The Expansion of the Comma Butterfly From County Wexford Up Four River Corridors
THE EXPANSION OF THE COMMA BUTTERFLY FROM COUNTY WEXFORD UP FOUR RIVER CORRIDORS
Pat Bell (PB), Jesmond Harding (JH), Andrew Power (AP) and Brian Power (BP)
The first confirmed report of the Comma butterfly Polygonia c-album in the Republic of Ireland was from the Raven Nature Reserve, Co. Wexford in August 2000 and it was not recorded again in Wexford until September 2005. Total sightings for Ireland increased to 40 in 2008 and remained around the half-century mark for the next five years before increasing strongly in 2014 and 2015. The authors detected a particular expansion up four river corridors which is the subject of this article.
It was first recorded in Enniscorthy (River Slaney) in July 2009, Waterford City (mouth of River Suir) in August 2009, St. Mullins, Co. Carlow (River Barrow) in August 2010 and New Ross (mouth of River Nore) in August 2011. By the end of 2015 it has been recorded near the source of the River Slaney in the Glen of Imaal, Co. Wicklow; Carrick-on-Suir, Co. Tipperary, on the River Barrow, Maganey, Co. Kildare and on the River Nore on the outskirts of Kilkenny City.
The expansion of the Comma up the river corridors of the Slaney, Barrow, Nore and Suir was studied. A possible eastern corridor was considered. The methodology consisted of the authors’ own field studies and observations and an analysis of public records was undertaken. Maps and river systems were examined. The records were primarily from Butterfly Conservation Ireland (BCI), Butterfly Ireland (BI) and the National Biodiversity Data Centre (NBDC). These records were also documented and quantified by Ian Rippey of Butterfly Conservation Northern Ireland (www.bcni.org.uk). Some aspects of the Comma’s expansion in Britain were also reviewed. Two of the authors (JH & BP) published the first verification of Commas breeding in Ireland in 2014 and this situation is updated here.
While the expansion of the Comma is not confined to river corridors, this arguably has been the most dramatic and measurable aspect of their expansion and is summarised below.
The Slaney is the first river system that Commas would encounter if they moved inland from the Raven Co. Wexford. Sightings began to be reported west of the Raven in the Eden Vale/Crossabeg/Castlebridge area and in July 2009 a sighting was reported from Enniscorthy. Two of the authors (AP & BP) first observed them in Kildavin, Co. Carlow in 2011 and have continued to study this population at this site. The first recording from Altamont Gardens Co. Carlow was in 2014.
The first recording from the Barrow came from St. Mullins, Co. Carlow in August 2010. Sightings were reported from the foot of Mount Leinster, Co. Carlow in 2012 and 2013. The first recording from New Ross was in 2011. BP searched further upriver and found them near Borris in July 2014 – the large numbers subsequently recorded by him near Ballytiglea might indicate that they had already been here before 2014. These sightings, and some from Palatine, north Co. Carlow in April 2015 prompted PB to look for them in Kildare and one was duly found in August 2015 north of Maganey Bridge. PB had recorded three individuals a few weeks previously between Carlow Town and the border with Co. Kildare.
The first recording from New Ross, Co. Wexford at the mouth of the Nore, was in August 2011. A further sighting at Bennettsbridge followed in August 2012. In August 2015 a record came in from Lavistown on the outskirts of Kilkenny City. There have been an increased number of sightings coming from Co. Kilkenny in 2014 and 2015 in addition to records from the Nore valley.
A sighting was reported from Waterford City in August 2009. AP recorded the first Comma from the National Biodiversity Data Centre, Co. Waterford in July 2011. They have continued to move westwards and were recorded from Mount Congreve, Co. Waterford in July 2013 and Carrick-on-Suir, County Tipperary in August and October 2015. At nearby Wellingtonbridge Co. Wexford there have been records since 2010. It is possible that this is the route they took westwards.
East coast corridor
There has been a discernible expansion northwards up the east coast of Co. Wexford and up into Wicklow. The first sightings from Ferns and Courtown Wood were in July 2010, from nearby Arklow in August 2012 and several sightings were reported from Mount Usher Gardens just west of Wicklow town in 2015. However, there have been sightings north of here. The Comma is expanding in Britain, and it may be the likely reason why it is initially appearing along the east coast of Ireland. They may have arrived at different points at coastal locations where no breeding habitat exists so that individuals may have dispersed without finding a mate, while the suitable breeding habitat at the Raven may have resulted in individuals remaining and breeding successfully, providing a base from which to expand in Ireland.
