Heath Fritillary – the science behind the management

Heath Fritillary - Thrift Wood

The Heath Fritillary Melitaea athalia is a priority species with a restricted distribution in the south of England, occupying a total habitat area of less than 1km2 in the UK.

The butterfly was considered on the verge of extinction in the 1980s and this may have become a reality were it not for concerted conservation work in the ensuing years. However, despite these great efforts, the status of the Heath Fritillary has continued to fall and remains of great concern. The butterfly has suffered a decline of 68% in distribution between 1976 and 2014 and a decline in abundance of 87% over the same time period1. Analysing the data over a shorter ten year period, the distribution change has slowed to a 12% decline but the abundance change is still worrying and the UK Butterfly Monitoring Scheme reported the Heath Fritillary to have suffered its worst year on record in 2016.

 

Heath Fritillary long-term trend 


The species is only found in four landscapes: The Blean Woods complex in Kent; woodlands in southern Essex; heathland combes of Exmoor in Somerset and the Tamar and Lydford valleys of Cornwall and Devon. Butterfly Conservation has a long history of working with landowners and conservation partners in these landscapes and currently has funded landscape-scale projects running in three of the four areas and staff working with key partners in the remaining area. In recent months we have been working to review the status of the species in each landscape to understand what the drivers are behind the recent declines in abundance. 

The ecology of the Heath Fritillary has been extensively researched since the 1980s and it is known to be reliant on early successional habitats. Targeted management is required to provide the ideal conditions for the species where the principal larval host plants of Common Cow-wheat Melampyrum pratense and Ribwort Plantain Plantago lanceolata thrive. Traditional management practices such as coppicing and ride management are required in woodlands and on moorland sites extensive grazing along with burning to tackle the gorse and scrub encroachment are necessary, to create ideal conditions for egg laying and larval survival. 

In the Blean Woods the occupancy of the butterfly is dynamic and targeted management is required to provide a regular resource of suitable habitat patches (coppice coupes or sections of rides) for the butterfly to colonise and occupy while they remain suitable (usually <6 years for coppice). Targeting habitat management is key to maintaining occupancy and abundance. 

Our detailed habitat patch mapping and monitoring data has been utilised to develop a simulation model called ‘MANAGE’ to assist in helping to answer conservation questions for dynamic species2. Researchers at the University of York used Heath Fritillary data to build a simulation model that would help to target the area and spatial location of management. The resulting model confirmed that the observed rates of coppicing were not enough to meet conservation targets, under a ‘worst case scenario’ model the populations could be sustained if the coppicing area increased to approximately 30 ha per year. Targeting of the management is vital as the four woodland blocks are interdependent, with movement occurring between woodland blocks. This research has helped us, working with the various landowners, to target our advice and management priorities. 

In the Blean the long-term population trend is stable, although there have been declines over the last four years along with a reduction in the size of colonies. The cause of this decline is possibly related to a recent reduction in management levels within some parts of the woodland complex. This has now been reversed and we would expect the species to respond positively again which will continue to be monitored. Anecdotal evidence is suggesting that climate change is impacting on the abundance and phenology of the host plant and this is an area for further research. 

On Exmoor the number of colonies and sites has remained relatively stable since the retraction in range from the western sites on Exmoor during the 1990s due to a decline in grazing. The ideal management for these heathland combes is through a combination of cattle and pony grazing but often rotational winter burning (swayling) is also required. The ideal management for these sites following burning was examined to establish the most effective way to maximise the abundance of Common Cow-wheat and to control the increased growth of bracken3. The results of this work have formed the basis of management advice for these sites. 

However, analysis of UKBMS data from transects and timed counts demonstrates severe declines in peak population sizes for some Heath Fritillary colonies despite their distribution remaining stable.  What may be the reason behind this trend? Further work is now required to establish the reasons for this decline on Exmoor in recent years. 

Caroline Bulman

Head of Species Ecology, Butterfly Conservation

 

References

  1. Fox, R.; Brereton, T.M.; Asher, J.; August, T.A.; Botham, M.S.; Bourn, N.A.D.; Cruickshanks, K.L.; Bulman, C.R.; Ellis, S.; Harrower, C.A.; Middlebrook, I.; Noble, D.G.; Powney, G.D.; Randle, Z.; Warren, M.S.; Roy, D.B. (2015) The state of the UK’s butterflies 2015. Wareham, Dorset, Butterfly Conservation. http://butterfly-conservation.org/files/soukb-2015.pdf 

  2. Hodgson, JA, Moilanen, A, Bourn, NAD, Bulman, CR & Thomas, CD (2009) Managing successional species: Modelling the dependence of heath fritillary populations on the spatial distribution of woodland management Biological Conservation, 142: 2743-2751. doi:10.1016/j.biocon.2009.07.005 

  3. Brook S., McCracken M., Bulman C.R., Camp P. & Bourn N.A.D. (2007) Post-burn bracken Pteridium aquilinum control to manage habitat for the heath fritillary butterfly Mellicta athalia on Exmoor, Somerset, England. Conservation Evidence 4: 81-87. https://www.conservationevidence.com/collection/4