localised dispersal |
regional expansion |
altitudinal migration |
climate change |
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.
Copyright © Ingo Arndt from "Nomads of the Wind".
The story of the
Monarch migration, and further pictures from the book : see
next page >>
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
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.
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
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.
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,
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.
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.
and losers in temperate zones
The distribution and range of many
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
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.
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 15°C lower than
that of surrounding rural or urban land.
Enormous swathes of Amazonian
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
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
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.
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 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.
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
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.
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
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
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.
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
Migration in Britain
In Britain the most well known
migrants are the
Painted Lady, and the
Admiral, all of which originate in North Africa and the Mediterranean coast,
but migrate northwards each year, usually arriving in Britain in May or
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
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 >>