Species Focus: The Wall Brown
The Decline of the Wall Brown in Ireland (Lasiommata megera (L.)) (Lepidoptera: Nymphalidae) by J.Harding.
This article examines the phenology, brood structure, breeding habitat, status and possible reasons for the decline of the Wall Brown Lasiommata megera in Ireland. This butterfly occurs in most of Europe, North Africa, Turkey, Israel, Lebanon, Syria, Iraq, Iran, Transcaucasus and Turkmenistan. It may be extinct in southern Latvia (Tolman 2009).
Phenology and Brood Structure
There are two generations in Ireland annually. The first brood flies from around April to early June with the second flight typically from mid-August to mid-September. The first brood is less numerous than the second. The larval stage can last a month in summer and seven or eight months when it over-winters in this stage (Harding 2008). It is possible that some over-wintering occurs in the pupal stage when some larvae from the second brood develop more quickly (Nash et al. 2012). In some years, a very small third brood may fly from late September (Thomas and Lewington 2010).
The Wall lay eggs on a wide range of mainly finer-bladed grass species including Annual Meadow Grass Poa annua, Sheep’s Fescue Festuca ovina, Red Fescue Festuca rubra and Common Bent Agrostis capillaris. Some broad-leaved grasses, such as Yorkshire Fog Holcus lanatus, Cock’s-foot Dactylis glomerata and False Brome Brachypodium sylvaticum are also used. The butterfly is very particular over egg-sites. Grasses abutting bare ground such as on the edge of tracks, ruts, rabbit burrows and steep banks highlight the species need for warm, dry situations (Thomas and Lewington 2010). In spring the larva basks, sometimes away from the food plant, highlighting its need for insolation (Heath et al. 1990). These requirements are met on unshaded grassy areas with bare ground such as eroding sand dunes, quarries, limestone pavement, eskers, rocky outcrops on mountains and hills, spoil heaps, steep banks, cliff walks, broad sandy or stony rides at the edges of plantation forestry, stony lakeshores, tracks through dry heath and drier areas of patchily vegetated peat on cutaway bogs (Harding 2008).
Status of Wall Brown Population
The status of the Wall in Ireland is a cause for concern. Hickin (1992) described the Wall as “widely distributed and often abundant throughout Ireland.” However, the population has been in a state of continuous decline for many years, probably since the mid-1980’s. The threat category assigned in the Ireland Red List No. 4 Butterflies 2010 is Endangered. It was assessed as Critically Endangered in Northern Ireland (Regan et al. 2010). Nash et al. (2012) shows the declining figures for recorded sightings in 10km squares over three five year periods (see Figure1).
|Species||No. 10 km records 1995-1999||No. 10 km records 2000-2004||No. 10 km records 2005-2009|
Figure 1.No. of 10km square records for the Wall for three successive 5-year periods
(Nash et al. 2012)
From 2013-2016 there is some evidence of a further decline (see Figure 2). In 2016 the highest number recorded at any site was eight, on Inisbofin Island, Co. Galway, in May (Butterfly Conservation Ireland 2016). The Wall was not observed in Northern Ireland in 2016 (Ian Rippey pers. comm.) where it is/was on the brink of extinction (Nash et al. 2012). Regarding the Republic of Ireland in 2016, records exist for only seven counties. The majority are from coastal locations, highlighting the more severe declines inland (Butterfly Conservation Ireland) (Dublin Naturalists’ Field Club).
|Counties reporting Wall Brown 2013-16||2013||2014||2015||2016|
|Totals per year||226||139||67||74|
Figure 2. No. of county records for the Wall Brown 2013-2016 (Butterfly Conservation Ireland) (Dublin Naturalists’ Field Club).
Historically the butterfly is known to be subject to periods of decline followed by recovery; the Wall disappeared from vast areas during a series of cold, wet summers in the 1860’s but recovered over the next 50 years. It declined after World War II owing to the widespread use of fertilisers and herbicides and the end of grazing of unfertilised pastures. It recovered on these following the recovery of the rabbit population (rabbits scrape/disturb soil, leading to the development of fine swards, higher ground temperatures and improved drainage) only to decline since the mid-1980’s. However, the species is highly mobile and able to recolonise areas that become suitable (Thomas and Lewington 2010).
Discussion of Reasons for Decline
The reasons for this widespread and prolonged decline are poorly understood. The habitat assessments from National Parks & Wildlife Service (2008) which cover the Republic of Ireland assess the following habitats associated with the Wall Brown (see Figure 3).
|Annex I Habitat||Future prospects||Overall|
|1230 Vegetated sea cliffs||Poor||Poor|
|2130 Fixed (grey) dunes||Bad||Bad|
|4030 Dry heath||Poor||Poor|
|6210 Orchid rich/Calcareous grassland||Bad||Bad|
|7120 Degraded raised bogs||Poor||Poor|
|8240 Limestone pavement||Poor||Poor|
Figure 3. Assessments of habitats on Annex I of the Habitats’ Directive associated with the Wall Brown (Regan et al. 2010).
