increase grazing or allow for cutting so that a greater variety of plants can flourish. Some scarification and reseeding may also take place.