Fritillary monitoring results 2020 Dartmoor and Exmoor Jenny Plackett South West Regional Conservation Manager Butterfly Conservation Company limited by guarantee, registered in England (2206468) Registered Office: Manor Yard, East Lulworth, Wareham, Dorset. BH20 5QP Charity registered in England and Wales (254937) and in Scotland (SCO39268) www.butterfly-conservation.org 1 Contents Summary of fritillary results across Dartmoor and Exmoor Page 3 High Brown Fritillary Page 3 Marsh Fritillary Page 4 Heath Fritillary Page 4 Pearl-bordered Fritillary Page 5 Small Pearl-bordered Fritillary Page 5 Acknowledgements Page 5 Appendices Appendix 1 Dartmoor High Brown Fritillary raw data and graphs Page 6 Appendix 2 Exmoor High Brown Fritillary raw data and graphs Page 7 Appendix 3 Dartmoor Marsh Fritillary raw data and graphs Page 8-10 Appendix 4 Exmoor Heath Fritillary raw data and graphs Page 11-12 Appendix 5 Dartmoor Pearl-bordered Fritillary Page 13-14 Appendix 6 Dartmoor Small Pearl-bordered Fritillary Page 15 Appendix 7 Exmoor Small Pearl-bordered Fritillary Page 15 Front cover images Clockwise from top left: Small Pearl-bordered Fritillary, Marsh Fritillary, Heath Fritillary, High Brown Fritillary, Pearl-bordered Fritillary 2 Summary Monitoring of the fritillary butterflies on Dartmoor and Exmoor was severely hampered by the covid pandemic, which resulted in fewer surveys being carried out, and fewer butterflies recorded overall. Travel restrictions coincided with the peak in the flight periods of the Pearl-bordered, Small Pearl- bordered and Heath Fritillary, so it’s likely that the abundance of these butterflies was not fully captured. Recording of adult Marsh Fritillary butterflies was also affected, however by August movement restrictions had been eased and the Marsh Fritillary larval web surveys were unaffected. Overall, the fritillaries had a mixed year in the South West, with High Brown Fritillary having an exceptional year, particularly on Dartmoor, whilst Marsh Fritillary appeared to have a poor year with abundance at the lowest point since 2013. Heath Fritillary did better than in 2019, but did not achieve the excellent numbers seen in 2018. The data should be read with extreme caution, however, since the reduced survey effort, and likely mis-timing of surveys outside of the peak in the flight period, will have certainly affected the results. Each species is presented in turn with a brief report: High Brown Fritillary, Marsh Fritillary and Heath Fritillary, with graphs showing the population indices for each species in Dartmoor and/or Exmoor, set against the national trend for that species. Results for Pearl-bordered Fritillary and Small Pearl-bordered Fritillary are also presented, although there was insufficient data to enable a regional trend to be calculated. Graphs for species abundance in individual landscape areas are displayed in the appendices, along with the raw data. High Brown Fritillary Population trend over the period 2002-2020 Dartmoor STABLE Exmoor 1021% increase Figure 1. Population trend of High Brown Fritillary on Dartmoor, Exmoor and the UK 2002-2020 In Dartmoor, High Brown Fritillary had an exceptional year, exceeding any in the 18 year monitoring period. Numbers have been increasing year on year since 2017, and the overall population trend averaged out over the full period remains stable. In the Dart Valley network, Aish Tor, Blackadon and Luckey Tor had very good numbers, which have been rising since 2017. Dartmeet, Leigh Tor and Great Combe fared less well, with numbers down on 2019, and none recorded at Great Combe. The Walkham Valley sites did exceptionally well, with higher numbers recorded than any in the 18 year annual monitoring period, and a comment from the surveyor that numbers were as he had remembered them being in the nineties! See Appendix 1 for raw data and graphs. 3 In Exmoor’s Heddon Valley, the butterfly also had a good year in 2020, increasing on the 2019 abundance, and doing reasonably well following the declines seen in the first half of the monitoring period. Over the full 18 year monitoring period, the butterfly has seen a 1021% increase in trend. Ladies Mile saw higher numbers than have been recorded over the last ten years, as did Trentishoe Combe (with the exception of 2014), whilst Mill Farm and King’s Lane Slope fared similarly to the last 3 years, but down on numbers recorded in 2016 and 2017. See Appendix 2 for raw data and graphs. Marsh Fritillary Population trend over the period 2005-2020 Dartmoor 423% increase Figure 2. Population trend of Marsh Fritillary on Dartmoor and the UK 2005-2020 2020 was not a good year for the Marsh Fritillary on Dartmoor, however the long term population trend for