Project

Our butterfly conservation work on the Holnicote Estate

Heath fritillary butterflies on the Holnicote Estate, Somerset — Heath Fritillary butterflies | © National Trust Images/Matthew Oates

The Heath Fritillary butterfly is a rare species found only in a few places in the UK, and is now endangered. Find out how the rangers on the Holnicote Estate have been tackling this by managing their habitat for over 30 years, to ensure that this threatened species can thrive once more.

Thirty years of the Heath Fritillary

The Heath Fritillary (Melitaea athalia) is arguably one of, if not the rarest butterfly in the UK, when you consider its limited geographic distribution. It has one of its main strongholds on Exmoor, with most colonies located on the Holnicote Estate.

The species has seen its distribution shrink by 68% in the period from 1976 to 2014. As a result, the Heath Fritillary is a species of high conservation importance: it is listed in Schedule 5 of the 1981 Wildlife and Countryside Act and identified as a priority species in the UK government’s Biodiversity Action Plan (BAP).

With it being so rare it is important that its numbers are checked every year, to enable effective conservation measures to be put in place to prevent any further decline. Monitoring of the Exmoor Heath Fritillary populations has been carried out since 1984 and is co-ordinated by our partners, Butterfly Conservation.

A close up of a small pearl-bordered fritillary butterfly on a green plant in Killerton, Deon — Fritillary butterfly | © National Trust Images / Matthew Oates

Managing the land: bringing back cow-wheat

After observing such a dramatic decline, it was apparent that there was little option other than direct and robust habitat management to try and restore the fortunes of the Heath Fritillary on Exmoor.

Key to the butterfly’s recovery would be the abundance of cow-wheat which had been shaded out by competing vegetation and bracken litter due to under-management of their favoured sites.

Cow-wheat is an annual plant, semi-parasitic on bilberry and the key larval food plant for the Exmoor Heath Fritillaries.

Positive results

Since 2005 the Heath Fritillary has continued not only to increase in numbers at key sites but also re-colonised at some historic sites, particularly in the combes on the lower slopes on the northern side of Dunkery Beacon.

In 2020, not only were good numbers recorded at their key sites but there has also been widespread dispersal of individuals searching out new habitat.

In 2021, the Heath Fritillary had a great year on Exmoor, with most sites having higher abundance than recorded in 2019 and 2020, and some sites faring even better than in the very good year of 2018. The Exmoor population trend for the butterfly during the monitoring period 2004-2020 is stable, set against a declining trend for the UK as a whole. The Holnicote Estate sites did well overall, with big increases noted at Aller Combe and Hollow Combe. Sweetworthy also recorded good numbers. Nutscale (National Trust), which was recolonised last year following no records since the early 1990s, remains occupied, and singletons were also recorded within the grounds of the adjacent SWW reservoir site. A singleton was recorded at Bagley Combe, which is only occasionally occupied. No sightings were made at Spangate in 2021 for the second year running, and no surveys were undertaken at Mansley Combe.

Heath Fritillary butterfly larva | © National Trust Images/Matthew Oates

Modern technology lends a helping hand

The Bromford flailbot has quickly become a valuable addition to the Holnicote Estate. This remote-controlled flail mower has allowed the rangers to take a different approach to the management of some of our most valuable sites, from orchards to heathlands.

Flailbot can access remote sites which are hard get to with vehicles and traditional machines. Many of these places would be unsafe to use a tractor on, whereas the flailbot can be used when working on steep slopes and coastal areas with a reduced risk to safety.

On the Holnicote Estate our Health Fritillary population is found on steep slopes in woodland edge habitat. The rangers use the flailbot to manage their habitat by breaking up the vegetation and cutting back scrub which allows light in for common cow-wheat, the main food source for the caterpillars.