Case Study: Adding Value to Chalk Grasslands: Creating Chalk Banks to benefit butterflies and other insects.

Author: Lynne Roberts . Farm: The RSPB’s Manor Farm, Newton Tony, Wiltshire

Aims and setting:

Manor Farm is a 296 ha working farm strategically placed between two of the largest tracts of semi-natural chalk grassland in the British Isles – Salisbury Plain and Porton Down. The RSPB purchased Manor farm in 2006 and have been reverting former arable land back to species-rich chalk grassland to create a landscape-scale stepping stone between these two areas.

Whilst this reversion has been very successful in establishing flower-rich grassland, even after several years the ex-arable fields still have unsuitable soil conditions for the plant species typical of very thin, nutrient-poor chalky soils. The fields are also rather flat as a result of historical cultivation, lacking the humps and hollows of natural grassland which help to create a variety of microclimates for a wider range of plants and insects.

For these reasons, in 2013 it was decided to create two large mostly south-facing banks to support viable populations of species characteristic of thin, chalky soils. Butterfly banks and scrapes can provide ideal conditions for butterflies, with sheltered sunny spots and bare ground for basking, alongside a mosaic of nectar-rich flowers for foraging adults and specific foodplants for growing caterpillars.

Funding for the creation of the banks was provided by the SITA Trust (now SUEZ) “Conservation of the small blue butterfly at RSPB Winterbourne Downs” project, and the Biffa Award “Saving a special place for Wiltshire’s endangered butterflies” project. Appropriate permissions for the work were also obtained.

Construction:

The site for the butterfly banks was flat terrain with a 20cm layer of flinty loam over chalk. Two large banks measuring 150m x 12m and 180m x 12m were constructed in two different fields. The loam topsoil was bulldozed into an S-shaped mound, oriented so that most of its length faced south. The mound was then covered with the chalk from the beneath the soil, leaving a wide 'scrape' area of bare chalk alongside.

The S-shape ensures that there are sides facing all possible directions, creating a variation in topography and diverse range of micro-climates. The idea is that this helps to increase the resilience of both plants and insects to the more extreme conditions predicted as a result of climate change. In the mornings, butterflies, bees and other warmth-loving insects can warm up on the east-facing side of the banks, which catches the early morning sun. During the main part of the day, they can use the warm, sunny, south-facing side. If it gets too hot on the south side, the insects can retreat to the north-facing side. As expected, during the middle part of the hottest July day on record in England in 2015, all the bee activity was on the banks’ north-facing slopes.

Construction begins

The first bank was initially seeded with a generic chalk grassland and wildflower mix as well as kidney vetch and horseshoe vetch, which are vital food plants for small blue, chalkhill blue and Adonis blue butterflies. On the second bank, only the kidney vetch and horseshoe vetch were introduced. The banks could have been left to regenerate naturally, but the butterfly foodplants were not growing in the vicinity and therefore seeding with specific species was preferred to ensure that the right plants established.

Development over time and ongoing management

The vetches were the first plants to establish and were flowering in the first summer after autumn sowing. Over the following two years other species, such as small scabious, ribwort plantain, harebell and quaking grass appeared, the coverage developing into a patchwork of abundant flowering species interspersed with bare areas - the ideal structure for butterfly habitat.

Vetches establish quickly on the bare chalk

The vegetation on the banks has become denser over time but has required little ongoing management as the tough conditions prevent many unwanted species from colonising. Sheep are our management tools, grazing from August onwards, when most flowering is over. This helps us to keep on top of any scrub encroachment and keeps the sward open, with some disturbance of the ground surface to create germination opportunities. Cattle would probably cause damage to the banks, so are avoided.

Where necessary, brush cutting is carried out in the summer and the arisings removed and taken to other areas of the farm which are species-poor and could benefit from wildflower sowing.

Brush cutting the coarser vegetation

Achievements:

Colonisation of the banks by breeding small blue butterflies was impressively quick - within the first three years, as the kidney vetch established readily. The abundance of suitable foodplants is particularly important for sustaining viable populations of butterflies, so the fact that small blues were seen on the banks in August, probably a second brood from eggs laid in June, was a good indication that suitable habitat for breeding had been achieved.

Small blue on its larval foodplant, kidney vetch

Other chalk grassland butterflies have also been recorded on the banks in the last few years: common blue, brown argus, marbled white, dark green fritillary and, excitingly, the marsh fritillary, a species in severe decline which has been attracted to Manor Farm by the abundance of scabious plants in the grassland. Good numbers of these species are now being seen on the farm as they are moving in from the neighbouring strongholds on Salisbury Plain and Porton Down. We have yet to record chalkhill blue, but the Adonis blue was a new record in 2018, so hopefully it will just be a matter of time before the chalkhills arrive.

The diversity of flora species continues to develop, with some of the less common chalk specialists such as devil’s-bit scabious, starting to appear on the banks.

In addition, we have shown that the fields containing the banks can still be used as grazing land, albeit in a carefully managed way.

Our experience and lessons learned:

Five years on we found that some plants had been harder to establish on the banks than others. The pioneer species kidney vetch and horseshoe vetch, two of our key butterfly food plants, established readily from seed and plug plants were not required. However, we didn’t have the same success with common rock rose, the food plant of the brown argus and cistus forester moth. This was easier to establish from plugs, although the sown seed may germinate eventually when conditions are favourable and the seed coat has weathered a little. Plugs are more expensive, so a pragmatic approach may be to try sowing seed first and then supplementing with a few plugs after a couple of years if the seed has not germinated. Germination rates of common rock rose can be improved by scratching the seed surface (scarifying) prior to sowing.

Where we sowed the generic mix, some of the taller plants have become dominant, greater knapweed particularly, shading out some of the foodplants, such as horseshoe vetch. We have therefore had to manage some of the less desirable species in order to help the key plants thrive.  This was not such issue on the second bank, where we had just sown the foodplants, with just the odd thistle to deal with.

We suspect that by creating the base of the banks from the flinty loam topsoil, the longer rooted chalk grassland species may be accessing nutrients from this base and gaining an advantage. Ideally, the bank would be created from pure chalk and the removed soil utilised elsewhere. A chalk-only bank would create the harsher alkaline conditions which favour the more extreme chalk specialists, including the kidney vetch and horseshoe vetch, and would slow the rate of encroachment by scrub species.

Although our butterfly banks are very large it is important to note that banks of any size can make a big difference to the survival of butterfly populations - even just moving soil around to create variation in the topography of reverted arable fields or pasture can help. Butterflies exist in small populations which are linked to create larger ‘metapopulations’, so even quite small areas of suitable habitat could provide a vital link in the chain.

All photos supplied by Patrick Cashman.   
For more information, contact: patrick.cashman@rspb.org.uk

Further examples of butterfly banks

Organisations such as Butterfly Conservation and Buglife have been constructing butterfly banks on several sites, both rural and urban, as a way of creating breeding habitat suitable for a number of different butterfly species. For further information see the following links:

https://butterfly-conservation.org/our-work/reports-and-factsheets/habitat-creation

https://farmwildlife.info/2017/12/15/bare-ground-butterflies-moths/