The Comma can be seen at any time of the year even occasionally awakening on warm winter days. In January 2016, the first sighting for the year had already been recorded in England. It usually emerges from hibernation in March giving rise to the first generation which appears at the end of June and early July. Part of the first brood, which is known as the normal or dark form because of their dark undersides, goes on to hibernate. The remainder of the first brood is lighter in colour and known as the form hutchinsoni. These breed immediately producing a second generation consisting of normal/dark form Commas that feed for a few weeks before entering hibernation. As a result, there is another peak emergence in late summer/early autumn (August/September).
There is much ongoing discussion and speculation about their amazing dispersal or colonising behaviour. It is a highly mobile and nomadic butterfly especially during July, August and September. It is also long lived. One UK academic referred to colonists as not being a random selection of the source population but that they “share a suite of traits associated with increased dispersal ability.” Most of the boundary-pushing sightings have been in late July and August when the non-direct breeders are attracted to flower filled areas, including gardens, but nearer winter they seek suitable woodland where they feed until cold conditions arrive. Two boundary lines were drawn for the known limits of their expansion, one for 2011 and one for 2015 (Figure 1). The rate of expansion between these two boundary lines over these four years is approximately 10 km per annum which is similar to reported rates for their expansion in Britain.
Population size would appear to be another factor influencing expansion. Research from the University of York has concluded that much of the variation in range expansion rates can be accounted for once population trends are known. Study of Irish records suggests that there have been a number of population build-ups in places such as Eden Vale/Crossabeg/Castlebridge, Kildavin and Borris/Ballytiglea and that these populations became bases for further expansion. Overall population has increased exponentially in the past two years from a period of about six years where total number sighted remained around the half-century and then jumped to 198 and 414 records in 2014 and 2015 respectively as referred to in the introduction (Figure 2).
Having reached the source of the River Slaney will it expand by railway line and/or roadside? For example the old disused Sallins-Tullow railway line and the N81 road could be zones for expansion. This could possibly bring the Comma to the River Liffey. On the River Barrow, the next town upriver is Athy and after that Monasterevin. The Barrow Way towpath makes this the most walkable of the four rivers. It may already be in Kilkenny City on the River Nore and continuing upriver will bring it to Ballyragget and on into County Laois. It may possibly be already present in this county as the western bank of the northerly sightings on the Barrow is in County Laois. The fact that the Comma’s expansion into Kilkenny is not confined to the Nore is an interesting development and one that is likely to be repeated if the Comma spreads up into the country from these river corridors. Clonmel is the next large town upriver on the River Suir. Most of the East Munster Way between Carrick-on-Suir and Clonmel is on the Suir so perhaps this will yield future sightings.
Following the confirmation of breeding at Cuilaphuca Wood, near Kildavin, Co. Carlow in 2014, breeding has since been confirmed south of Ballytiglea Bridge, Co. Carlow, and at Ravenwood, Co. Wexford. In the latter two locations summer breeding was confirmed with hutchinsoni adults present and ova found on Common Nettle, the main larval host-plant. The increase in the range and distribution of the Comma in parts of Counties Wexford, Waterford, Carlow and Wicklow and reports of the butterfly in successive years in places like Altamont (Carlow), Ferns (Wexford) and Shillelagh (Wicklow) suggests that breeding may be widespread in suitable habitat in these areas.
The factors that may be influencing the expansion of the Comma in Ireland are not clearly understood and data is limited but favourable climatic conditions probably linked to climate change may be relevant. The very suitable weather conditions during the spring egg-laying period during April 2014 and April 2015 is likely to have contributed to the increases in sightings in those years. The Comma responds well to warm, sunny springs when larvae develop rapidly and are more likely to produce direct breeding hutchinsoni, which means that a second, dispersing/nomadic brood is produced. On the other hand April weather in 2012 and 2013 was much cooler which corresponds with lower population figures for those years. The population may be larger and might be more likely to expand following a sunny April.
Other climatic factors may be relevant. Winter humidity and temperature have both been proposed as causal factors in the contraction and subsequent expansion of the Comma in Britain. Winter temperature trends appear to affect populations with milder winters being linked with changes in distribution while successive cold winters coincide with periods of decline. The Comma is regarded as less able to survive cold winter weather than the Small Tortoiseshell. This lower survival rate may be related to the more exposed locations chosen by the Comma as overwintering sites.
Another factor is habitat availability. While the larval host-plant is common and widespread, the Comma prefers plants growing in fertile soil in damp situations that receive full sun. These conditions are present in all of the sites where breeding has been confirmed and are available along stretches of the waterways referred to in this article. The presence of breeding habitat along the rivers in the south-east might be the main reason that the Comma seems to be moving northwards along these routes. The species is however highly mobile and will readily fly across open countryside.
Mild winters, sunny springs and the abundance of suitable habitat are likely to result in further expansion in the coming years. Whatever the reasons behind its colonisation of parts of Ireland, we are delighted to see the Comma’s addition to Ireland’s butterfly list.