1 - Random dispersal, daily commuting, seasonal
migration, distribution and range
The migration of the Monarch butterfly
possible to find a few butterfly species straying into city parks and
gardens, investigating weedy areas of wasteland or flying in other
'unnatural' habitats, but despite such apparently cosmopolitan
lifestyles butterflies are extremely choosy about where they lay their
Butterflies have strict requirements in terms of minimum / maximum
temperature tolerance. The habitats they occupy are determined by
where their larval foodplants grow, and by the availability of adult
food sources and roosting sites. They are unable to survive and breed
unless these and numerous other vital conditions are precisely met.
Suitable habitats are
often highly localised so consequently many species have an extremely
never stray more than a kilometre away from their established breeding
grounds - it would be wasteful of their short lives to ramble across
barren habitats where there were no suitable plants on which to lay
their eggs. Nevertheless all species whether humans or butterflies are
to a greater or lesser extent genetically programmed to 'leave home',
dispersing to explore new areas.
Migration or dispersal ?
It is important to understand the
difference between dispersal and migration. The term dispersal is used
to describe random and aimless movement away from the site where a
Dispersing butterflies are easily diverted from their course by minor
changes in wind direction or obstacles in their path. They will for
example fly around the edge of a block of forest rather than fly
through it or over it. When they encounter hostile habitats such as
arable farmland, lakes, rivers, roads or buildings they steer left or
right to try to find a route around them.
The term migration refers specifically to
medium or long distance directional movements. Migrating butterflies
have a strong purposeful flight and are unaffected by obstacles or
hostile landscapes. If e.g. they encounter a building they will fly
over it rather than take an easier route around it. Their flight path
is not affected by wind:- C.B.Williams analysed 470
Vanessa cardui migrations in various
parts of the world and found no correlation between flight direction
and wind direction. Other studies involving migrations of
Catopsilia florella and
Eurema hecabe in Africa,
Pieris brassicae in England and
Ascia monuste in North America have
arrived at the same conclusion.
Migrations usually involve mass movements
- a flight of Vanessa cardui in
California in 1924 was estimated to contain about 3000 million
butterflies. It has been estimated that a swarm of migrating
Libytheana carinenta in Texas in 1921 was
passing at a rate of over a million butterflies per minute across a
250 mile wide front. Migrations often begin spontaneously across a
wide area, but once in the air the insects all head towards a common
destination. It is generally accepted that migration is a 2-way
process e.g. butterflies migrating south in autumn will return north
again in the spring. In some cases an individual butterfly will take
part in both legs of the journey, but in many species a 'parent'
generation flies in one direction, and it is the offspring that
undertake the return journey.
above illustrates the differences between dispersal and migrational
flight patterns. The blocks of green represent 'hostile' habitats,
e.g. a dispersing grassland butterfly would consider a forest to be
hostile so would avoid flying into it or over it. On the other hand a
migrant species such as Danaus plexippus,
Vanessa cardui or
Pieris brassicae would have an extremely directional flight
pattern and would fly over a hostile habitat to maintain its compass
Nothing in nature is constant. Habitats
are continually changing. Woodlands become overgrown and shade out
herbaceous plants on which the caterpillars of many butterflies
depend. Heathlands catch fire, grasslands become overgrown with scrub,
deserts expand, cliffs crumble away.
Species at the edge of their natural
distribution range tend to inhabit sites which are sheltered from bad
weather. Pearl-bordered Fritillaries for example occupy open meadows
in mainland Europe, but in Britain such habitats are too cold for them
so they form colonies in sheltered woodland clearings instead.
Unfortunately these habitats are ephemeral. They fast become overgrown
and the violets on which the Fritillary caterpillars feed rapidly get
shaded out. Within 2-3 years the habitat can no longer support the
species, so the adult butterflies disperse soon after mating,
radiating out along forest trails in search of more suitable breeding
Many other factors can trigger dispersion
e.g. after a particularly successful breeding season
populations can reach explosive levels.
