Daily Commuting

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.

The male Purple Emperor Apatura iris commutes daily between feeding and courtship sites.

Montane butterflies

In 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.

Swarms 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.

Other 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.

Another 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

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 mass Pangaea. Nature fills every available niche, so ancestral butterflies were probably nomadic, their populations drifting seasonally from one area to another in search of suitable habitats.

The migratory behaviour would have been interrupted as tectonic activity caused mountain ranges and seas to appear, splitting up formerly contiguous areas of breeding terrain. These geological movements took place over millions of years, during which butterflies in all likelihood 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 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 mainly by climatic phenomena, although depletion of food resources, over-abundance and other factors may also be relevant.

Route-finding

Research on 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.

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 overall area in which a species can be found is called it's 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.

The 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.

An 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.

The Amazon is often referred to as the "lungs of the world". It also functions as a vast watershed and a powerful thermo-regulator - temperatures within the shade of the rainforest are at least 15°C lower than those of surrounding rural or urban land.

Vast swathes of Amazonian rainforest continue to be deliberately burned to make land available 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 equally destructive demand for vegetarian food have prompted major international companies to buy up millions of hectares of cheap rainforest in the Amazon and elsewhere in the tropics, burn it to the ground, and replace it with vast soybean or oil palm plantations.

Matters are exacerbated because higher temperatures and much lower humidity in the deforested areas affect the climate of the entire region. The vegetation structure in the remaining forest areas consequently deteriorates causing more extinctions, even in allegedly protected areas. The domino effect of climate warming causes the forest to shrink further, until what little remains has lost it's original character. 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, and the butterflies and other wonderful creatures of the rainforest become extinct.