BUTTERFLY FUTURES

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Suffolk Natural History, Vol. 42

BUTTERFLY FUTURES ROB PARKER Everyone wants to know whether a warmer climate will bring more butterflies to Suffolk. The changes that have already happened are described here, along with the results of research seeking to predict future developments. Starting with an overview of European butterfly distribution, the opportunities for species to adjust their geographic range are described, taking a few flights of fancy with an optimistic view on what could develop. A number of case studies follow – firstly the positive developments of the past century, and then the predictions for the remainder of the 21st century. Some unwelcome realities are also explained – the limitations which might curb expansion.

European Overview All the exciting butterflies seen on Mediterranean holidays are the ones we want to see at home in future, so a European overview is a good starting point.

Iceland has no resident butterflies, but Suffolk is a little better, with 32 we count as “regulars” out of the 60 British species. If we lived in France, Spain, Greece or Italy – a country with a Mediterranean coastline and some real mountains, then we would have about 240 species, out of the European total of 440. Our own White Admiral is a totally European butterfly, with its whole range falling entirely within Europe. As its northern limit lies across U.K., we class it a “southern” species, and in Suffolk we are well placed to watch its progress.

We can divide the opportunists into 4 categories. Warmer weather has already led many of the species that live along England’s south coast to venture further north – and some more will reach Suffolk soon. Many of the migrant species now find they can withstand our milder winters and may soon establish breeding populations in this country. Some we count as extinct from Britain are still common enough on the continent, and we have a good chance of recovering a few of them. Not far south of the English Channel are some more that like it hot, and might be enticed to cross the Channel.

Flights of Fancy Typical of the south coast species is the Marbled White, which has been expanding its range during the last 30 years of a century that has warmed by just 0Â6 °C. It could be in Suffolk soon. Two degrees of frost is all it takes to kill hibernating Red Admirals, and fifty years ago we counted them only as migrants, but in recent years those overwintering in well protected spots have re-appeared in Suffolk on bright days in February. The proportion that survive our winters will go up as our frosts diminish in severity. The Painted Lady is a regular migrant that breeds here in summer – thistle is an abundant foodplant, but the adult can not tolerate frost, so those that attempt to hibernate here simply die at present, but this is a butterfly we can expect to see more of in future. The Bath White is a much less regular migrant, making it to the South Coast only in exceptional years, but its opportunities are improving. Some will remember the welcome few years when the Queen of Spain Fritillary appeared at Minsmere and seemed to be breeding. We all hoped that it would stay, and Richard Stewart hopefully put it on the front cover of his book. It will come again, and on some future occasion might find the climate to its complete satisfaction.

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We lost the Black-veined White from Britain in the 1920s, and we do not really know why. It remains relatively common in orchards in France though, and it requires only a little optimism to hope that we might see it back in the years to come. The Mazarine Blue has been extinct from Britain for over a century, and yet we see it abundantly in clover fields on the continent. Why should it not return? It would be nice to have the Scarce Swallowtail too – after all, it is a mobile species, and already flies quite close to the Channel.

Small butterflies are not so mobile, but the Map did enjoy a short stay in Britain when a small colony was introduced. The summer form looks rather like a miniature White Admiral, but it is seasonally dimorphic, with the spring form orange in colour, and resembling a map sufficiently to justify its common name. It feeds on nettle, and would be another welcome addition to the British list. Its scientific name – levana, is now used for Butterfly Conservation’s distribution mapping software. It is nice to enjoy such flights of fancy, but we should not go too far.

Temperature & Latitude It has been suggested that one degree Centigrade is worth about 80 miles of southward move, so Ipswich has effectively moved to Kent, with the climate our grandfathers enjoyed in Canterbury. Another 3°C would take us south of Paris – to Fontainebleu, but to reach the Mediterranean at Marseilles would require 5°C. Such a shift would bring us into the domain of the Nettle-tree butterfly, which certainly likes it hot. Its larval host plant is Celtis australis – the Nettle-tree, and I think we may assume that it will be some centuries before the tree makes its way north to Britain. This is one example of a reality which will put a firm check on this butterfly’s ability to spread. If there is no suitable habitat to the north, it has nowhere to go.

Clearly, butterflies are faced with three choices: either to adjust their range as the climate changes, to stay put and adapt, or to go extinct. This is hardly rocket science, but it is fundamental to understanding how butterflies are responding. Let’s try to look at things from a butterfly’s point of view.

Consider the Brown Argus, essentially a sedentary species, but during the 90s it showed an ability to adapt and to move, galloping across East Anglia in the space of a decade. To achieve that, it had to adapt, abandoning its traditional host plant of rock-rose in favour of more widespread stork’s-bill and cranesbill.

