The Heath Fritillary (Mellicta athalia) on Exmoor: Changing Status 1980-2000 and Conservation Recommendations Butterfly Conservation Report No. S01-30 Katherine Stewart, Nigel Bourn, Martin Warren and Tom Brereton. May 2001 Master Copy - Butterfly Conservation Office, Wareham National Trust - Cirencester Office, Holnicote Office English Nature - Taunton Office, Exeter Office, Head Office MAFF & FRCA - Taunton Office, Exeter Office, London Office Exmoor National Park Dartmoor National Park Butterfly Conservation Branches - Devon and Somerset South-West Fritillary Action Group (SWFAG) members Exmoor Natural History Society (ENHS) Tenants/farmers consulted during the survey Wessex Water - Bath Office RSPB - Exeter Office Environment Agency - Exeter Office, Bridgwater Office Wildlife Trusts - Devon & Somerset CONTENTS TABLES AND FIGURES EXECUTIVE SUMMARY 1.0 INTRODUCTION 1.1 BACKGROUND 1 .2 ECOLOGICAL REQUIREMENTS OF THE HEATH FRITILLARY ON EXMOOR. 1.3 SURVEY HISTORY AND DISTRIBUTION 1.4 LAND ENHANCEMENT SCHEMES RELEVANT TO THE CONSERVATION OF THE HEATH FRITILLARY-THE EXMOOR ESA 1.5 AIMS OF CURRENT STUDY IN 2000 2.0 METHODS 2.1 TIMED COUNT SURVEYS AND POPULATION ESTIMATES 2.2 HABITAT ASSESSMENT 2.3 HISTORICAL POPULATION DATA AND MANAGEMENT INFORMATION 3.0 RESULTS 3.1 NUMBER OF SITES, DISTRIBUTION AND POPULATION SIZES IN 1999/2000 3.3 COMPARISON BETWEEN THE 1989 AND 2000 SURVEYS 3.4 CHANGES IN POPULATION SIZES FROM 1984 TO 2000 3.4.1. Collated Index from all Heath Fritillary sites on Exmoor 3.4.2 Fate of 1989 sites with respect to site size 3.4.3 Effect of Isolation and Area of Sites on Population Persistence 3.5 CHANGE IN VEGETATION STRUCTURE 3.6 MANAGEMENT HISTORY ON EXMOOR 3.6.1 Grazing regimes on Heath Fritillary sites 3.6.2 Changes in the burning regime on Heath Fritillary sites 3.6.3 Management agreements on Heath Fritillary sites 3.6.4 Impact of Deer Activity 3.6.5 Case Studies of Exmoor Heath Fritillary Sites and Overview of Management Changes 4.0 DISCUSSION 4.1 POPULATION RESULTS 4.2 THE IMPLICATIONS OF METAPOPULATION DYNAMICS FOR HEATH FRITILLARY CONSERVATION 4.3 MANAGEMENT CHANGES 4.4 EXAMPLES OF OTHER PRIORITY INVERTEBRATES THAT DEPEND ON EXTENSIVE GRAZING -IMPLICATIONS FOR THE ESA SCHEME 4.5 FULFILLING SAP OBJECTIVES AND CONTRIBUTING TO THE EXMOOR BAP 4.6 FUTURE RESEARCH AND MONITORING 4.7 GENERAL RECOMMENDATIONS 5.0 ACKNOWLEDGEMENTS 6.0 REFERENCES 7.0 APPENDICES 7.1 APPENDIX ONE - HEATH FRITILLARY POPULATION AND VISIT DATA 7.2 APPENDIX Two - TRANSECT DATA FROM BIN COMBE AND HALSE COMBE 7.3 APPENDIX THREE - AREA AND ISOLATION DATA USED IN THE STATISTICAL TESTS 7.4 APPENDIX FOUR - POPULATION ESTIMATE DATA 1984-2000 FOR ALL SITES 7.5 APPENDIX FIVE - VEGETATION DATA 7.6 APPENDIX Six- MANAGEMENT DATA 7.7 APPENDIX SEVEN- QUESTIONS ASKED TO FARMERS/LANDOWNERS DURING THE MANAGEMENT SURVEY 7.8 APPENDIX EIGHT - CASE STUDIES OF EXMOOR HEATH FRITILLARY SITES 7.9 APPENDIX NINE - PRIORITY BUTTERFLIES AND MOORLAND MANAGEMENT ON EXMOOR AND DARTMOOR TABLES AND FIGURES TABLES TABLE 1.1: EXMOORESA TIERS AND PAYMENT LEVELS TABLE 2.1: SIZE CATEGORIES FOR POPULATION ESTIMATES TABLE 2.2: COW-WHEAT ABUNDANCE SCALE TABLE 3.1: ESTIMATED POPULATION SIZES IN 1999/2000 TABLE 3.2: UNOCCUPIED SITES IN 1999/2000 TABLE 3.3: SIZE POPULATION CATEGORIES IN 1989 AND 2000 TABLE 3.4: FATE OF 1989 SITES IN 2000 WITH RESPECT TO EXTINCTION AND PERSISTENCE TABLE 3.5: MANN-WHITNEY TEST RESULTS COMPARING EFFECTS OF ISOLATION AND AREA ON OCCUPIED AND EXTINCT HEATH FRITILLARY SITES ON EXMOOR 1989-2000 TABLE 3.6: COMPARISON IN VEGETATION DATA BETWEEN 1989 AND 2000 TABLE 3.7: COMPARISON BETWEEN OCCUPIED AND UNOCCUPIED SITE VEGETATION DATA BETWEEN 1989 AND 2000 TABLE 3.8: OVERALL CHANGES IN GRAZING FOLLOWING THE INTRODUCTION OF THE EXMOOR ESA ON THE 8 STUDY SITES TABLE 3.9: FREQUENCY OF BURNING ON HEATH FRITILLARY SITES (LISTED IN DECREASING ORDER OF POPULATION SIZE) TABLE 3.10: SUMMARY TABLE SHOWING SITES ENTERED INTO MANAGEMENT AGREEMENTS TABLE 3.11 SUMMARY OF HABITAT AND MANAGEMENT ON CASE STUDY SITES FIGURES FIGURE 3.1: MAP SHOWING THE STATUS OF HEATH FRITILLARY COLONIES IN (A) 2000 AND (B) 1989 FIGURE 3.2: COLLATED INDEX GRAPH, 1984-2000 FIGURE 3.3: THE EFFECT OF ISOLATION AND SITE AREA ON THE OCCUPANCY OF HEATH FRITILLARY COLONIES IN 2000 FIGURE 3.4: THE EFFECT OF ISOLATION AND SITE AREA ON THE OCCUPANCY OF HEATH FRITILLARY COLONIES IN 1989 EXECUTIVE SUMMARY The Heath Fritillary (Mellicta athalid) is a species of high conservation importance: it is listed on Schedule 5 of the 1981 Wildlife and Countryside Act and is identified as a priority species in the UK Government's Biodiversity Action Plan (BAP) (DOE, 1995). Exmoor is a major stronghold for the species containing over half of all UK colonies in 1989. The principle aim of this study was to determine possible reasons for the recent decline of the Heath Fritillary on Exmoor, including changing habitat management and the metapopulation status of the species. A