The larvae scour their habitat, devouring every available leaf of
their foodplant. Ultimately they sense imminent starvation, and
undergo a chemical change which switches them into a high activity
phase causing them to swarm across the surrounding countryside in
search of food.
In Britain adults of
Clossiana euphrosyne nectar almost
exclusively on bugle - Ajuga reptans.
They lay their eggs on dead bracken or dry grass stems, but their
caterpillars feed on Viola. Fortunately
for the butterfly all of these plants can be found in the same habitat
- recently cleared woodland, so it is here that the butterflies spend
their entire lives.
Many other species however are not so
lucky - their larval foodplants may grow in entirely different places
from the adult food sources, so they need to commute between breeding
and feeding sites.
Species such as
Apatura iris which occur at low density in woodland habitats
can't easily locate the opposite sex, so they have evolved
'hill-topping' - a strategy whereby both sexes fly to the highest
point in the vicinity, typically a tall oak tree on a ridge, where
courtship and copulation take place. After mating the females disperse
to lay their eggs on sallow bushes which typically grow alongside
ditches on lower ground. The males also disperse to low lying areas
where they feed by imbibing mineralised moisture from the paths or
patches of mud. Next morning they commute back to the 'master' oak
tree to mate with other females.
Apatura iris commutes daily between
feeding and courtship sites ©
temperate regions mountainous areas such as the Alps, Pyrenees,
Rockies and Tien Shan, land above about 1800m is covered in snow for
much of the year. In the Andes, the Himalaya and the mountain ranges
of New Guinea areas as low as 3000m are subject to seasonal snow
cover. During the short montane summer however, the meadows and
pastures become carpeted in vast swathes of flowers, attended by
hordes of butterflies.
of Blues, Fritillaries and Skippers aggregate to imbibe moisture from
patches of mud; while Coppers, Ringlets, Apollos and Heaths nectar
avidly at the abundant flowers. Most of these species are sedentary
insects, forming highly localised breeding colonies. Because they are
'stay-at-home' species, their flight season is limited to 2-3 weeks in
mid-summer, and they have to spend several months of their lives
hibernating as either eggs or caterpillars.
species such as Clouded Yellows, Whites and Swallowtails are nomadic,
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.
form of altitudinal movement takes place on a daily basis.
In alpine regions one side of a
mountain may remain in the shade until late morning. Sunlight reaches
the east facing slopes first, but may not reach steep west facing
slopes until late afternoon. Consequently butterflies commute around
the mountainsides to keep pace with the sun.
Butterflies commute altitudinally too,
moving from peaks to valleys and back again, to areas where the
temperature is most suitable. Sometimes these journeys take them to
mountain passes. Over the millennia these become established routes by
which species migrate seasonally from one valley to another.
Seasonal migration is an entirely different phenomenon from commuting
or random dispersal. It tends to occur spontaneously and involves the
mass movement of hundreds, thousands or even millions of butterflies.
The evolution of migratory behaviour
When butterflies first
evolved the present day continents were united to form the giant land
In the centre of such a
massive continent there would have been huge seasonal extremes of
temperature and humidity. Consequently butterflies would have had to
find some way to survive when their larval foodplants and nectar
sources wilted in summer, or when temperatures were too high or too
low for normal activity.
choice would be to remain
and diapause, i.e. to aestivate. In a diapaused state large numbers
would perish due to predation by insectivorous birds. A better
alternative would be to fly to another location e.g. close to a river,
where foodplants would continue to be available long after the
surrounding land had dried out. Further foodplants would then be found
by migrating up or down river. Natural selection would ensure that
butterflies which survived as a result of such migratory actions would
be able to pass on the trait genetically. It is perhaps no coincidence
that so many species in the tropics still use rivers and streams as
In subsequent geological
periods the migratory behaviour would have been interrupted as
tectonic activity caused mountain ranges and seas to appear, thereby
splitting up formerly contiguous areas of breeding terrain. These
geological movements took place over millions of years, during which
butterflies probably 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 would have been unable to overcome the new natural barriers,
particularly the ever widening oceans. Their populations therefore
became permanently divided, gradually taking on new characteristics,
and ultimately evolving into new species. Other species however
managed to cross the barriers regularly, and migratory behaviour
became genetically imprinted in them.
Individual migrations appear to be triggered primarily by climatic
phenomena, often in association with over-abundance, and depletion of
larval food resources.
C.B.Williams in his book 'Insect Migration' quotes Skertchley, who in
March 1869 witnessed the commencement of a migration of
Vanessa cardui in desert behind Suakin by
the Red Sea :
"He noticed that
the whole mass of grass, through which they were riding on camels,
was in a state of violent agitation although there was no wind. When
he dismounted he found that the cause was the emergence from the
chrysalis of myriads of Painted Lady butterflies, which dried their
wings and about half an hour later flew off together eastwards
towards the sea".
climatic differences on opposite sides of a mountain range will cause
butterflies to migrate between them, ascending to cross over high
alpine passes. Beebe's observations include accounts of over 250
butterfly species migrating through the 1000m high Portacheulo Pass in
Venezuela. 22 of these species were said to have occurred in
"thousands". In late May 1946 he and 4 colleagues used stop watches
and counters to try to estimate the numbers of
Eunica monima migrating through the pass. In Beebe's own words
they "completely failed to keep up with fast enough estimates of
numbers, but at a minimum clocked 1000 per second going past in the
face of a gentle breeze".
Monarchs has shown 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 the antennae. 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.
circadian clock employs rhythms of
biochemical, physiological or behavioural processes that control
time based behaviour including the daily mate location / rest /
feeding cycle of males and the oviposition / rest / feeding
cycle of females. The clock also triggers one-off activities
such as emergence, and annual / seasonal activities including
migration. The precise timings are modified by climatic factors
such as temperature, humidity and sunlight levels.
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
Vanessa cardui and Pieris rapae
fly very low over the sea when migrating from the European mainland to
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".
A team of neurobiologists led by Steven
Reppert of the University of Massachusetts spent several years
investigating Monarchs. In 2010 they published a paper indicationg
that they had discovered that Monarchs have 2 types of photoreceptor
proteins which not only allow them to see UV light but also enable
them to detect the Earth's magnetic field. A series of experiments on
Drosophila fruit flies whose genomes were
engineered using Monarch Cry1 and Cry2 cryptochrome proteins proved
that they respond to magnetic fields when under the influence of
Expansion and contraction of range
overall area in which a species can be found is called its range.
Within that range there will be areas of unsuitable habitat so the
distribution within a butterfly's range is always patchy. The range
and distribution are limited by climate, geology, aspect and altitude,
all of which affect the type of larval foodplants and adult nectar
sources that will grow in an area.
distribution of a species within its range is also greatly affected by
human intervention - urban expansion has the greatest impact, but
governmental policy on farming, forestry and road planning also has a
very profound effect on the distribution and abundance of butterflies.
example is the High Brown Fritillary Argynnis
adippe which was widely distributed and common in England until
the 1950's. It's range then contracted rapidly as a result of habitat
fragmentation, and a change from traditional coppice woodland
management to the mass planting of conifers in English forests. By the
turn of the 21st century the butterfly had disappeared from virtually
all of its former habitats. Now in 2012 it clings to its existence at
just a handful of sites in western England.
other forest butterflies have also declined as a result of being
shaded out of the darker and cooler modern woodlands. Only one British
species Pararge aegeria has benefited, as
it survives better in shadier conditions. It has even been able to
increase it's formerly patchy distribution in the UK to the point
where it is currently widespread and common over almost its entire
Pararge aegeria, a species expanding
its range ©