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Migration and dispersal
 
localised dispersal | regional expansion | altitudinal migration | climate change | mass migration

Butterfly migration occurs at several levels - localised dispersal, regional expansion, and mass migration. It also takes place on varying time scales ranging from daily movements within a home area, to seasonal or annual events over large distances.

Monarch Danaus plexippus. photograph Copyright Ingo Arndt from "Nomads of the Wind".
The story of the Monarch migration, and further pictures from the book : see next page >>

Localised dispersal

Most species have very strict requirements in terms of temperature, humidity, altitude, habitat, larval foodplants, adult food sources and other factors. They are unable to survive and breed unless all of these conditions are precisely met.

Consequently, although the natural range of a given species might for example cover the whole of Europe, that species might have a very patchy distribution within Europe, being found perhaps only on areas of dry heathland, or at certain altitudes on particular south facing mountain slopes.

Most butterfly species are sedentary in behaviour. They rarely stray away from their established colonies - it would be wasteful of their short lives to ramble across the "wrong" habitats where they could not find suitable plants on which to lay their eggs.

 

However in particularly warm summers, when butterfly populations are highest, there is a tendency for a small percentage of the population to disperse beyond their existing breeding sites in search of suitable new habitats. In such cases the females usually still lay most of their eggs close to their emergence sites, but later disperse to lay the remainder elsewhere.

 

Unfortunately even in years when dispersal is high, most species will wander no more than a kilometre or two from their emergence site, so colonisation of an area takes place in a series of hops and jumps, and over a period of many years.

Regional expansion and contraction

The natural range of a butterfly is limited by climate and geology, which both affect the type of larval foodplants that will grow in an area. The distribution within that range is however greatly affected by human intervention - governmental and commercial policies on farming and forestry for example can have a very profound affect on the distribution and abundance of butterflies.

 

An example is the High Brown Fritillary Argynnis adippe, which was widespread and fairly common in England until the 1950's, but then contracted it's range very rapidly as a result of habitat fragmentation, and a change from traditional coppice management to the mass planting of conifers in English forests.

 

Most other woodland butterflies have also declined as a result of being "shaded out" of the darker and cooler modern woodlands, but one species - the Speckled Wood Pararge aegeria has actually benefited, as it survives better in shadier conditions. This species has even been able to increase it's formerly patchy distribution in the U.K. to the point where it is currently very widespread and common over almost it's entire range.

Altitudinal migration

In mountainous areas such as the Alps, the Rocky Mountains, the Tien Shan and the Himalaya, altitudes above about 1500 metres are covered in snow for most of the year. During the short summer however, the meadows and pastures become carpeted in vast swathes of flowers, attended by hordes of butterflies.

 

Blues, Fritillaries and Skippers swarm to imbibe moisture from damp ground, while Coppers, Ringlets, Marbled Whites, Apollos and Heaths nectar avidly at the abundant flowers. Most of these species have a very short flight season - typically no more than 2 or 3 weeks, and spend several months of their lives as either eggs or caterpillars.

 

Other species, such as the Clouded Yellows, Whites and Swallowtails, are less sedentary in nature, and migrate down to the lowlands in late summer to breed. In the early spring their progeny produce a further brood in the lowlands, but the habitat there becomes too hot and dry in summer, so they then return to the mountainsides where there is cooler air and an abundance of flowers for nectaring.

 

Another form of altitudinal migration takes place on a daily basis. Many butterflies tend to spend the early morning egg-laying in the lower meadows, but in late morning migrate up to the higher slopes where nectar sources are more abundant and temperatures more comfortable. As the high meadows and pastures cool down in late afternoon, they undertake a return migration, and roost overnight at lower altitudes. I have observed  Black-veined Whites Aporia crataegi and Mountain Clouded Yellows Colias phicomone behaving in this manner many times at Vanoise national park in the French Alps.

