Yarner Wood, Bovey Valley and Trendlebere Down, located within the East Dartmoor Woods and Heaths National Nature Reserve, are fabulous hotspots for butterflies! Yarner Wood was one of the first sites to take on the annual recording of butterflies back in 1976, when the UK Butterfly Monitoring Scheme began. https://ukbms.org/methods
Volunteer butterfly surveyors walking the Yarner Wood transect
The ‘transect walk’ is quite an undertaking, requiring surveyors to walk a fixed route, in good weather, on a weekly basis between April and September, and record all the butterflies they see within 5m of them. In that first year, there were 30 different species recorded on the Yarner Wood transect, and a few years later the Bovey Valley transect was set up. Since then, Natural England staff and volunteers have made weekly walks on set routes across the two sites, counting and recording all the butterflies they see along the way. Aside from it being a very enjoyable way to spend an hour or so each week, the resulting data has enabled a valuable resource to be built up, enabling analysis of the butterfly trends locally, providing information on the distribution and abundance of species, and helping to inform management decisions.
Butterfly species numbers across the years from transect records
Individual butterfly number totals across the years from transect records
Alongside the transect recording, monitoring of some of the rarer species of fritillary butterfly has been carried out through timed count surveys. Counting the number of butterflies seen during a fixed period of time creates population trend data which can flag up when a species is undergoing long term changes in population size.
Happily, the two rare fritillary butterflies found in the reserve, the Pearl-bordered Fritillary and Small Pearl-bordered Fritillary, are both faring very well, thanks to the considerable management work carried out there by Natural England.
Pearl-bordered fritillary Photo Credit: Neil Hulme
Pearl-bordered fritillary counts across the years
The Pearl-bordered Fritillary, in particular, has very niche habitat requirements, being reliant on the Common Dog-violet plant (the caterpillar’s foodplant) growing in association with bracken. The density of bracken, and the depth of the dead bracken material, or ‘litter’, in wintertime are quite critical to its survival. The butterfly overwinters in its caterpillar stage, and when it comes out of hibernation in the early spring, the temperature in the micro-climate within the bracken is considerably warmer than the ambient air temperature. This differential enables the caterpillar to develop more quickly – in time to complete its larval development, pupate and emerge as an adult butterfly by early April. Getting the bracken management right for this species is very important for its continued survival here at Yarner. The Dartmoor ponies in Yarner Wood help create suitable habitat, whilst the livestock on Trendlebere Down carry out the same function.
High Brown Fritillary butterflies Photo Credit: Amanda Hunter
It was very pleasing to hear of a sighting of a pair of very rare butterflies at Bovey Woodland last year. A mating pair of High Brown Fritillary butterflies were recorded and photographed in the woodland, the first seen here in many years. Only 28 records have been recorded on the Bovey Valley transect since its inception, and the last sighting (off transect) was made in 2006, so this was a very exciting discovery!
Practical management for fritillary butterflies on wider Dartmoor
Alongside the survey work, volunteers and staff have been crucial in the delivery of the practical management work at the reserve. For two years the Natural England regular Tuesday group of committed volunteers worked alongside BTCV ‘and Butterfly Conservation’s volunteers (making up the ’Wildlife Hit Squad’), to carry out practical work tasks of benefit to the fritillary butterfly population both on the reserve and more widely across other Dartmoor sites. Joining forces in this way has enabled a coordinated approach to monitoring and has enabled the volunteer activity to be directed at sites where it has been much needed.
Pearl-bordered fritillary butterfly workshop at Yarner Wood
Butterfly Conservation has been working with Natural England since 2010, helping to train volunteers in identification and surveying of the fritillary butterflies, and many of the volunteers have gone on to carry out monitoring more widely across other sites on Dartmoor.
Small Pearl-bordered fritillary butterfly Photo Credit: Peter Eeles
The latest training took place on 13th April 2022 to help anyone interested in taking up surveying for Small/Pearl-bordered Fritillaries. Here we ran through the status of these butterflies across the UK and Dartmoor, why they are declining, how to distinguish the two species from each other, how to carry out timed counts, and explain what makes good habitat!
Written by Jenny Plackett, South West Regional Conservation Manager, Butterfly Conservation
With thanks to Ian Middlebrook, Butterfly Conservation for the transect data graphs