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BUTTERFLIES MOBILITY - Distribution and migration
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Paul Smart - The illustrated encyclopedia of the Butterfly world |
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Butterflies are to be found all over the world, but the greatest number of species live in the tropical regions. Butterflies are D particularly abundant where tropical rain forests abound; for example in Africa (south of the Sahara), in the Oriental region (Far East and India) and in the Neotropical region (South America). Some large genera of Butterflies occur throughout the world, for instance Papilio (the swallowtails), Danaus (the monarchs) and Eurema (the 'yellows' from the family Pieridae, which also includes the 'whites'). A few genera are common to the Palaearctic (Europe North Africa and Asia) and Ethiopian regions, - these are mainly in the Pieridae. the Satyridae ('browns'), the Nymphalidae and the Lycaenidae (the 'blues' and 'coppers'). Among the Lycaenids the small copper (Lycaena phlaeas) occurs not only in the two regions but also in the Nearctic (North America which is a remarkable distribution for one species. The Ethiopian region shares many species with tropical Asia (Oriental region) but has few affinities with the tropics of South America (Neotropical region).
Despite the fact that butterflies are one of the more mobile groups of animals, which considerably aid them in colonizing such large areas; many species are extraordinarily localized in their distribution. Some may be found only on a certain group of mountains, or one small part of a forest, and nowhere else in the world.
Ecological factors affecting distribution The distribution of a species is dependent not only on the geography of the area and the ability of the species to move around within it, but also on the ecological demands of the species. Each species of butterfly has its own set of clearly defined preferences concerning the environment in which it lives. These requirements not only limit the overall distribution of that species but also the distribution within its range. Thus in Africa the largest number of species occur in the forests in and around the Congo basin. The number of species becomes smaller to the north and south of the equatorial forest and also towards the east coast, which is somewhat drier. But even within the rain forest there are marked differences in the species found in lowland and highland or montane forest. Still more different species are to be -found in dry savanna conditions, derived savanna (cultivated land turned wild) and swamps. In Britain the chalk-hill blue (Lysandra coridon) and the adonis blue- (Lysandra bellargus) are restricted geographically by being inhabitants only of chalk or limestone hills in southern England. But this does not mean to say that they are found throughout these hills, because they only occur where the food plant of their larvae, the horse-shoe vetch, is growing. However, one cannot be certain of finding either species in a chalkland locality where this vetch is growing because many other factors also affect the ecology and therefore the distribution of these species. The ecological requirements of a butterfly are unique to each species, and so these two have slightly differing distributions which are localized even though the butterflies involved are mobile animals which can move easily from one place to another at will. Other downland Butterflies, whilst preferring chalk or limestone; are not entirely restricted to such areas. The grayling (Hipparchia semele), for instance, occurs not only on downs but also on dry heaths; it therefore has a wide distribution in Britain even up to the north of Scotland, but it is mainly found in coastal areas except on the downlands of the south.
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The Red-spot jezebel Delias descombesi - Picture: Phùng Mỹ Trung |
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The ecological preferences of a species will largely govern its distribution within its range, and the permanence of the habitats which that species occupies will affect its migratory activity. On the whole insects living in permanent habitats (like established woodland) tend to migrate less than species in temporary habitats (like fields of agricultural crops). Even apparently permanent habitats may be temporary for a given species because either the weather is only suitable at certain times of the year (a species at the northern end of its range may not be able to survive the winters), or its food plant is only available in certain seasons. In order for any insect species to survive in habitats which are 'temporary' in nature it must develop either a resistant resting stage in the life cycle or be able to move out of the habitat before conditions become unfavourable (emigration) and migrate back again when conditions are favourable (immigration). During the course of evolution the species which are good at migrating have become the most successful colonizers of temporary habitats, and as a result have been able to
extend their range as more of these habitats become available, often as a result of human activity.
The movement of whole populations of Butterflies from place to place in the form of mass flights provides one of the most spectacular examples of insect migration. These migrations often take the form of persistent flights in one direction over a long distance by large numbers of Butterflies and are clearly different from local flights around the breeding area, which are associated with feeding, mating and egg laying. Anyone who has witnessed the mass migrations of the painted lady (Vanessa cardui) from the Mexico/California border northwards as far as San Francisco Bay will have been struck by the vast numbers of insects moving purposefully north west 'as if guided by a compass'. However this butterfly does not establish itself permanently after the migration and there is no return flight.
In other animals, such as mammals and birds, migration is understood to include a movement of the population from the breeding place to some other site and the subsequent return of the same individuals. In the case of insects (including butterflies) the concept of migration needs to be modified somewhat: the return flight is rarely made ' by the same individuals because insects are relatively short lived. Usually it is the next or even later generations which make the return trip, and in many cases the movement is one way only, and no return journey is made at all.
Why do butterflies migrate ?
Many theories have been put forward to explain what causes butterflies to migrate and most of them centre round the idea that migration occurs in response to unfavourable conditions such as lack of food, insufficient food of the right quality, overcrowding or the onset of winter. In some insects, such as houseflies, flight seems to be a simple response to the environment-there is a shortage of food so they fly away. In butterflies the long migratory flights are almost certainly brought about by environmental factors which act in anticipation of unfavourable conditions before they actually happen.
These triggers, acting through the hormone system of the developing larvae, change the physiology of the next generation so that specifically migrant adults are produced.
Systematic and quantitative information on butterfly migrations is scarce despite the many occasions when migrations have been observed. Individual marking (similar to ringing birds) is difficult. Small tags can be attached to the tip of the wings but the proportion of marked butterflies which are recovered during or after a migration is usually very small indeed. This is the result of mortality during migration, damage to wings resulting in the loss of the tag, and the fact that not all the population will necessarily migrate to the same place. Added to this is the fact that the marking cannot be done until just before migration commences (ie when the adults have emerged) since it is useless marking a larva or pupa as the mark will be left on the cast skin after the moult. In the light of these problems it is easy to see why most records of butterfly migration are of an observational or even anecdotal nature. Nevertheless butterflies do feature prominently in the literature on insect migration.
Lepidopteran migration has been divided by one authority into two main types, short to medium-range displacement and long-range displacement. He points out that although many butterflies appear to exert considerable inÝ1uence over the direction in which they move, the longer they remain airborne the greater will be the effect of winds and other air movements. Much discussion has taken place about the influence of the insect versus the wind on the actual route taken during a migration. However there are clear examples of butterflies which direct their own movement over short to medium distances (less than 150km) and equally clear examples of long-range migrations of moths influenced almost entirely by meterological conditions. The Butterflies which migrate long (often inter-continental) distances have been alleged to have some control over their direction.
One very good example of a short to medium range migrating butterfly is the great southern white Ascia monuste. This butterfly breeds on the islands off the east coast of Florida where there is a plentiful supply of its food plant, vidrillos (Batis maritima). The plant is a woody perennial and common on coastal salt marshes. The butterfly breeds all the year round in Florida but only for about four months in any one particular place and at different times of the year according to latitude. Populations of Butterflies tend to spread
from the breeding site in one of two ways. They either diffuse slowly through the country as a result of daily movement in the form of short random 'non-migratory' flights to find flowers on which to feed, or periodic mass migrations occur. The latter involve large populations at a time when food for the larvae is still abundant; the butterflies which join in one of these migrations are less than 2 days old and the females are sexually immature. The butterflies fly from the breeding site to their feeding site in the morning as usual, and after feeding set off in large clouds along the coast, following the line of the shore or nearby roads. In calm weather they appear to be capable of controlling their direction; in windy conditions they may be blown off course somewhat, but some will fly in sheltered situations such as the leeward (phía dưới gió) side of sand dunes. Some of these migrating butterflies have been followed by car for over 20km and some are known to have travelled 130km in one day (downwind). By no means all the individuals cover the whole distance travelled by a group, many fall by the wayside and others lose their direction. Those which succeed find new breeding sites and revert to their normal 'non-migratory' activities of feeding, flying short distances, and egg laying. The adults only live for about ten days, so any subsequent migration from the new breeding site will involve the following generations.
The above account is based on observations made on individual populations of Butterflies which were followed as they migrated. Most often the migrations of Butterflies have to be pieced together like a jig saw puzzle from the isolated observations of many individual observers at different times of the year and in many different places within the range of a species. In just such a way C. B. Williams, who contributed so much to the study of insect migration, was able to build up a picture of the migrations of the large white butterfly (Pieris brassicae) in Europe. This species can be found migrating during the months of June, July and August in central Europe. By examining the records from many countries Williams was able to establish that migrations occur from breeding areas in Scandinavia and the Baltic through Germany to the Alps in the south and to Britain in the west. Like Ascia monuste, which is in the same family (Pieridae), the adults migrate when freshly emerged and immature and they do not return. They appear to migrate only when the temperature is high enough and in light winds; to some extent the Butterflies can control their track while they fly, but more often than not the final route taken is a compromise between the course chosen by the butterflies and the direction of the wind.
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The Fivebar swordtail Pathysa antiphates - Picture: Phùng Mỹ Trung |
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The migration of the monarch
The supreme example of long distance displacement among butterflies is the annual migration of the monarch (Danaus plexipus) in North America. The northern race of this beautiful insect migrates the length of the sub-continent-a distance of some 3200km between its breeding areas in Canada and the northern United States and its over-wintering sites in the southern United States, California and Mexico.
Adult monarchs start to move south from the northern breeding sites in July and by September vast numbers can be seen on the move. They apparently move by daylight only, feeding on the way, and on the whole do not fly at a great height but maintain a southerly direction. Mark and recapture. experiments have proved beyond doubt that this butterfly regularly covers distances in excess of 1,900km in only a few days, and at average speeds of up to 130km per day (though 25-35km is a more typical day's flight). The longest recorded flight by one insect is just under 3,000km in a period of 130 days. Opinions have differed as to how far most individuals fly and how much the migration is really made up of successive short migrations by different individuals. The mark and recapture experiments emphasize the fact that individuals can cover great distances, as do sightings of monarchs several hundred kilometres out to sea and the occasional landings in Britain of these American butterflies which have presumably strayed off course during their southerly flight.
At night and when the weather is poor, the Butterflies roost in trees, often in large groups. Most of the movement occurs down the eastern United States and also on the west coast, but migrations through mountain passes, several over a thousand metres high, have been observed. There are many records which suggest that the insects are capable of directing their own flight, even in the face of a 15kph head-wind. On the other hand they are often seen being blown south by a strong north wind, which suggests that in the long run it is the wind direction and strength which determines where the migrating Butterflies will end their journey. Unlike the butterflies previously mentioned, some of the individual monarchs which complete the long journey south to overwinter make the hazardous return journey the following spring. But the flight back follows a completely different pattern. Males with ripe sperm and females with ripening eggs set off northward separately and not in large groups. They fly high and fast, not bothering to feed on the way but using up their fat reserves, and they apparently rarely stop, flying by
day and night. Some females may pause to lay eggs on the way and it is not at all certain whether they continue to migrate once their eggs are mature. It is also uncertain what proportion of the Butterflies which arrive back at the northern breeding sites really have overwintered in the south and how many are from successive generations which may have been produced during the overwintering period. But there is little evidence of breeding at the overwintering site and certainly many of the butterflies which arrive back at the northern breeding sites have a very battered appearance, suggesting that they have overwintered.
The speed of flight
The remarkable distances covered by some migrating butterflies pose questions about the speed at which they fly, how long they can remain in the air, what sustains them in flight and what makes them stay airborne for such a long time. Butterflies are not among the fastest flying insects by any means: Ascia monuste was reported by Nielson to fly at about 8kph against a strong wing and 20kph with it; the painted lady, Vanessa cardui, has been observed flying at speeds in the region of 8-15kph, and probably the fastest known migrating butterfly is the monarch which can keep up with a car travelling at 30-40kph. To put this in perspective some horseflies and honey bee workers can fly at approximately 50-65kph and one large dragonfly, Anax junius, has even been reported keeping up with an aircraft travelling at 140kph. Even the slowest flying butterfly can cover considerable distances during migration if it is carried along with the wind. Flying at 8kph in a light breeze of similar speed it could cover perhaps 130-160 kilometres in a day.
Most migrations are usually characterized by the long periods which the insects spend in the air instead of resting or feeding on the ground. Flying for such long periods involves the use of enormous quantities of energy. This is provided from fat stored in the body, mainly in the abdomen in the form of a large mass of tissue called the fat body. Butterflies are well endowed with fat body when they emerge, perhaps 30 percent of the total body weight after drying. After a long migration this figure may be down to 1-2 percent in the case of the monarch, even though the insects will almost certainly have fed en route in order to partially replenish their energy reserves. In females the fat body is also used to nourish the developing eggs and this is partly why migrating Butterflies often do not mature their eggs until after their long flight is completed.
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