The Clouded Yellow is obviously a much more mobile species that can arrive with us in a favourable migration season and reproduce in fields of alfalfa or clover. The hitch is that it has to pass the winter as a tiny caterpillar, and generally does not survive. In the relatively mild winter of 1998/9, an observer monitored the progress of a number of wild larvae laid in protected sunny spots along the undercliff at Bournemouth. Most of them continued feeding sporadically and survived to adulthood. That really is progress! Note too, the move from speculation to fact. The Adonis Blue is seriously sedentary; it often can not be bothered to cross a hedge to get into the next field of suitable habitat. At present it is staying in its familiar haunts around Dorset, and we can expect considerable inertia with such lazy species waiting for their populations to build up a pressure before moving on.

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U.K. Species-richness The Species-richness map from the Millennium Atlas, shows most obviously that there are more species in the south of England. But it shows a lot more than that. In places, the underlying geology is discernible. Lines of species richness radiate outwards towards East Anglia – these are the chalk downlands, rich in flora, so rich in butterflies generally, but also home to specialist species like the Chalkhill Blue and Silver-spotted Skipper. The high ground of the Grampians, the Pennines and the Welsh hills also show clearly due to their poor butterfly numbers. Finally, every blob on the map also reminds one that butterfly data may be used as an indicator of change. Sir David Attenborough described butterflies as “the canary in the coal mine” – they are very responsive to environmental change, highly visible and well recorded. Some 30 “Southern species” reach the northern or north-western extremity of their range in the U.K., making them very suitable for study. We also have an interesting group of 4 montane or “Northern species” that live up mountains, and reach the southern limit of their range somewhat north of Suffolk.

What Butterflies Need As well as mild winters, butterflies appreciate higher temperatures for larval development, longer flight seasons for better reproduction, and of course, access to pastures new. Caterpillar metabolism seems to work better at higher temperatures, and in many cases, the foodstuff is growing better too. The Marsh Fritillary larvae live in a communal nest which is spun in an optimum position for mutual warmth and sunbathing in spring. It is a species in serious decline, but perhaps extra warmth will serve to offset the effects of habitat loss. On the continent, the Swallowtail larva feeds on roadside fennel in a wide variety of habitats, whereas subspecies britannicus demands milk parsley in the Norfolk Broads. Improved feeding at higher temperature for longer seasons might strengthen our population and allow them to be less fussy over their habitat and foodplant. Interbreeding with incoming continental Swallowtails is another interesting possibility.

Are Species Moving North?

Are longer and warmer seasons allowing butterflies to move north? Well, decisively “Yes” for some species, as can be seen from distribution maps. In the 20 years leading up to the Millennium, the Comma and Gatekeeper made impressive northward gains. I would just point out that these are both reasonably mobile species, with abundant food plants in nettle and grass respectively. The Peacock is another strong-flying nettle-feeder. Its progress across Scotland has been rapid, even when seen on the short timescale of the 5 years since the Millennium Survey. The Orange-tip too, is moving north. A phenogram recording its appearance shows clearly that the flight dates are earliest in the south and later in Scotland, with lower butterfly density in the north. Apart from a few stragglers, the main flight period is contained in a 7 week band for this single-brooded species. The phenogram for the Small Copper differs, showing it to be double brooded, with a clear gap between broods. Furthermore, the second brood is much more numerous than the first.

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In southern Europe, there can be three or even four broods, building to very high numbers. Many of our less mobile species are multiple brooded, and stand to benefit in this way from the longer warm season. An interesting situation exists for the Common Blue, which is double brooded in the South of England, but can only manage a single brood in the north. We can expect the transition point to move north as we get deeper into the 21st century.

Fresh Pastures?

All this moving north assumes that suitable habitat is available to move into.

Fresh pasture is a nice concept, but man’s activities have destroyed most natural habitat. If the next field on is as nice as the existing habitat there will be no problem, but sadly, that is unlikely. Even so, grassland of some sort exists in every tetrad, and grass feeding species will find better continuity than those with precise habitat needs. Grass feeds the entire family of Browns. One of them made an impressive advance from Epping about 15 years ago, and the Speckled Wood was welcomed to Suffolk with some delight. The speed of its advance surely had a lot to do with the flatness of East Anglia. It is now established close to the top of our league table for common garden butterflies.

Grass suits a good proportion of the Skippers too, and several of them are making progress. One that has been doing well for thirty years is the Essex Skipper, and its gains have continued in the 5 years since the Millennium survey. The Silver-spotted Skipper lives only on the chalk downs. It suffered in the 60s, when myxomatosis terminated rabbit grazing, and the butterfly retreated to the most sheltered, south facing short grass hillsides. And yet, in recent years, there has been a recovery. Progressively, each small site has strengthened its colony and expanded its boundaries outwards into less optimal habitat. In each niche, the microclimate is improving, and the populations are flourishing as a result.

Research Results So much then, for encouraging facts from the recent past. Now I will turn to the results of some research conducted by Butterfly Conservation in conjunction with Jane Hill of York University. Using computer climate models based on 3 parameters: Annual temperature sum >5 °C, Coldest month mean temperature and Moisture availability, she modelled patterns of climate suitability. Her maps used 50 km squares, and covered the whole of Europe.

She achieved excellent correlation between the model’s theoretically suitable area and the actual distribution of each species. Her projections for the end of the century were then generated and displayed to show the areas likely to become suitable, the areas remaining suitable, and in the areas expected to become climatically unsuitable. The prediction for the Ringlet is typical and shows great voids in Bulgaria and the south of France! This loss of territory in the south may come as a bit of a shock to anyone wishing only to hear good news. The theoretically suitable area extends to Iceland, and therein lays another problem: some of the theoretical gains will be unachievable, whilst most of the losses will be all too real. The criteria used for this study took HADCM2, one of the Hadley Centre’s climate models which assumes a gain of 3 °C in the course of the present century. This is by no means extreme, as other models use 5 °C or more.

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Moving Uphill So far, we have considered moving north as if across a 2-dimensional map, but there are mountains to be climbed. We can think of a tide of warm air lapping at the foothills, and sweeping up the valleys. In meteorological terms, 1000 ft of climb is normally equated to a temperature drop of about 2 °C, so the 0Â6 ° C warming of the last century ought to be worth about 100m in elevation. The northern or montane species live at high altitudes, and four of these, Mountain Ringlet, Scotch Argus, Large Heath and Northern Brown Argus, were studied alongside 11 southern species. It comes as no surprise to find that the 2 groups behave differently. Most southern species are found from sea level up to some maximum elevation. The temperature rise of just point six of a degree over the past century has lifted the average elevation of their domain by 22Â3 m. The northern montane species live within a height band, and are very sensitive to temperature change. The same increase lifted their domain by an average of 40Â7 m. As they are forced uphill, the area available reduces until they reach the top, after that, there is nowhere to go. The prediction for the Large Heath is more extreme than the Ringlet, with few achievable gains and massive losses, even within U.K. Obviously, some do not like it hot! Fortunately those rather unexciting northern species are in the minority.

Range Adjustments The overview begins to suggest that the southern species will make an overall gain of just 2%, whereas the northern species will be forced off 40% of their mountain domain. And this is based on the optimistic assumption that species will move absolutely in step with climate change. Sadly, appropriate habitat is simply not available for most British species. Taking the pessimistic view that no ground will be gained in the north leads to the gloomy losses of 24% for southern and 65% for northern species. In practice, we can expect something between these 2 extremes, with wildly differing outcomes for different species. Their mobility and adaptability is already being put to the test.

More Realities The extreme weather events of 2005 are a reminder that butterflies flourish best in stable, predictable conditions; hurricanes, inundation, and drought are all bad news. The more we discover about climate change, the more we realise that it is not progressive global warming, but a riot of unpredictable change with plenty of localised disasters. Natural predation is another variable factor, and man’s impact on the environment has been seriously unhelpful, leaving all too little suitable habitat for Lepidoptera. Natural enemies of butterflies are many and varied, and parasitoids are particularly serious predators of caterpillars. But the parasites are also moving north, and our Small Tortoiseshells have been suffering at the hands of Sturmia bella, a parasitic Tachinid fly that only crossed the Channel five years ago. So we must expect these extra predators to disturb the balance of nature. And then there is man.

Monoculture, insecticides, habitat destruction and more; we have modified our countryside in such a way that recolonisation will be interrupted. Clearly butterflies have to face a future loaded with uncertainty.

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Conclusion In summary then, global warming is a positive factor and has already brought improvements for many species. Southern species are moving north and making progress uphill. Migrants will become residents, and multiple brooded species will flourish. Those likely to succeed in Britain outnumber those that do not like it hot. The bad news has been mentioned in passing. If the European overview looks gloomy, we simply have to be selfish and say that Bulgaria’s loss is Suffolk’s gain!

Acknowledgements The author gratefully acknowledges assistance from many colleagues who shared the results of their own work, including many of the images displayed (with credits) at the SNS “Some Like it Hot” Conference.

References Asher et al. (2001). The Millennium Atlas of Butterflies in Britain & Ireland.

Oxford University Press, Oxford.

Fox, R. Unpublished updates including Butterfly Conservation’s Atlas +5 Provisional maps (2000–2004).

Hill, J. K., Thomas, C. D., Fox, R., Telfer, M. G., Willis, S. G., Asher, J. & Huntley, B. (2002). Responses of butterflies to twentieth century climate warming: implications for future ranges. Proc. R. Soc. Lond. B (2002) 269: 2163–2171.

John, E. (2004). Sturmia bella (Meigen) (Dipt.: Tachinidae). New to Wales.

Entomologist’s Record 116: 75–77.

Skelton, M. (2000). Successful overwintering by Clouded Yellow Colias croceus (Geoff.) in Southern England. Atropos 8: 3–6.

Stewart, R. (2001). The Millennium Atlas of Suffolk Butterflies. Suffolk Naturalists’ Society, Ipswich.

Rob Parker 66 Cornfield Road Bury St Edmunds Suffolk IP33 3BN

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