Climate change

Few would now argue with the scientific evidence that global warming is taking place. The average temperature of the planet is increasing, partially the result of "natural" fluctuations, but certainly exacerbated by the destruction of the rainforests, and the release of "greenhouse gases" from the burning of fossil fuels.

 

The fact that global temperatures are increasing however does not necessarily mean that local temperatures will increase. Some areas will become hotter, some will become cooler, some will become wetter, some will become drier. Ocean currents such as the Gulf Stream, and air currents such as the Jet Stream will almost certainly change course and speed, and even a tiny change of direction could for example mean the difference between Britain's climate becoming as cold as Alaska or as warm as North Africa.

 

Current evidence shows that the trend is for average temperatures in Britain to increase, and for the climate to become more volatile - the stability of our climate is being lost, and we are likely to experience more floods and droughts, less predictable temperatures, and more severe storms.

Winners and losers in temperate zones

The distribution and range of many Holarctic species is changing as a result of climate warming. The Orange tip Anthocharis cardamines, Speckled Wood Pararge aegeria and several other species have for example been able to extend their ranges northwards as average temperatures in northern England and Scotland have increased.

 

Other species, such as the Adonis Blue Lysandra bellargus, and the Silver-spotted Skipper Hesperia comma have also begun to increase their ranges. The Glanville Fritillary Melitaea cinxia, which is currently restricted in Britain to a narrow strip of undercliff on the south coast of the Isle of Wight, is also likely to be able to extend it's range and colonise the mainland within the next few years.

The news for many species however is far from positive :

 

While climate warming will allow several species to expand their ranges northwards it will also cause them to abandon their habitats at the southern limits of their current range, where the climate will become too hot. The trend therefore, at least in the short term, will be for species in the Holarctic region ( Europe, North America and temperate Asia ) to adopt a more northerly range.

 

In the case of British species such as the Mountain Ringlet Erebia epiphron, Northern Brown Argus Aricia artaxerxes, Large Heath Coenonympha tullia, and Scotch Argus Erebia aethiops, all of which prefer cooler and damper climates, their existing habitats are likely to become too warm for them.

They will initially respond by moving to higher altitudes where temperatures are lower, but as they are forced to move higher the amount of available habitat will decrease and populations will shrink and destabilise. Extinctions are inevitable. These species are already showing signs of contraction, and will probably be lost from Britain within 50 years if current climate trends continue.

Mass extinction in the tropics

The effects of climate change in the tropics are terrifying, and are largely the result of the mass destruction of rainforests. The Amazon is rightly referred to as the "lungs of the world", but also functions as a vast watershed and a powerful temperature regulator - the temperature within the shade of the rainforest is at least 15C lower than that of surrounding rural or urban land.

 

Enormous swathes of Amazonian rainforest have already been deliberately burned to make way for cattle pastures. The nutrients in the soil become exhausted very quickly, and within the space of a few years desertification begins.

 

The headlong rush towards biodiesel fuel, and the dramatically increased demand for vegetarian food have caused several major international companies to buy up cheap rainforest, burn it to the ground, and replace it with vast soybean plantations.

 

These actions are crimes against the Earth, bringing mass extinction of birds, butterflies and all other wildlife.

 

Matters are made even worse because higher temperatures and much lower humidity in the deforested areas affect the climate of the entire region. The vegetation structure in the remaining rainforest areas consequently deteriorates, causing yet more extinctions, even in the allegedly "protected" areas.

 

The domino effect of climate warming causes the forests to shrink further, until what little remains has lost it's original character. Evergreen rainforest trees are unable to survive in the hotter drier conditions, their place being taken by deciduous species which shed their leaves in the dry season. As the climate warms further thorn scrub takes over, but long before then the wonderful creatures of the rainforest have long become extinct.

 

Within the next 50 years, virtually all of the world's rainforests will have been destroyed by mankind. A few butterfly species will be able to migrate to new areas, but most will be unable to find alternative habitats. By halfway through this century, if deforestation and climate change follow predicted paths, at least a third of the world's butterfly species could become extinct.

Mass migration

Mass migration is an entirely different phenomenon from the types of dispersal and expansion discussed above. It occurs spontaneously, and involves the mass movement of hundreds, thousands or even millions of butterflies.

 

Butterfly eyes can see polarized light, which enables them to determine the position of the sun, even when it is partly hidden by cloud. This enables them to relate their position to the sun, and use it as a compass when migrating in overcast conditions.

 

Recent research on Monarchs has revealed that their annual migration from Canada to Mexico is controlled by a "time-compensated sun compass" that depends on light receptors and a circadian clock built into their antennae. A circadian clock employs rhythms of biochemical, physiological or behavioural processes which control daily, seasonal and annual activities - including migration. When scientists removed the antennae from one group of Monarchs they flew strongly but in random directions, but a control group with their antennae intact all flew in the same direction - their south-westerly migration route.

 

In another experiment the antennae of some were painted with black enamel, and these butterflies when placed in a flight simulator all flew together, but in the "wrong" direction compared to their normal migration route. Another group had their antennae painted with transparent paint, and these all migrated together in the right direction.

 

Research by Chapman suggests that migratory butterflies and other insects are programmed to seek out "wind highways" in the sky, which they use to enable them to travel quickly during their migratory flights. This may be the case with certain species, but it is well documented that species including Colias crocea, Vanessa cardui and Pieris rapae fly very low over the sea when migrating from the European mainland to Britain. There is for example a famous account by Rev. Harrison, who in 1868, from a cliff near Marazion, Cornwall, observed "a yellow patch out at sea, which as it came nearer showed itself to be composed of thousands of Clouded Yellows, which approached flying close over the water, rising and falling over every wave till they reached the cliffs".

 

The causes of mass migration are not scientifically proven, but are almost certainly triggered by phenomena including drought, over-abundance, or sudden changes in weather.

A personal theory :

When butterflies first appeared on Earth, the present day continents were connected to form the giant land mass Pangaea. Nature tries to fill every available niche, so butterflies would then have naturally been nomadic, their colonies migrating seasonally from one area to another in search of suitable habitats.

The nomadic behaviour was interrupted as tectonic activity caused mountain ranges and seas to appear, dividing formerly contiguous areas of breeding terrain. These changes however took place over millions of years, so butterflies and birds instinctively continued to migrate along the same routes, crossing mountain ranges via low passes, and hopping from island to island to cross seas and oceans.

Many species of course were unable to overcome the new natural barriers, particularly the ever widening oceans. Their populations therefore became permanently divided, gradually taking on new characteristics, these being the origins of "sub-species".

Butterfly Migration in Britain

In Britain the most well known migrants are the Clouded Yellow, the Painted Lady, and the Red Admiral, all of which originate in North Africa and the Mediterranean coast, but migrate northwards each year, usually arriving in Britain in May or early June.

 

These species disperse across Britain, sometimes breeding as far north as Scotland, and produce a new generation of adults which emerge in late summer. As autumn approaches they begin a return migration southwards. Individuals are sometimes observed flying south across the English Channel but the majority remain and attempt to hibernate in southern Britain. Most die with the arrival of harsh winter conditions, but in very mild winters a few Red Admirals and Clouded Yellows survive.

 

Other migratory species include the Camberwell Beauty which tends to arrive on the east coast in the autumn; the European race of the Swallowtail which occasionally turns up along the south coast in mid-summer; and the Long-tailed Blue which on rare occasions breeds in southern England.

 

The most famous migrant of all is the Monarch Danaus plexippus, which migrates each year from Canada to Mexico. The butterfly has extraordinary powers of dispersal and has become established as a breeding species in South America, Africa, Asia and Australia. In Britain it's occurrence is very sporadic, but at least one or two individuals are recorded each year in south west England.

 

Read more about the Monarch >>

 

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