Winter workdays are in full swing and our fantastic team of DPA Conservation Volunteers have been busy on various sites, undertaking seasonal tasks including gorse clearance and sapling removal. On Wednesday the 14th of January the team made their yearly winter visit to Roborough Common, accompanied by members of Butterfly Conservation. Jenny Plackett, South West Conservation Manager for Butterfly Conservation, joined DPA Volunteer Co-Ordinator Stephen Barrow to guide habitat improvement tasks aimed at maintaining favorable breeding conditions for the threatened High Brown Fritillary butterfly.
The focus of work on this site was removing saplings of Alder Buckthorn, which had begun invading the open area on this sunny slope, where High Brown Fritillaries have been recorded breeding in previous years. This site is one of only a handful on Dartmoor which offers the very specific conditions which this butterfly needs to thrive. The caterpillars feed exclusively on Dog Violet, which is reasonably common, but this plant must grow alongside bracken on south facing slopes below 300m in order for breeding to be successful. Encroaching scrub risks cutting out light from the understorey of bracken and Dog Violet, which is why it is vital to keep this special site clear.
The High Brown Fritillary reproduces at a rate of one generation per year, with adults flying from mid-June until early August. These butterflies overwinter as eggs, which are laid singly, often on dead bracken, with the caterpillars hatching in early spring. After hatching, they spend time basking on dead bracken and eating Dog Violet beneath a thatch of dead bracken stems and leaves, where the temperature can be 15-20 degrees higher than in surrounding grassy vegetation. These warm microclimates, hidden away under a tangle of dead bracken, are the secret to these caterpillars being able to develop quickly in otherwise cool spring weather. As a mark of bracken’s importance in this butterfly’s life cycle, the caterpillars are even decorated with feathered brown spines that make them look like dead bracken leaves. When they’re ready to pupate they do so close to the ground, often tucked away beneath the leaves they resemble.
During the workday, volunteers removed numerous Alder Buckthorn saplings, a fast growing scrub tree with striking yellow wood beneath its bark. They used special tools, including a tree popper (the yellow tool which can be seen in the header image) which levers saplings out from the root, removing the risk of re-growth. Larger saplings were cut using a handsaw and the cut wood was piled a short distance from the slope, where it will naturally break down or be utilised by animals.
Even the movement of volunteers around the site was useful, as human feet can provide a similar service to this habitat as ponies, cattle and other grazing animals. By moving through the bracken, breaking down standing patches of dead plant material and creating a network of paths, people help make open areas where violets can receive light and germinate. This mosaic habitat, of dense bracken interspersed with grassy patches and canopy gaps, with abundant violets growing through broken bracken litter where there is limited grass cover, is exactly what the High Brown Fritillary butterfly needs.
This specific spot on Roborough Common is so vital as it has just the mix of characteristics which is required. Most bracken stands on uplands and moorlands are not viable for this type of butterfly, as they are too acidic and do not contain the violets which are needed as a food source. We are extremely grateful to have the help and guidance of Jenny and her team from Butterfly Conservation, who will return in the summer to monitor butterfly numbers at the site. Collaborating on land management projects in this manner allows us to deliver sustainable plans over many years and track the impact which our actions have.
Our dedicated volunteers make a real difference on Dartmoor and we’re always delighted to welcome new recruits. All the training and tools required are provided, plus we have wet weather gear to lend out if needed. The Conservation Volunteer team are a happy and welcoming bunch who benefit from fresh air, outdoor exercise and socialising with like-minded people, all while helping to improve and maintain habitats on the moor. All our upcoming workday opportunities can be found on our online calendar.