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The Gatekeeper - from an Irish perspective

































by Ken Bond

 

Irish butterflies generally fall into two distribution categories.  There are those that are generally distributed throughout the country, such as the Meadow Brown or the Green-veined White, and those that are confined to certain parts of the country, because of foodplant or habitat limitations.  Examples of the latter include the Brimstone, confined to midland and Western areas with buckthorn, or the Large Heath, confined to extensive areas of raised bog or blanket bog.

 

There is another category, which is surprisingly poorly represented here, those that are confined to certain geographical areas, apparently for climatic reasons.  The only obvious Irish representative of this group is the Gatekeeper, the distribution of which shows a distinct northern limit running through Kerry and Cork, and then reappearing at almost entirely coastal sites in Waterford, Wexford and Wicklow.  This is in marked contrast to the situation in Britain where the great majority of butterflies have a well-recognised northern limit. A limit such as Bristol to the Wash is typical of many species.  This situation is also found in mainland Europe (along with a southern limit).

 

Why does this situation occur with only one species in Ireland?

 

What does the distribution tell us about the Gatekeeper's habitat requirements - is its distribution limited by climatic factors, such as temperature or sunshine duration?

 

The answer to the first question may be found in the relatively uniform habitat and climate found over Ireland from north to south.  In addition, it is probable that most of our species arrived over a land-bridge in the vicinity of north Donegal.  Once they arrived there, they were able to gradually expand southwards to the rest of the country, where equally suitable habitat occurred.  If they had arrived over a land bridge from the south, as in Britain, many more species might have arrived here, but at least some of these would have found acceptable conditions only in the south.  This would have produced a situation similar to that in Britain, with northern limits for most species.  We can speculate that the Gatekeeper occurred earlier in the north of Ireland in a period of more favourable climate.

 

It's likely that the Gatekeeper's distribution is controlled by both climatic and habitat factors.  If we combine mean air temperature with sunshine duration, we can produce a climatic zone with approximately the Gatekeeper's distribution.  However, some of the sites in West Cork and Kerry can scarcely be described as sunny, and this is where the habitat aspect must also play a role. Colonies of Gatekeepers are known to exist in relatively cloudy and wet locations, for example near Dunmanway or Kenmare, but here they seem to favour rocky, south-facing locations.

 

When we take a closer look at the Gatekeeper habitats, and in particular those at the northern edge of distribution, we find certain features in common.  During August 2002 a colony was found at Mount Desert, just west of Cork City, which represents the local northern limit for this species (and the closest to the city as far as I know, grid ref. W637718).  This site is steeply south-facing, and within a disused (abandoned?) field of long grass and abundant gorse scrub.  There is also abundant bramble blossom, on which the butterflies bask in sunshine.  The butterflies occurred only in certain shaded parts of this field, where gorse sheltered them from the breeze.  Repeated transects of the field showed that they remained in these small localised pockets of habitat.  The nearest colony to the west known to me, and also one on the northern limit, is at Burnt Mill Station, in the Shournagh Valley (W584767).  Here a colony of eleven Gatekeepers was found in late July 1999.  This is again south-facing, but along an old hedge bank, with a good road verge of long grass.  As at Mount Desert, the Gatekeepers were strictly confined to one area, a section about 200m long.  A colony of Gatekeepers was found in 2001, and again in 2002, just west of Halfway (W584612).  This is another steeply south-facing locality, with abundant bramble blossom.  There may be similar colonies elsewhere along this hill slope, which is also the site of the disused Cork-Bandon railway line, but the dense bramble growth makes access difficult.  Gatekeepers were also observed here about 20 years ago, and as the 6-figure grid reference had been noted then, it was possible to locate the site exactly.

 

The common features of these sites are: south-facing, abundant bramble blossom and undisturbed grassland with at least some long grass.  The steepness and southern aspect must also mean that they are relatively dry, which may be an important factor for overwintering survival of the larva.

 

The Millennium Atlas (2001) showed most recent period records only for 1995-2000, and this gave the impression that the Gatekeeper had disappeared from many of its older sites.  In fact, since 2002 it has been rediscovered in many of these, and also found in several new Cork sites, thanks to targetted recording.  Up to summer 2002 it had appeared that the species no longer occurred in East Cork, but it has been rediscovered at Little Island (Courtstown Industrial estate), and Tom Gittings reported it from Ballyannan Wood.  In addition a good colony still exists at Rostellan, but it would be very useful to know of any colonies further north and east.  There is also an apparent gap in distribution in the triangle roughly between Macroom, Bandon and Ovens, but is this simply due to lack of recording here?  Records are also sparse in the area north of Clonakilty, and this is probably due to the lack of  "good" hedgerows and sheltered sites.  If anyone knows of other colonies in any of these areas, please let me know.  We need to get as much information as possible now for a forthcoming atlas of Irish Butterflies, so that the gaps revealed by the Millennium Atlas are fully investigated.

 

It may seem strange not to have mentioned the foodplants up to now.  Butterfly Ireland web site (see below) lists "wide range of grasses including - Fescues Festuca spp. Bents Agrostis spp."  This would suggest that foodplant preference is not a limiting factor for the Gatekeeper in Ireland, but in reality we know little or nothing about the larval behaviour in Ireland.

 

Although the Gatekeeper seems to be holding its own around Cork, there is no doubt that loss of habitat is threatening its isolated colonies here.  Loss of undisturbed grassland is certainly reducing populations of the grass-feeding butterflies such as Ringlet and Small Heath in particular, but even the most common species such as Meadow Brown are becoming scarce locally in many parts of the county with intensification of agriculture and insensitive trimming of road verges.  It is sad that the best urban sites, i.e. waste ground and quarries, are deemed fit only for clearance and development.  These are exactly the sites that hold some of the healthiest populations of our grass-feeding butterflies.

Butterfly Ireland Website