Carmarthenshire – the extraordinary movements of Marsh Fritillaries in 2018.

A wonderful post from Richard Smith:

It’s not often that a long-threatened and declining species turns up in 73 additional fields in a county, even one the size of Carmarthenshire. That’s what happened in 2018 and that’s just the part we know about. So what has lead to this amazing phenomenon? It’s a fascinating story and made all the better that we still don’t know all the answers. Wildlife Conservation is still a modern science and precise life-stage requirements (“auto-ecologies”) in the wild for conservation priority butterflies in the UK, were mostly only fully researched within the last 25 years. Since then, climate change, air, water & land pollution by enrichment from intensive human activity, including agriculture, have also impacted. Speaking to leading plant geneticists in Wales, the general public is probably not aware of the increased competition for suitable butterfly habitats from “invasive” species – not just the aliens like Japanese Knotweed & Himalayan Balsam but Bramble is thought to grow a third more vigorously than a decade ago.  Then there are the almost universal ryegrass monocultures with sheep and/or silage. If it were not for EIA Uncultivated Land Regulations, farms would probably have ploughed and re-seeded even more natural and semi-natural wildlife habitat. Cue the smallholders and others with land which doesn’t have to be cropped to provide income – there is a significant and mostly positive role played by these in Carmarthenshire.

Since 2000, there has been an initiative by the then Countryside Council for Wales (now subsumed within NRW) to assess local landscapes for habitat suitability for Marsh Fritillaries across Wales to help fulfill EU, UK and Wales government biodiversity duties. Thus in South Wales, Glamorgan has been almost completely covered, plus parts of west Wales. So saying, the immediate area around Cross Hands was assessed in 2001, then expanded to a larger radius area (Mynydd Mawr) in 2009 and again in 2018. Meantime, with support of local biodiversity partnerships, we got Amman valley and several localities around Brechfa Forest assessed in last 5 years. These studies identified several thousand hectares of suitable habitat still extant across these locations but usually in private ownership. Of these only a small fraction revealed occupying populations of Marsh Fritillary butterflies, so with increasing member and volunteer interest, we set about trying to visit some of the most promising sites across Carmarthenshire. 

Over the years, we’ve had some early successes, notably an initiative in 2005 to visit Small Pearl-bordered Fritillary sites in SE Carms, brought records of Marsh Fritillaries for a few sites!  The 2009 Mynydd Mawr habitat assessment had revealed a dozen more occupied sites. Then for a few years most of our volunteer effort in Carms went into helping to manage our reserve at Caeau Ffos Fach and (later) the adjoining Median Farm. Meanwhile BC Wales’ MF Surveillance programme concentrated on regular visits to assess MF population on a selection of known sites across Wales, allied with an initiative to re-visit all known MF sites every 5 years – which is where volunteers could help. However, the Carms CC/Wales Gov’t funded habitat study around Brechfa Forest in 2016 showed habitat and MFs across a much wider part of the county than previously known, which gave us opportunities to involve volunteers and smallholders from wider Carms, not just the Mynydd Mawr & Amman valleys area. 

It’s worth reflecting on MF metapopulation theory at this point. MFs occupy sites/fields from time to time but need a network of suitable habitat patches within easy reach to move to/from as their parasite and habitat condition changes often year on year. Some models suggest that roughly every seven years on average, local MF populations will “Boom” with up to 200 caterpillar webs present per hectare! A famous example in history is thousands of MFs erupting in Ireland in the 19thcentury, around time of potato blight. Seldom are such densities found in Wales!  We know from hard experience in South Wales that this movement is unpredictable for particular localities (metapopulations) and by no means synchronized across metapopulations. However, if parasite activity on caterpillars in spring is low (often attributed to dry, sunnier spring) and weather in flight period (late May into June) is favourable, movement of MF butterflies can happen then. Its success, of course, depends on the conditions being suitable in the local “arrival” sites. They need continuous Devil’s Bit Scabious, caterpillar hibernation opportunities and minimal disturbance to persist into subsequent years. Cue 2018!

Flight period highlights, June of 2018.

It is difficult in a post like this to provide accurate maps or grid refs for individual fields, given data protection and privacy of many sites. However, some of the flight period highlights included: a timed count, by five of us volunteers (Alan, Dai, Gareth, Paul & I), of 554 Marsh Fritillaries across 8 fields in Harford in north of county; 70 counted by myself & Rob Parry in a known small site near old railway in Cwmgors; 40 counted in another small meadow in Harford and Amanda Evans & I counting 46 at a new site near Llannon. 

  • New fields with adults only found in 2018 in Mynydd Mawr metapopulation = 4 new fields
  • New fields with adults only found in 2018 in other Carms metapopulations = 22 new fields

Larval web count highlights, July to September 2018.

As it didn’t rain between end of flight period in late June and early August, we had the unprecedented luxury of finding larval webs right through this period, in addition to normal period of mid August to late September. (The rain usually washes away webbing from early caterpillars clusters, but not in 2018, so they were very visible). Again, some highlights: Rob Parry got to 187 webs in those Cwmgors old railway fields and ran out of time; 50 webs at a new site near Trapp (Francis, Kath, Paul & I with Meryl from NRW – and now a new SSSI); 52 webs at that small meadow in Harford; Dave Bannister finding 9 webs at a new site near Brechfa; 25 webs in a lovely pingo field in Harford where none found previous two years and finally Amanda, Francis, George & I finding 3 webs in a great looking pasture north of Ammanford where we’d failed to find evidence last four years.  

  • New fields containing larval webs found in 2018 survey in Mynydd Mawr area but o/s current SPG (project area) boundary = 31 new fields
  • New fields containing larval webs found in 2018 survey in Mynydd Mawr area but within current SPG boundary = 5 new fields
  • New fields with larval webs found in 2018 in other Carms metapopulations = 10 new fields

Keeping going into 2019.

Even with a generous volunteer mileage budget agreed by our branch committee for 2018, we just didn’t get to all the sites we would have liked to. Given that larval webs were found in so many new sites, we hope that they will spill over into sightings in 2019, which means it should be possible to visit additional Carmarthenshire sites this year and to find MFs present. We hope to start in late May and first half of June with visits to find adults in flight period and then continue initiative through the larval web season of August & September. So if you might be available to help and join like-minded people, do let us know. 

Incidentally, although we concentrated on Carmarthenshire, given its already recognized importance for MFs in Wales, a handful of new sites were also found in both Ceredigion and around Seven Sisters in Neath Port Talbot. Quite a stunning episode in times of Marsh Fritillaries in Wales!

Richard Smith

Photographs showing not Marsh Fritillaries but some of the other experiences that we had during those search days. 

Early flowering Devil’s Bit Scabious in Brynamman hay meadow, 11 June            

  View over one of the fields towards Carreg Kennen

Time out with butterfly orchids in Pencader cemetery

                                             Gareth & Kath from another field

Owners joining in with web searches  

Marbled Whites in Trapp field with larval webs, July

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