Butterfly transect summary 2019


Butterfly Transect Data Summary 2019

On Fleam Dyke, Transect walks were completed for every week of the 2019 season and on the Roman Road, just one week was missed. However, its absence at the beginning of the season will have had little impact on the overall picture. All completed counts met required criteria and were accepted by the Transect Walker 2 software.  A total of 26 species were recorded on Fleam Dyke but on the Roman Road, just 22 were recorded. Total butterfly sightings were approximately 10% higher than in 2018 on Fleam Dyke but 5% lower on the Roman Road. The difference is mainly attributable to the large increase in Chalkhill Blue numbers on Fleam Dyke (see below).

Looking at the data for individual species, it was a season of mixed results but our three habitat specialists, with the exception of the Chalkhill Blue on the Roman Road, had a good season.
On Fleam Dyke, the Chalkhill Blue had its best year since 2015, the index having increased by 58% compared with 2018. On the Roman Road, the very small population of this species is just hanging on, unfortunately showing a decrease from an index of 20 in 2018 to just 9 this year, the lowest since 2011. The maximum index achieved was 50 in 2015.

Our second habitat specialist species on Fleam Dyke, the Green Hairstreak, had a better season this year with an index of 17, compared with just 3 in 2018. The highest index was 34 in 2011. The extensive scrub clearance, particularly in section 3, has probably affected the population in its previous hot spots but interestingly, this year, 12 were recorded in sections 7 and 8, where we had not seen any previously. The number recorded in sections 2 and 3 was just 5.

The Dark Green Fritillary, which appeared in our Transect on Fleam Dyke for the first time in 2010 with one sighting, and showed modest increases to an index of 13 in 2013, was not recorded in transect counts for the subsequent four years but reappeared in 2018 with an index of 3. This year, the index increased to 10.
Moving on to the wider countryside species, numbers of Small/Essex Skippers were near average on both sites with a slight reduction on Fleam Dyke and a modest increase on the Roman Road. After an excellent season in 2018, there were significant reductions in the number of Large Skippers on both sites.

The Brimstone showed an increase in numbers on Fleam Dyke, almost reaching the level achieved in 2014, our best year to date. On the Roman Road, however, the increase was relatively small. After a very good season in 2018, the Large White was present in much smaller numbers on Fleam Dyke in 2019 and showed a slight decrease on the Roman Road. Small White and Green-veined White numbers were down on both sites after almost record indices in 2018. The Orange-tip had a good year on Fleam Dyke with a very significant increase in numbers relative to 2018 but on the Roman Road there was a reduction of about 30%.

In 2018, the Common Blue had its best year on both sites since we started monitoring in 2007 but this year, there were 85% and 87% reductions on Fleam Dyke and the Roman Road respectively. After a very good year in 2018, the Brown Argus showed reductions of 51% on Fleam Dyke and 34% on the Roman Road. Holly Blue numbers dropped to a near average level on Fleam Dyke but rose by 165% on the Roman Road, equalling our best year to date. No Small Coppers were recorded on the Roman Road but on Fleam Dyke, we had an index of 3. This compares with just 1 in 2018, which was our first record on this site since 2007.

It was a mixed year for the vanessids. The Red Admiral showed a modest increase on Fleam Dyke but did not reach the levels achieved in 2016 and 2017. On the Roman Road, numbers were similar but this represented a large percentage increase from the very low numbers recorded in 2018.

On both sites, the Small Tortoiseshell showed a further reduction from the rather poor figures obtained in 2018 but Peacock numbers improved slightly. The Comma, which occurs in very small numbers on our sites, showed little change. It was a good year for Painted Ladies, by far the best for this migrant species since 2009, but numbers did not reach the very high levels recorded in that season.

The Marbled White was one of the highlights of the year. The 2018 season had been our best to date for this species but this year, the indices increased from 15 to 68 on Fleam Dyke and from 11 to 65 on the Roman Road. Hopefully this is an indication that this species is becoming established on our two sites, after many years of zero or very low numbers in the Transect counts. The Speckled Wood was down on both sites to a near average level on Fleam Dyke but below average on the Roman Road. The Gatekeeper and Meadow Brown showed increases on both sites, the latter having a particularly good season on the Roman Road, where an index of 262 equalled our previous best season of 2009. The Ringlet, on the other hand, had a relatively poor year, particularly on the Roman Road, where the index was down by more than 60%. The Small Heath had particularly good seasons in 2017 and 2018 and on Fleam Dyke showed a further increase of around 20% in 2019, our best year so far. On the Roman Road, where the population has always been smaller, there was a reduction of about 36% compared with 2018.

Fortunately, the scrub clearance in sections 1-4 on Fleam Dyke does not seem to have had any obvious impact on butterfly numbers so far. The Green Hairstreak was the species we were most concerned about and although numbers declined in 2018, there was a good recovery in overall numbers this year, as described above.

Chalkhill blues on carline thistle, Fleam Dyke