Fen Bog YWT

Gouthland

Site Description

Recorders:  Gary Savill                      Distance:   1352m                      Walk Time Approx 45mins

Fen Bog is a fen and moorland nature reserve situated between the summits of Tom Cross Rigg and Crag Stone Rigg and the summit of the NYM railway.  It takes the form of a long curving piece of land that is bounded at both ends by moorland streams. The 19 hectare site comprises of two main sections; the main valley mire and the higher ground leading down to it from the parking area which is primarily wet heath and moorland and is separately fenced from the rest of the site. 

Sections

S1 from near the car park   and then down to the entrance style and is quiet good for many grassalnd species. S2 enter heathland and Green hairstreak is found. S3 -S4 along the east rim of the bog is heath and is the best area to see Small Pearl-bordered Fritillary  as well as Small Heath abound along with Emperor moth. S5-S6 enter the bog where the Large Heath is found. 

Results 2024

 Countywide, after a warm winter  April turned  cold, very wet and dull and everything just about stopped and it was near impossible to transect walk all month. By May  there were already  losses  among our spring species being  down a third in numbers.  June brought Arctic winds, a complete reversal of  the Hot June of 2023 when nine  species reached all time highs here and across the UK.  For the  first two weeks of this June  temperatures  were 8’C cooler than  in 2023 around  a frigid  9’C  with  only 5 good days of sun at the end of the month.  The  first three weeks of July had  the same theme of  cool and damp with 5 warm sunny days at the end. Transect walking for many  was very difficult.  The  persistence of the cold and damp over such a long period had a devastating effect with numerically two thirds of our butterflies in the critical later stages of their development; mortality was very high. It was  also one of the longest June  ‘Lulls’  lasting from late May to the beginning of July. When the main flight  season did arrive it  was  slow to build up  and there was no usual peak. Better weather in August helped  save the second generation and very unusually numbers actually went up at the start of September.

  Losers

        For many species it was a triple whammy after  the heat and droughts of the previous 2 years and  last July’s poor flight period this year made it a perfect storm. Overall, annual numbers were down 41% against the average and 45% down on last year.  Half our species were down more than 50% and a quarter down more  than 70%. The Lycaenids, Vannesids and the Fritillaries were amongst the worst affected. Many transect walkers did not see  a single Common Blue, Small Copper, Holly Blue, or Brown Argus until late into their 2nd broods..  Four of our long term declining species Common Blue, Small Heath, Small Skipper and Green-veined White  had  their worst year ever recorded. 

The Winners and less bad losers 

      Northern Brown Argus  had an exceptional year up near 50%  Other species bucking  the trend were Brimstone and Marbled White. The  damp  loving Ringlet on many limestone/dry sites  were up a half  while on most wet sites they fell by a half.  However if it wasn’t for Ringlet holding up the overall numerical losses, this year would have been significantly worse. It often happens when Meadow Brown has a very poor year, like this one, Ringlet does proportionally better, and vice versa in a warm year. 

It has all happened before… many times.    

It was our worst year since 2016 but nearly as bad as 2009  when numbers  were  down 50%.  2024 comes after a run of 6 relatively good years since 2018  along with  rapidly rising temperatures including 2 of our hottest summers which were followed by two of our best butterfly years in 2019 and 2023. It is  likely 2024 was a blip

Fen Bog is just one of three sites that had a better year, in fact its best year so far.  It is possible that teh site is experiencing a resurgence after the extensive fire damage to the site. Unfortunatly Large heath has not been seen at thsi site for 2 years now which is worrying although it survives not far away on May beck.  The improvemnt in teh grasses is reflected in meadow browns and Ringlets

Transect reports

Results 2023

County wide 2023 results reflect  the 2022 and  2023 spring drought with Small Tortoiseshell and Peacock nearly halved for the second year against their 5 year average. Red Admiral arrived in force in July and took advantage of soft nettle growth of the rains and had their best year ever. Drought sensitive species on thin soils were badly hit,  particularly Dark -green Fritillary and Northern Brown Argus but also Ringlet, Green-viened White and Small Heath.  Less drought affected species along with the hottest June on record built even more on gains last year leading to Comma, Brimstone, Holly Blue and most Browns  having a fantastic year reaching all time highs. A increase of 9% overall was mostly due to sheer numbers of Meadow Browns and Gatekeepers ment 2023 nearly pipped 2014 as best year in modern times.  Most noticeable was the large differeces between mositure retaining mineral soils of the valleys and thin, dry limestone or sandy soils. A large number of damp grassland, hedgerow and woodland  dominated sites benefitted hugely with 3 sites seeing more than 50% increase . A smaller number of thin, chalky, sandy or craggy sites did badly some down up to 25%.

Fen Bog has had a torrid year with wildfire breaking out burning off teh bog area and surrounding and has had a very big influence on results along with last years hot weather some  things have boomed amonst the embers.  Total numbers were 30% down which is high but not unusual  on upland dry sites but still he pooreset year since 2015.  Smal Copper and Common Blue seem to be quick to respond to the more open conditions and likely their foodplant have taken advantage of the flush of nutrients from the fire.  Unfortunately no  Smal Pearl -bordered Fritillary or Large Heath were seen on transect but one LH was seen off transect and we hope they survive. 

Results 2022

Somewhat down on the recent average similar to other sites . The species trends however were very much the reverse of the rest of the county!  The big losers in the county were the skippers and the vanessids but not here where they both boomed which might be related to this being a wet site and being somewhat cooler with the altitude, The browns were  down while elsewhere they were almost universally up.  A wandering marbled white was seen for teh fist time this year as it was a dispersal year for this species.