However, these negative habitat ratings do not conclusively explain the butterfly’s decline. Habitat specialists including the Grayling Hipparchia semele a species that also requires dry conditions and even higher temperatures at egg-sites (Thomas and Lewington 2010) and Dark Green Fritillary Argynnis aglaja aglaja share the same habitats but none of these have suffered the severe decline that afflicts the Wall Brown which has undergone a population reduction of over 50% 1995-2010 (Regan et al. 2010). Furthermore, a species that is sun-loving/warmth-loving might be expected to do well in a warming climate, especially when some of its habitats, especially in the Burren region in County Clare, remain highly suitable but numbers recorded there (10 in 2016) and in many other good habitats remain low (Butterfly Conservation Ireland 2016).
The start of the current decline is traced to four very wet summers in the mid-1980’s by Thomas and Lewington (2010) when farmers were unable to save hay and a greater concentration on silage production followed together with inputs of synthetic grassland fertiliser. The increase in chemical inputs is blamed for the decline of butterflies because fertilisers change the character of grassland vegetation, helping the more nutrient-demanding species to flourish at the expense of butterfly food plants (Asher et al. 2001). The fine-leaved species of grass used by the Wall in its warm habitats are replaced by more vigorous tufted grasses (Regan et al. 2010) which may also quickly invade bare ground needed by the adult and larva (pers. comm.).
Higher rainfall levels may be a factor behind the Wall’s decline (Asher et al. 2001). Met Éireann data shows the past two decades have seen increased rainfall levels and this coincides with the current decline in the Wall’s distribution. While rainfall shows great inter-annual variability, a 30-year mean of the national annual rainfall indicates an increase in average national rainfall of approximately 70mm over the last two decades. All seasons show a small increase in totals over the last few decades. Extreme rainfall leading to severe flooding especially in early winter in some recent years may result in over-wintering larvae drowning during floods and habitats taking longer to dry out. In addition, flooding may change habitats by depositing nutrients onto higher ground where the butterfly breeds (pers. comm.).
The butterfly is not abundant in areas of high rainfall; in Co. Donegal, where strong populations remain it is less common in the far south of the county, including the coast around the south side of Donegal Bay where it is found in four of the nineteen 10km squares (Aldwell and Smyth 2016). Interestingly, the rainfall map show this area to be in a zone of high rainfall, receiving a mean annual rainfall of 1600-2400mm from 1981-2010. The coastal areas to the north and west receive much less precipitation, showing a mean annual rainfall of 1000-1400mm from 1981-2010 (Met Éireann). The distribution map in Aldwell and Smyth 2016 shows the Wall’s presence in all but one of the seventeen 10km squares along the coast that receive the lower rainfall, apart from Inishowen.
While the distribution pattern of the butterfly in Co. Donegal suggests a relationship with rainfall levels the distribution pattern in Co. Wexford does not suggest this correlation. The Co. Wexford coast is in the 800-1000mm annual mean rainfall zone 1981-2010 while inland areas are in the 1000-1200mm zone (Met Éireann). The distribution map for the Wall in O’Donnell and Wilson 2009 shows a decline from a presence in twenty-three 10km squares before 1989 to fifteen by the end of 2008 (out of a total of forty-two in the county)-a decline of 35%. In addition, it was absent from eight 10km squares with a coastline during 1989-2008 (O’Donnell and Wilson 2009). Perhaps declines in Co. Wexford are more attributable to agricultural intensification or other land use changes (pers. comm.).
Nevertheless, the species is strongest in coastal locations in Donegal (Aldwell and Smyth 2016) Wexford (O’ Donnell and Wilson 2009) Galway (Butterfly Conservation Ireland 2016) and Dublin (Dublin Naturalists’ Field Club 2016) where free draining, low fertility soils and lower rainfall levels exist.
The State of the UK’s Butterflies 2015 (Fox et al. 2015) which records a 77% decrease in occurrence and an 87% decrease in abundance in the UK since 1976 has suggested reasons for the Wall’s decline there. Fox et al. (2015) suggests that nitrogen pollution might be involved in the Wall’s decline. Fox et al. (2015) raises the issue of climate change, posing the question of whether the shift to a third generation in the autumn that produces few offspring might be part of the problem. The resulting larvae may be too small to survive the winter, leading to few or no butterflies particularly in inland areas the following spring.
There is evidence that this may happen in Ireland with four reports of suspected third brood individuals reported to the Dublin Naturalists’ Field Club in 2015. If a third brood was having the impact suggested by Fox et al, it is possible that natural selection would ensure that the genotype of Wall Browns that are more likely to produce a third brood are selected out of existence (Barkham, 2014). In captive indoor breeding of second generation Walls by this author, some larvae developed directly to become third brood adults in October/November while same age siblings reared in the same container over-wintered in the larval state, suggesting a genetic basis for third brood production (pers. obs.).
Conclusion and Outlook
Given that the Wall is scarce or absent from many habitats that remain in good condition, it appears that climate change may be playing a more significant role than habitat quality in the Wall’s demise. However, while absent from or very rare in areas of high rainfall it is not present in some areas of low rainfall where apparently suitable habitat exists. A research study to investigate the cause/s of the Wall’s decline should be commissioned. Targeted monitoring of the Wall at sites owned by the National Parks and Wildlife Service and on transects established under the auspices of the National Biodiversity Centre might generate important data concerning abundance, distribution and brood structure. In the absence of a determination of the factors driving the Wall’s decline and the availability and implementation of amelioration measures one can only hope that there is an upturn in its fortunes as occurred during the second half of the nineteenth century. Otherwise we could be lamenting the extinction of one of our formerly common and widespread butterflies.
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