The dramatic decline of the Wall, previously a common species across England and Wales, is one of the biggest butterfly mysteries of the last forty years. Now, scientists in Belgium think they might have the answer. Richard Fox, Butterfly Conservation Surveys Manager, picks up the trail.
In the first butterfly atlas of Britain and Ireland, published in 1984, there was little sign of anything amiss for the Wall. The authors noted that it was limited climatically in Britain, its range barely extending across the Scottish border and that its numbers had been hit, along with those of almost all other butterflies, by both the 1976 summer drought and by the destruction of unimproved grassland for agricultural intensification.
During the 1980s, in south-east England, the Wall took a sudden turn for the worse. Recorders in Buckinghamshire, Hampshire, Oxfordshire and Surrey noticed that this widespread butterfly, often encountered in farmland, roadsides, woodland edge and gardens, was no longer widespread.
The intensive, sustained national recording instigated by Butterfly Conservation’s Butterflies for the New Millennium project from the mid 1990s, clearly showed the collapse of Wall populations spreading out westwards and northwards from this initial area (see maps below). Now, some 20 years on from the first worrying signs of decline it is now experiencing a slump in Derbyshire and south Yorkshire. Intriguingly, the losses have typically affected inland populations; around the coast, Walls seem to be fine.
Maps show the recorded distribution of the Wall in each five-year time period (red dots) and the areas where Wall had been recorded in the 1970-1982 survey but not recently (blue dots).
The Wall’s woes are clear from transect counts undertaken for the UK Butterfly Monitoring Scheme too. The number of Wall butterflies at these regularly monitored sites has decreased by 86% since 1976.
Something has gone badly wrong for the Wall, posing a puzzle for conservationists. The timing and pattern of decline doesn’t seem to fit with the all too familiar losses of butterflies as a result of habitat loss or degradation due to the abandonment of traditional farming and forestry practices. So, some have suggested that the changing climate may somehow be the cause, inland areas of southern Britain experience greater extremes of temperatures (both heat and cold) than the relatively mild coastline.
This is where the newly published research comes in. Trends of the Wall butterfly in Belgium mirror the situation in the UK. The butterfly has declined severely over recent decades, with losses clearly biased towards inland areas. Researchers, led by Professor Hans Van Dyck of Louvain University and Dr Dirk Maes of the Flemish Research Institute for Nature and Forest, suggest that the Wall has fallen victim to a ‘developmental trap’.
They propose that the warming climate has driven Wall populations to produce an increasingly frequent and larger third brood, late in the year, when there is little time for the development of caterpillars before winter. This narrow window of opportunity makes the third brood a high risk strategy, to the extent that in some years, the offspring suffer from a very high mortality rate. This ‘lost generation’ is, the scientists argue, sufficient to cause the severe population declines and colony extinctions that have been witnessed in Belgium.
Their theory, published in the international journal Oikos, is backed by three strands of evidence. First, they showed that in Belgium, there had indeed been an increase in the occurrence of third generations of the Wall since 1981. Second, in a field experiment they placed enclosed pots of foodplant with captive-bred Wall caterpillars at coastal and inland sites of similar latitude during the summer (concurrent with the butterfly’s second generation). All of the caterpillars placed at inland sites went on to develop directly into third generation Wall butterflies during the same year. At coastal sites, fewer than half (42.5%) developed in this way, with the majority overwintering and emerging in the following year. Finally, the researchers measured the micro-climate experienced by the experimental caterpillars and found that those at the inland sites were on average 1.2°C warmer than at the coastal sites.
While the Belgium team acknowledge that this evidence does not amount to a ‘smoking gun’ in the mystery of the Wall, it certainly provides a plausible theory and opens up an avenue for further research.
Many butterfly species in Britain possess the ability to alter their voltinism (number of generations per year) from year to year or from place to place. Some, such as the Wall are known to produce a small extra generation only in the warmest years. In others, such as the Comma, we witness this flexibility every year with the variable proportion of individuals that become ‘hutchinsoni’ adults destined to breed again in the late summer versus those that quickly enter hibernation and breed the following spring. Another example is the Common Blue which is reliably double-brooded in southern Britain but single-brooded in the north.
It’s easy to imagine that such life-cycle flexibility would be a benefit for ‘cold blooded’ creatures living in a country with notoriously fickle weather patterns. Less obvious is the potential for the sort of developmental trap proposed for the Wall in Belgium. But the Wall may not be an isolated case. In Britain, small numbers of White Admirals have appeared unexpectedly as an extra, autumn generation in recent years. It is extremely difficult to imagine that these ‘out of time’ individuals would leave any viable offspring with winter closing in.
The future for the Wall is unclear even if the development trap explanation is correct. In theory, as the climate continues to warm, we’ll reach a stage when the butterfly can reliably complete three generations each year, as it does currently in warmer parts of Europe. We’d also expect the affected inland Wall populations to experience strong natural selection to bring them back into sync with the climate. Whether they have the time or ability to do this prior to local extinction is unknown.
Ironically, while all the attention has been focused on the Wall’s decline in southern Britain, the species has been slowly spreading northwards along the coastline of southern Scotland, also, presumably, in response to the warming climate.
Read The lost generation hypothesis: could climate change drive ectotherms into a developmental trap?
Follow Richard Fox on Twitter @RichardFoxBC
Reference:
Van Dyck, H., Bonte, D., Puls, R., Gotthard, K. and Maes, D. (2014). The lost generation hypothesis: could climate change drive ectotherms into a developmental trap? Oikos. DOI: