Butterfly effect: a cumulatively large effect that a very small natural force may produce over a period of time
Cover image: the white-letter hairstreak has undergone serious declines in recent years. © Peter Eeles
The sun’s heat turns the air thick and soupy. On the horizon, bruise-purple thunderclouds roil. Closer to hand, stalks of cow-parsley push up through swathes of waist high grass, and wild roses blush in the hedgerows. Bees bustle between the blooms, marmalade hoverflies skim and dip faster than the eye can follow, ladybirds drone through on too-small wings. And above it all, immune to the frenetic energy of the air traffic, butterflies dance from flower to flower like wind-borne petals.
Butterflies matter. Most obviously they act as pollinators, dispersing pollen from the flowers they visit on their nectar-driven wanderings. Their immature stages, leaf-munching caterpillars, are beloved by birds and loathed by gardeners across the country. Showy, colourful, and largely harmless, butterflies hold a unique place in our cultural consciousness, and their reliance on specific habitats and climatic conditions makes them a good tool for scientists to study how humans are changing the environment.
Unfortunately, the news isn’t good for Britain’s butterflies. Since 1976, 57% of our sixty species have become less common, and 70% have seen their distributions shrink . In the worst examples of these trends, the white-letter hairstreak has lost 96% of its population, and the high brown fritillary has lost 96% of its range, now found in just a handful of colonies in Devon, Cumbria, and South Wales. Specialist species, which use only a few specific habitats, are faring worst, but many wider countryside butterflies are also declining. For decades, two questions have dominated British butterfly conservation: what is causing these declines, and how can we reverse them?
Many butterflies are strongly tied to specific habitats. Often, caterpillars feed on only one or two plant species, so the adults are restricted to areas where these plants grow if they want to reproduce. If the host plants are rare, then so are the butterflies.
Looking at a map of the UK’s habitats, such as this one produced by the Centre for Ecology & Hydrology in 2015, it becomes apparent that most of Britain’s landscape isn’t natural. The eastern half of England is dominated by arable land for growing crops, and much of the rest of the country is used for farming livestock. It’s been this way for thousands of years, and despite what you might think, this actually benefited many of our butterflies. To understand why, we need to look at the British Isles’ geological history. Up until about 17,000 years ago, at the end of the last ice age, most of the archipelago was under several miles of ice cap. With so much of the world’s water contained in ice, sea levels were lower, and the English Channel was a dry plain linking Britain to mainland Europe. When the country was locked away beneath the ice, there were no butterflies anywhere. As the ice melted, sea levels rose, and by 4,000 years ago, Britain was cut off from the mainland. Most of the butterflies that live here are the ones that survived the ice age on the edge of the ice sheet, in the tundra and boreal woodland habitats that then occupied the Mediterranean, and followed the ice as it retreated northwards. Most of our native species don’t do well in dense woodlands – they need open spaces that would previously have been maintained by cold weather and large herbivores, like wild horses and mammoths. Farming, and woodland management practices like coppicing, provided these open spaces when the climate warmed and the large herbivores disappeared.
Since the 1950s, British agriculture has undergone a series of revolutions that have proved disastrous for many of our butterflies. In order to feed the nation after two world wars, agriculture was intensified, using a combination of fertilisers, pesticides, and new cultivation methods to maximise the land’s output. This led to many hedgerows being ripped out, and the scrubby edges of fields clipped back to the fence to maximise space for growing crops, while upland fields were inundated with nitrogen fertilisers to encourage grass growth for feeding livestock. Many butterflies which did well in these marginal habitats were pushed out into less suitable habitats, leading to population declines . The increased use of herbicides meant that many of our common weeds, like cornflowers, corn marigolds, thistles and dandelions, have become rarer. These plants provided a bounty of nectar which, when cut off, made habitats less suitable for all pollinators, including butterflies .
While some areas were becoming increasingly overmanaged in an attempt to maximise productivity, other parts of the country suffered the inverse problem. As larger farms became increasingly profitable, the owners of small operations were forced to abandon their land. Without livestock and machinery routinely disturbing the growth of native plants, areas of open land converted to scrub and stands of bracken, and then to closed-canopy woodland: good for some species, but not for our grassland butterflies .
Combined, these changes have led to remaining high-quality habitat becoming fragmented, as large patches are carved up for agriculture and the links between smaller patches are pruned away. When habitats are fragmented, populations become isolated; with the adults unable to move around, inbreeding occurs, and traits which decrease survival build up over the generations . Species which can disperse across large distances, or survive in small populations, tend to do better in fragmented landscapes , and these species are overwhelmingly generalists . As the niches exploited by specialist species close up, they are replaced by species with broader requirements. Butterfly communities in different areas become more alike, and the number of species overall declines .
Alongside shifting cultivation practices, the greatest change to British farming brought about by the post-war agricultural revolution was a massive increase in the use of artificial chemicals. The most obvious of these chemical inputs is pesticides, especially insecticides: artificial chemicals designed specifically to kill pest insects which are damaging crops. Many common agricultural pesticides, like organophosphates and pyrethroids, are not specific in which insects they target, making them lethal to all manner of beneficial species, including butterflies. Insecticides are often sprayed over wide areas of crops, and the tiny droplets are easily carried by a breeze into neighbouring hedgerows and field margins , where they reduce the abundance and richness of non-target insect species . Even where insecticides are not lethal, they can still be damaging to butterfly populations: neonicotinoids, which mimic the insecticidal properties of nicotine and have been widely implicated in bee population declines, have been linked to delayed development and smaller body size in North America’s monarch butterflies . Fortunately, there is hope: applying buffer strips, a few metres of bare ground between the crops and the field margin vegetation, reduces pesticide drift into adjacent habitats , meaning that beneficial invertebrates like crop-pollinating flies and pest-eating spiders can survive.
Many of Britain’s butterflies live in what are termed ‘early successional’ habitats. The continuum of ecological succession covers the range of habitats from bare ground, through the first colonising plants, to grasslands, shrublands, and eventually woodlands, which in the UK form the ‘climax’ community in most areas. Climax communities are stable: when they are disturbed, they often return to their healthy climax state, and it takes a lot of disruption to change them in any meaningful way. Early successional communities, on the other hand, are very unstable, and even a small amount of disturbance can tip the balance in favour of a different habitat, or a different set of plants and animals. This is the case where fertilisers are applied to grasslands, increasing the availability of nitrogen. Lack of nitrogen is the main nutritional factor limiting plant growth, so nitrogen is applied to arable and pastoral fields to boost growth of crops and fodder. Unfortunately, much fertiliser remains unabsorbed, and is carried into nearby environments by water the next time it rains. Many wild plants respond to nitrogen in the same way as domesticated varieties, putting on a spurt of growth. This can lead to species-rich grasslands becoming dominated by a few species, like the fern bracken, plunging the understorey into shade. By lowering the ground temperature by a few degrees, this shading effect can push some butterflies towards extinction, preventing their eggs from hatching or their caterpillars from growing .
Agricultural change has been the driving factor behind British butterfly declines over the past century, but looking into the future, climate change is likely to play an increasingly important role. Throughout their lives, the speed at which butterflies grow is strongly constrained by temperature. The warmer it is, the faster they develop – but only up to a point. Beyond this limit, which varies between species, rising temperatures start to slow development down, and eventually they kill. Average temperatures in the UK have risen by 0.9 °C, with a further 0.9-5.4 °C rise forecast by 2070 . As temperatures rise, species move northwards and uphill, into cooler areas, attempting to follow shifts in their ideal temperature range. For species which occur in the lowlands of southern England, this is usually manageable; but for those which live in the north, like the Scotch argus, or at the tops of mountains, like the mountain ringlet, there is little that can be done. If temperature ranges keep moving where butterflies cannot, such species will be consigned to extinction. Even species with broad climatic tolerances may be vulnerable to climate change reducing their distributions , and as high-altitude species move uphill, species richness in low-lying areas decreases . As well as raising average temperatures, climate change will lead to an increase in the frequency and severity of extreme weather events like droughts, heatwaves, and floods. Such events can have a massive impact on butterfly populations, and the increasing instability might push small and fragmented populations to extinction.
Winners and Losers
It’s not all bad news: most of the UK’s butterfly species are predicted to increase in distribution under moderately warmer climates , and species which thrive in warmer climates tend to be doing better overall. The brown argus, previously limited to southeast England, can now be found as far afield as Cornwall, Anglesey, and Scarborough, while the silver-spotted skipper, once found only on the warmest south-facing slopes of Kent’s chalk hills, is now 943% more abundant than in 1976 . Across the board, it is the generalists which are faring best: the nettle-feeding butterflies (peacock, comma, and small tortoiseshell) and white butterflies (large, small, and green-veined) are all expanding their ranges as the northern part of the UK becomes increasingly suitable for them. Species which once went through two or three generations per year, like the common blue and speckled wood, are now managing five or six in the far south of England, where conditions are suitable for breeding almost year-round. Even for habitat specialists, warmer weather means that butterflies can fly further, making up for habitat fragmentation at a local scale .
While some species are increasing naturally, others will need help to overcome the challenges the past century has thrown at them. Fortunately, there are some well-recorded instances of British butterflies being brought back from the brink. Largely restricted to Hampshire and south-central England, a few colonies of the Duke of Burgundy butterfly have clung on at the southern edge of the North York Moors. Nationwide, the species declined by 46% in the decade from 1999-2009, a pattern that continued in the Yorkshire population as a series of extinctions rippled across its range. The Duke of Burgundy requires specific habitats to reproduce: it lays its eggs on primrose and cowslip plants, but only those growing in dappled shade on woodland edges, while the adult needs the full sun of open grasslands to survive. In Yorkshire, this was provided by scrubby limestone grassland, where its host plants grew under the shade of isolated hawthorn trees. Unfortunately, by the turn of the millennium much of the grassland had been abandoned by farmers, and the sheep which used to keep the trees to a minimum had gone with them. As the grassland turned to scrub and eventually woodland, the Duke of Burgundy was pushed out. Fortunately, interventions were applied just in time, with a programme of cutting and mowing the scrub on grassland patches still inhabited by the butterfly, and those close enough for the adults to disperse to. As the scrub was pushed back and the flower-rich grasslands returned, the Duke of Burgundy’s population stabilised, and then started to grow. By 2011, the Yorkshire population had increased by 395% compared to 1999, and today is still going strong.
The Duke of Burgundy isn’t the only butterfly that’s been given fresh hope. The chequered skipper, lost to England in 1976 when lack of coppicing caused its woodland habitat to grow too dense, has recently been reintroduced to sites in Northamptonshire, where early signs indicate that it is thriving. Extinct since 1979, after the ants it relies on to care for its caterpillars disappeared, the large blue butterfly was reintroduced to the UK in 1984, and self-sustaining colonies across Gloucestershire and Somerset now host a population of over 10,000 butterflies. The peat bogs across Lancashire and Greater Manchester are once again home to the large heath butterfly, a specialist of tundra environments, the populations re-established by caterpillars reared at Chester Zoo. Stories like this show what can be accomplished with dedication and sensitive habitat management, throwing a lifeline to our rare butterflies in the face of a century of coming uncertainty. The message of conservation is consistent: small actions, taken now, can reap big rewards in the years to come.
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