Worcestershire Record No. 26 April 2009 pp. 9-11

Mike Williams

As “cold blooded” animals, butterflies are particularly affected by changes in climate. Numbers are influenced every year by weather patterns but increases in mean temperatures over a sustained period can lead to significant changes in range, population size, the rate of colonisation or extinction, phrenology, number of generations each year, choice of larval foodplant and other ecological and evolutionary factors. It is important to understand and monitor change, not just because of its intrinsic interest but because these changes may have considerable implications for conservation management.

Because there is such a good data set and because of their biology, butterflies are very effective indicators of climate change. The Millennium Atlas of Butterflies in Britain and Ireland (Asher et al 2001). which was the most comprehensive and intensive period of butterfly recording ever undertaken in the UK, shows very clearly that butterflies are extending their distributions northwards. Many species, which have been formerly confined to southern Britain, have shown a significant extension to their range margin of 37km over a 21 year study period, nearly 2km per year (Hickling et al 2006). Because Worcestershire, historically, has been on the range margin of a number of these species, it is an exciting place to be and the county is well placed to see at first hand some of these changes in distribution. In any overview, however, it is important to recognise that 72% of British butterfly species have decreased in distribution between 1970-82 and 1995-2004. This is particularly the case with what can be described as habitat specialists which have shown a 93% decrease over this time period compared to the wider countryside species which have only declined by 56% (Fox et al 2006).

As well as expansions in range, there is also evidence that some species that historically have not been able to survive our winters are now doing so increasingly. The best example of this is Red Admiral which is now regularly recorded every month of the year and has been found over a wide area of southern Britain during the winter as both larva and pupa as well as an adult (see Map 1). It is now often the case that in January and February there are more sightings of Red Admiral than any other species on the British list. Good examples of significant range expansion nationally are Comma (see Map 2), Small Skipper, Essex Skipper, Holly Blue and, to a lesser degree, Speckled Wood. Distribution of the Comma, a butterfly that has always been well represented in Worcestershire, has virtually doubled since 1982 and the species is now recorded well into southern Scotland and has been recorded for the first time ever in Ireland. Over the same period, Small Skipper has expanded throughout Wales and has now reached the Scottish border, while the Holly Blue has undergone a similar expansion in both England and Wales and has also really taken off in Ireland.

Essex Skipper is a good example of a species where historically Worcestershire has been on the range margin, the first sighting in the county not occurring until 1997. Over the past 10 years, it has become extremely well established and, while undoubtedly generally under-recorded because of its similarity to Small Skipper, has been reported from the majority of 10km squares (see Map 3). With the Marbled White, when I first moved to the county in 1980, one could have drawn an east-west line through Worcester and say with confidence that the butterfly did not occur north of this line. Now the species is found pretty well throughout the county (see Map 4) occurring on most remaining areas of unimproved grassland not subject to annual cutting. A similar story can be told with the Brown Argus, a species in the early 80s pretty well confined to Bredon Hill, but now found over much of the county with the apparent exception of the north-east (see Map 5). This species has clearly benefited from the introduction of set-a-side and much of the expansion is associated with the utilisation of various species of geranium as an alternative larval foodplant to rockrose. In 1982, when Jack Green published his guide to the butterflies of Worcestershire (Green 1982). the White Admiral was so rare that the author kept the locations where it was found confidential. Now it occurs in virtually every area of woodland in the county right up to the edge of Birmingham.

What is difficult with some species is to distinguish the impact of climate change from other factors. The Brown Hairstreak is a really good case in point. As Maps 5 and 6 illustrate, the last 13 years show a major expansion in range within the county but is this really the result of climate change or is it increased recording effort or the impact of improved hedgerow management on the back of agri-environment schemes? I suspect that all these factors have played their part.

As well as range expansion, perhaps the other most striking impact of climate change has been with regard to emergence dates and flight periods. Generally species are being first reported much earlier in the year and are seen over a longer time period (Roy & Sparks 2000).. Back in the 1980s, the first Orange Tip was generally not seen until May, now it is unusual if it is not seen by mid-April and in some seasons e.g. 2005 it is recorded before the end of March. In recent years, there have been examples of normally single brooded species like White Admiral and Dingy Skipper apparently producing a partial second generation in southern Britain. Similarly, with normally doubled brooded species like Small Copper, there have been increasing records of specimens being on the wing in October suggesting a partial third generation.

It is perhaps easy to get carried away with the positive effects of climate change, perhaps even envisaging new species of butterfly queuing up at the channel tunnel to make the crossing, but it is not all good news. There has been a lot of concern in recent years about the collapse in numbers of the Small Tortoiseshell in Britain. Once one of our commonest and familiar garden butterflies, it has now suddenly become rather scarce. Research has suggested that this decline is linked with the arrival into the UK of a tachinid fly Sturmia bella which is a well known parasite of Small Tortoiseshells and other Nymphalidae on the continent. It first arrived in Britain in 1999 and has since spread to most areas having a major detrimental effect on Small Tortoiseshell populations. Certainly a less welcome impact of climate change on butterflies.

It will be interesting to see what the future has in store for Worcestershire’s butterflies. Developing our knowledge of the continuing impact of climate change will be key to ensuring that the right priorities and land management decisions are taken which makes recording and monitoring butterflies all the more important in the years ahead – so keep on recording! 2009 is the final year of the latest 5 year survey undertaken by Butterfly Conservation into the changing distribution of Britain's butterflies.  Recording forms can be downloaded from the regional BC website www.westmidlands-butterflies.org.uk.