Butterfly transect summary 2018


Butterfly Transect Data 2018

Transect walks were completed on both sites for every week of the 2018 season and all counts met required criteria and were accepted by the Transect Walker 2 software.

The season got off to a rather slow start but conditions improved and it was generally a very sunny, warm and dry summer, resulting in the early emergence of some species.  A total of 26 species were recorded on Fleam Dyke, two more than in 2017, but on the Roman Road, only 23 were recorded, one less than last year. Total butterfly sightings were approximately 22% higher than in 2017 on Fleam Dyke and 28% higher on the Roman Road, the highest numbers since 2015.

Looking at the results for individual species, it was a season of contrasts. Five species on each site had their best year since we started monitoring in 2007 but there were 12 species which showed a decline in indices relative to 2017.

Starting with our habitat specialists, the index for the Chalkhill Blue on Fleam Dyke showed a reduction of 23% compared with 2017 and was the lowest since 2012.  On the Roman Road, the very small population of this species showed an increase from an index of 15 in 2017 to 20 this year.  The maximum achieved was 50 in 2015.

Our second habitat specialist on Fleam Dyke, the Green Hairstreak, had a very poor season with an index of just 3, the lowest since we started recording.  The highest index was 34 in 2011.  The decline may be associated with the extensive scrub clearance last autumn, which particularly affected one of the hot spots for this species in section 3.  Apparently some Green Hairstreaks were sighted on scrub along the north-eastern boundary of the dyke, outside the Transect, but I was unable to find any there during my visits.

The Dark Green Fritillary, which appeared on Fleam Dyke for the first time in 2010 with one sighting, and showed modest increases to an index of 13 in 2013, had not been recorded in transect counts for four years but reappeared this year with an index of 3.

Moving on to the wider countryside species, numbers of Small/Essex Skippers were lower than in 2017 on both sites but they were around the average for the 12 years of monitoring.  In 2017, no Large Skippers were recorded on the Roman Road but this year we had an index of 37, almost equalling the maximum of 38, which was achieved in 2014.  It was also a better year for this species on Fleam Dyke, with an index of 15 compared with just 8 in 2017.

The Brimstone showed a slight reduction in numbers on Fleam Dyke but on the Roman Road, after a big increase in 2017, the index was down by more than 50%, close to that of 2016.  The Large White, Small White and Green-veined White all showed large increases compared with 2017.  The Small White index exceeded all previous years on both sites and on Fleam Dyke, the Green-veined White had its best year, with an index of 257, compared with the previous maximum of less than 200 in 2016.  The Orange-tip had a relatively poor year with reductions on both sites and on Fleam Dyke, numbers were the lowest since 2007.

The Common Blue result was one of the highlights of the year, with huge increases in the indices, from 15 to 136 on Fleam Dyke and from 11 to 116 on the Roman Road, the best year for this species since we started monitoring.  In terms of year on year increases, the Brown Argus did almost as well.  On Fleam Dyke the index rose from 6 to 45 and on the Roman Road from 4 to 29. This was the highest index so far on the Roman Road and the second highest on Fleam Dyke.  The highest was 69 in 2013.  From a near average level last year, Holly Blue numbers increased on both sites but numbers were still lower than in 2016.  A single Small Copper was recorded on Fleam Dyke but none on the Roman Road.

It was a very poor year for the vanessids.  After excellent seasons in 2016 and 2017, this year the Red Admiral showed reductions of 68% and 86% respectively on Fleam Dyke and the Roman Road.  The Small Tortoiseshell was also down in numbers, particularly on Fleam Dyke, where there was a 69% reduction.  The Peacock and Comma also showed a reduced index on both sites.  No Painted Ladies were recorded on the Roman Road this year and there were just two sightings on Fleam Dyke.

Among the browns, the Speckled Wood had a good year with increased indices on both sites.  The Gatekeeper showed a decline compared with 2017 to near average numbers.  The Meadow Brown increased slightly on Fleam Dyke but had a lower index than in 2017 on the Roman Road.  The Ringlet had a good season with substantial increases on both sites. Marbled Whites have never occurred in large numbers on either site, with indices normally in single figures but this season we recorded indices of 15 and 11 respectively on Fleam Dyke and the Roman Road, the best year so far.  In 2017, the Small Heath had the highest indices since we started monitoring in 2007 and in 2018 numbers increased again, only marginally on Fleam Dyke but on the Roman Road, where the population has always been smaller, the index increased by an impressive 47%.

With the possible exception of the Green Hairstreak, the impact of the scrub clearance on Fleam Dyke in sections 1-4 has been less dramatic than was expected but it should be noted that in sections 1 and 2, only the north-eastern outer bank has been completely cleared while scrub has been retained on the inner south-western facing bank. I understand that most of this will be cleared during the coming autumn/winter season.  The main impact, therefore, has been in sections 3 and 4.  In section 3, there were already extensive areas of chalk grassland with minimal scrub and section 4, around Mutlow Hill, has never been particularly high in butterfly numbers. We should continue to monitor the effects of the clearance in the coming years.

Chalkhill Blue on Fleam Dyke

Chalkhill Blue on Fleam Dyke