management survey was undertaken in autumn 2000 and the colony distribution data was analysed in terms of standard 'metapopulation' variables; the size and isolation of sites. The results show that the Heath Fritillary has declined by almost 50% on Exmoor since 1989, from 29 colonies to 15 in 1999/2000. The size of many colonies has also decreased, with the number of large colonies declining from 17 in 1989 to 5 in 2000. A collated index (based on timed counts of adults) calculated from 1984 to 2000 showed a decline in overall population size on Exmoor of 85%. Between 1989 and 2000 significant vegetation changes were noted. The vegetation height and the percentage bracken cover have both increased significantly, while the percentage cover of bilberry has decreased. The level of grazing has decreased on many Heath Fritillary sites, with a cessation of winter grazing on half of eight study sites where management details could be retrieved. There is seldom shepherding of stock and many combes have little keep to encourage animals to stay and graze, exacerbating the problems associated with a reduction in stock density. A conflict has been identified within the ESA, where a prime aim has been to improve heather quality by reducing grazing pressure. However this reduction appears to be leading to a severe decline in the quality of habitat in the combes for the Heath Fritillary and other threatened invertebrates. It is thus vital that methods are developed to allow appropriate grazing intensities to be maintained in these combes without compromising heather quality on the higher land. One mechanism recommended is to allow derogations to increase (or maintain) grazing levels on known or potential Heath Fritillary sites. To enable targeted management it is essential that information about the location and requirements of these threatened invertebrates are made known to the MAFF Project Officers responsible for adminstering the ESA scheme. There appears to be a strong correlation between regular burning and the size and persistence of Heath Fritillary populations. A more regular burning programme needs to be instigated on Heath Fritillary sites in conjunction with scrub removal, to eliminate some of the build-up in vegetation biomass that makes habitat unsuitable for breeding. The pattern of occupancy and extinction on individual sites shows that occupancy in 2000 appears to be determined, at least in part, by the size and isolation of a site. Extinctions have been more frequent on smaller, more isolated sites. The pattern of isolation and area of sites in 1989 and 2000 demonstrates that levels of isolation are increasing as sites are lost. This increased isolation then reduces the rates that a relatively immobile species like the Heath Fritillary can re-colonise former sites or colonise new habitats. Thus the Heath Fritillary on Exmoor is in an even more perilous condition than the loss of 50% of sites implies, as the metapopulations (comprising groups of nearby populations) break down and extinction rates escalate. The conservation of the Heath Fritillary needs to be considered from a broad landscape perspective, where metapopulation dynamics can be accomodated. Suitable management needs to be carried out in existing and potential habitats over a wide area, allowing populations to build up and naturally colonise new sites. Butterfly Conservation is collaborating with the National Trust on burning experiments and discussing the practicalities of increasing grazing/burning with MAFF and landowners. Continued detailed surveys are essential to determine responses of colonies to management and to monitor metapopulation dynamics. It is important that at least one transect is recorded each year for the duration of the Heath Fritillary flight period, according to the standard criteria. This allows the timed count data to be adjusted to peak flight period and population levels to be assessed accurately. More research needs to be undertaken into the impact of livestock and deer grazing on Heath Fritillary sites. 1.0 INTRODUCTION 1 . 1 Background The Heath Fritillary (Mellicta athalid) is a species of high conservation importance: it is listed on Schedule 5 of the 1981 Wildlife and Countryside Act and is identified as a priority species in the UK Government's Biodiversity Action Plan (BAP) (DOE. 1995). A study by Butterfly Conservation in 1999 showed that the Heath Fritillary had declined by almost 50% on Exmoor in the 10 years since the last thorough survey (Feber et a/., 2000). The number of known colonies on Exmoor had declined from 29 in 1989 to only 15 in 1999. Bad weather has hindered recent surveys and a thorough re-survey of all the past known sites was undertaken in June/July 2000. The principle aim of this study was to determine possible reasons for the decline, including changing habitat management and the break-up of metapopulations1 (natural extinctions exceeding colonisations).