Butterfly transect summary 2017

In 2016, we covered every week of the season but this year, a small number of transect walks were missed. We completed 24 weeks out of 26 on Fleam Dyke and 25 on the Roman Road.  One further count on the Roman Road was rejected by ‘Transect Walker 2’, because of a lack of sun and a temperature of just 16°C, whereas in similar weather conditions, an early-season transect walk was accepted. However, in that count, no butterflies were recorded.  It appears that different weather criteria from those stipulated in the guidance notes, apply to counts done in April.

The 2017 season presented a real mix of weather, with very warm sunny spells in early April, June and at the end of August.  A total of 24 species were recorded on each site.  Total sightings were approximately 10% higher than in 2016 on Fleam Dyke and about 7% higher on the Roman Road.  Combining numbers of Small and Essex Skippers, and ignoring the magnitude of changes, 11 species on Fleam Dyke and 10 on the Roman Road showed increased indices relative to 2016, which was a rather poor year.

In 2016 the index for the Chalkhill Blue on Fleam Dyke was the lowest for 4 years but 2017 showed a 20% increase.  However, it was still well below the peak year of 2015.  On the Roman Road, the population of this species is very much smaller and the index dropped to just 15 from 19 in 2016 and 50 in 2015.

Our second habitat specialist on Fleam Dyke, the Green Hairstreak, had a poor year with an index of just 9, falling from 11 in 2016 and 21 in 2015. Interestingly, however, one was recorded in each of Transect weeks 1 and 2, the earliest records to date.

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, has not been recorded in transect counts for the last four years.

On Fleam Dyke, Small/Essex Skippers had a good year, with an index only slightly down on the record year of 2016.  On the Roman Road, there was a significant increase in the combined numbers of these two species but the indices did not reach the higher levels recorded in 2015. On Fleam Dyke, the Large Skipper showed a slight reduction compared with 2016 but, for the first time since we started monitoring, none were recorded on the Roman Road.

After a poor season in 2015, the Brimstone increased in numbers on both sites, the index rising by just 10% on Fleam Dyke and by a very significant 114% on the Roman Road from 178 in 2016 to 381 this year.  The Large White, Small White and Green-veined White all had a relatively poor year, showing reduced indices compared with the previous season.  Numbers were particularly low in spring.  The Orange-tip showed a modest increase in numbers on both sites.  No Clouded Yellows were recorded this year.

The Common Blue showed increases on both sites compared with 2016, which was a particularly poor year for this species.  The Brown Argus had another poor season on both sites with indices still in single figures.  After an excellent season in 2016, Holly Blue numbers dropped to a near average level this year.  A single Small Copper was recorded on the Roman Road, the first since 2015.

It was a mixed year for the vanessids.  The Red Admiral had a very good season but on Fleam Dyke there was a slight reduction compared with the record numbers of 2016.  On the Roman Road, however, 2017 was a record year with an increase of more than 100% to the highest index recorded so far. The Small Tortoiseshell had an excellent season on Fleam Dyke mainly as a result of a high count in week 11 but on the Roman Road there were less than in 2016.  Similarly, the Peacock showed an increase on Fleam Dyke but a reduced index on the Roman Road was the lowest since 2012.  The Comma occurs in very low numbers on both sites.  It showed a slight increase on Fleam Dyke but a reduced index on the Roman Road.  Seven Painted Ladies were recorded on the Roman Road and eleven on Fleam Dyke. Interestingly these were the highest numbers since 2009, the only year with large numbers of this migrant species.

Among the browns, the Ringlet showed reductions on both sites but the ‘Transect Walker 2’ software was unable to calculate an index for Fleam Dyke because there were no data for the critical week 13. We had almost exactly the same situation with this species in 2014 and the explanation was that although the software will normally produce an estimated number, in this case it was predicting a peak in numbers that week which would have taken the estimate beyond 30% of the index.  When this happens, an estimated number is not shown.

After a poor season in 2016, the Meadow Brown showed a good recovery in numbers on both sites.  The Gatekeeper was also recorded in higher numbers than in 2016. Speckled Wood indices were slightly down on the previous season.  Although numbers are very small, the Marbled White was again recorded on both sites, with indices of just 2 and 3 on Fleam Dyke and the Roman Road respectively.  The Small Heath had an excellent year with the highest indices since we started monitoring in 2007 and many times higher than those recorded in 2016.

We can expect dramatic changes on Fleam Dyke in 2018, resulting from the 80% clearance of scrub from sections 1, 2, 3 and 4, currently taking place.  This is likely to have a negative effect on those species dependent on scrub, such as the Green Hairstreak, Holly Blue and Brimstone but in the medium to long term, grassland species such as the Chalkhill Blue may benefit, depending on how the site is managed.  Although a hedge has been left along the south-west and north-east boundaries, the cleared sections will now be much more exposed to the prevailing winds.  In order to monitor the effects of this clearance, it will be important to maintain the transect programme in the coming years.

Chalkhill Blue on Fleam Dyke

Chalkhill Blue on Fleam Dyke