the species in this landscape shows a 423% increase over the 16 year monitoring period. Numbers tend to fluctuate on around 5-7 year cycles of ‘boom/bust’; hopefully numbers will rise in 2021 from the low seen in 2020. Marsh Fritillary is found across four metapopulations on Dartmoor, and the butterfly fared quite poorly in all four, with one or two sites in each network bucking the trend with a more positive result. In the Tavy valley network, good numbers were seen at Hillbridge Farm; in the Postbridge West Webburn valley system Challacombe Farm saw good numbers; in the Fernworthy Long-Lane system Langaford Farm stood out with good numbers; whilst in the Tor Valley, Venton Farm held its own whilst other sites were unoccupied. See Appendix 3 for raw data and graphs. Heath Fritillary Population trend over the period 2004-2020 Exmoor STABLE Figure 3. Population trend of Heath Fritillary on Exmoor and the UK 2005-2020 4 Overall the Heath Fritillary had a reasonably good year on Exmoor, with many sites having higher abundance than recorded in 2019, although the extraordinary numbers seen in 2018 were not replicated. 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 (see Figure 3 above). The Holnicote Estate sites did well overall, with Heath Fritillary recolonising two sites, Nutscale Reservoir and Mansley Combe (the former site being extinct since the early 1990s), and strong numbers were recorded at core sites. The Grabbist Ridge sites did not fare so well, although numbers were at least higher than 2019. Hopcott Common and Perriton Combe were not surveyed but the three core breeding areas along the ridge were disappointing. It may be that the peak was missed on these sites, as the majority of surveys on Holnicote Estate were undertaken between 28th May and 2nd June, whilst the Grabbist sites were not visited until 15th June. Haddon Hill was also visited on 15th June, probably also missing the peak, as numbers were considerably down on the 2017-18 high (although still exceeded abundance during the years 2012- 2016). See Appendix 4 for raw data and graphs. Pearl-bordered Fritillary Spring started early in 2020, with warm and sunny weather throughout late March-May, and the Pearl-bordered Fritillary is likely to have emerged and peaked earlier than in most years. Unfortunately, the covid-related travel restrictions limited survey activity, as we were all obliged to stay very local to home during April. A couple of survey visits were made to sites in the Dart and Walkham Valleys, but the resulting data was insufficient to calculate a regional population trend. The Aish Tor count was slightly lower than numbers seen in 2018 and 2019, but slightly above average for the ten year period. It was pleasing to see numbers recover somewhat at Walkham Valley, following no records in 2019. Several visits were made to the Haldon Forest sites where the butterfly is doing reasonably well at the Haldon Forest site (Kidden’s Plantation), although numbers appear to be declining at North Wood. See Appendix 5 for raw data and graphs. Small Pearl-bordered Fritillary A small number of surveys were carried out for Small Pearl-bordered Fritillary across Dartmoor and Exmoor, although these records were often made whilst recording Pearl-bordered Fritillary (which generally peaks earlier than SPBF) or Marsh Fritillary (which generally peaks later than SPBF on Dartmoor), so the timing of the survey visits were not always optimal for the butterfly and the peak in flight period may have been missed. Insufficient data was collected to enable calculation of a population trend. See Appendix 6 for raw data. Acknowledgements With grateful thanks to all those who have helped with the 2020 monitoring programme – thank you so much for all your efforts, and please do help again in 2021! Thanks too to the many land managers, commoners and landowners who are working hard to keep their fritillary habitat in good condition to enable the butterflies to thrive on their land. Jenny Plackett South West Senior Regional Conservation Manager [email protected] 07918 073654 5 Appendix 1: Dartmoor High Brown Fritillary Wind (code) speed HBF Time adjusted Duration of search Duration of Mean temperature Mean Mean % sunshine sunshine % Mean Confirmed HB/DGF DGF Site Name Grid Ref. Date Aish Tor SX705705 01/06/2020 40 120 20.0 Aish Tor SX705705 24/06/2020 23 100 0 9 40 12 13.5 Blackadon SX713732 16/06/2020 20 50 2 2 40 2 7 3.0 Blackadon SX713732 24/06/2020 23 100 0 12 45 1 0 16.0 Dartmeet South SX673725 26/06/2020 20 50 2 2 36 0 5 3.3 Dartmeet South SX673725 24/06/2020 23 100 0 2 30 5 0 4.0 Deeper Marsh, Newbridge SX714712 24/06/2020 23 100 0 2 20 3 0 6.0 Dunsford SX785892 07/07/2020 18 50 2 0 101 0 2 0.0 Great