Butterflies of the World - Lifecycle, Ecology, Taxonomy, Conservation, Photography, Butterfly Holidays, Photo Galleries, Book Reviews and more.........
Text and photographs protected by Copyright © Adrian Hoskins 2007, and must not be published in part or in whole elsewhere without prior written permission from the author.
Ecology - the relationship between organisms and their environment
 
symbiosis | habitat management | predators & parasitoids | population dynamics

A few species of butterfly are generalists, able to exist in a wide variety of habitats. The adults can feed on nectar from a wide range of flowers, and the caterpillars are able to feed on the leaves of several different types of plant. Most butterflies however are far more specialised, each species having it's own particular requirements regarding habitats, temperature, humidity, larval foodplants, and adult food sources. In fact some species are so specialised, that they can only exist in symbiosis with other creatures or plants.

Symbiosis - an inter-dependent relationship between two different species of organisms

Large Blue Maculinea arion, has symbiotic relationship with Myrmica sabuleti ants

To give an example, the caterpillar of the Large Blue butterfly feeds when very small on the flowers of thyme Thymus pulegioides, but after a few weeks, when the larva has reached it's fourth instar, it secretes pheromones which induce a species of red ant Myrmica sabuleti, to carry it underground into the ant's nest. For the rest of it's life, the caterpillar eats ant grubs !

It is tolerated by the ants because they are able to "milk" the caterpillar, obtaining a sugary substance which is exuded from the caterpillar's "honey gland". The caterpillar hibernates, and later pupates in the ant's nest. When the butterfly emerges from the pupa, it uses another pheromone to appease the ants, and then crawls along the ant tunnels to reach the surface above, where it can dry it's wings and fly away.

The Large Blue is totally dependent on the ant Myrmica sabuleti for it's survival. The ants themselves have their own specialised requirements regarding habitat, which greatly limits the areas in which the butterfly can breed. If the ant colonies die out, the Large Blue colonies die out.

Most butterfly and moth species lie somewhere between the "generalist" and "specialist" categories.  A typical example is the White-letter Hairstreak which exists in very localised colonies, often based on a single elm tree. The butterfly lays it's eggs on the twigs of the elm, and the caterpillars hatch a few days after the flowers appear on the tree in early spring. When tiny, they feed within the flowers, but when the flowers have withered and died, they feed openly on the elm leaves. The adult butterflies emerge in mid-summer, and spend most of their lives at the top of the trees, but occasionally descend to feed on the nectar of thistles and other flowers.

In a "normal" summer the females lay all of their eggs on the same individual tree upon which they fed as larvae. The same happens with most other butterfly species - in normal seasons they are surprisingly sedentary in behaviour, so much so that many species never fly more than a hundred metres or so away from their emergence site.

In exceptionally warm summers, females still tend to lay most of their eggs on their home territory, but later disperse throughout the countryside in search of other suitable sites where they can lay their remaining eggs.

Unfortunately, fragmentation of the countryside greatly reduces the chance of any species locating suitable new habitats, and consequently almost all species in the developed world are declining at a frightening rate.

White-letter Hairstreak, a scarce species dependent on elms

Let's look at another example, the Pearl-bordered Fritillary. Sites for this species need to have an abundance of dog violets ( the caterpillar's foodplant ), and bugle ( the primary nectar source used by the adults ). The sites also need to meet fairly strict criteria regarding temperature and humidity.

Furthermore, because the caterpillars hibernate during the winter, the areas where they feed must be exposed to sunlight at the critical stage when they reawaken in early spring. The number of sites which fulfil all these criteria is very limited, so the butterfly is generally restricted to sunny clearings in woodland, where violets and bugle grow in profusion.

Pearl-bordered Fritillary Clossiana euphrosyne, depends on transitional woodland habitats

Habitat management

Woodland clearings are of course an unstable habitat - bare ground is quickly colonised by coarse grasses, then by bramble and bracken, or is replanted with new trees. Either way, the clearing quickly becomes overgrown, and the violets and bugle get shaded out. When this happens, the clearing is no longer capable of supporting the butterflies. If they are unable to colonise another clearing in the immediate vicinity, the butterflies die out.

Woodland management policy is affected by a multitude of factors including government policies, demand for timber / pulp / charcoal, timber extraction methods etc.

When woodland management changes on a national scale, and becomes incompatible with the requirements of a particular species of butterfly, the result is that the species follows a trend of decline.

Each species has it's own "catastrophe threshold" - once the amount of suitable breeding habitat falls below a certain level local extinctions occur and the species contracts towards areas where more extensive areas of suitable habitat remain.

If butterflies and other wildlife are to survive, habitat management must be coordinated at landscape level to ensure that sufficient areas of woodland are maintained in suitable condition in a given region. Vitally important also is the creation and maintenance of a network of natural corridors ( hedgerows, small copses, roadside verges etc ) to allow butterflies and other wildlife to easily migrate between sites.

For more information about the conservation and management of Britain's woodlands, grasslands, heaths and coastline see the Habitats in Britain page.

Predators and parasitoids

We looked earlier at Large Blue / Myrmica sabuleti symbiosis. Symbiosis is defined as a mutually beneficial relationship between organisms. There are also of course numerous non-beneficial relationships, such as parasitism and predation, where one species benefits at the expense of another.

All stages of the lifecycle are threatened by parasitoids - creatures which feed on other organisms and eventually kill them. There are for example minute wasps which inject their eggs into the eggs of butterflies, feed on the developing larva within, and emerge as adult wasps from holes in the butterfly eggs. Other wasps such as Apanteles and Ichneumon, and flies ( Tachina etc ) spend their larval stage feeding within the bodies of caterpillars, which die shortly after the parasitoids vacate them. Other tiny wasps, including the brilliant metallic green Pteromalus puparum, attack newly formed butterfly pupae.

Certain butterflies, particularly males of Meadow Brown Maniola jurtina, Marbled White Melanargia galathea and Common Blue Polyommatus icarus often have tiny red mites attached to them, usually on the thorax. Studies have shown that the mites have no detectable effect on the flight performance, orientation ability or lifespan of infested Meadow Browns, so this could be regarded as a non-harmful form of parasitism.

Adult butterflies however are killed in huge numbers by insectivorous birds, dragonflies and spiders. Orb spiders catch butterflies in their webs, and crab spiders lie in wait on flowers, ambushing butterflies which visit them for nectar. Vast numbers of caterpillars are eaten by birds, and lesser numbers by heteropteran bugs, solitary wasps, mantises, lizards, toads, mice and other predators.

For more information about predators and parasitoids see the Enemies of Butterflies page.

Population dynamics

Predation, parasitism and viral / fungal infestations cause very heavy population losses. A butterfly may be capable of laying 500 or so eggs, but in practice only about 50 are laid on average, as most females die before they are able to lay all their eggs.

Probably 95 percent of those eggs will hatch, but at least 90 percent of the caterpillars will be eaten by birds, or killed by parasitoids, and fail to reach pupation. At least half of all wild pupae will be consumed, be killed by parasitoids, or die from fungal attack. When the butterflies emerge from the surviving pupae, many will be killed before they mate or are able to lay eggs.

 

 

 

Marsh Fritillary

Euphydryas aurinia

 

 

 

 

 

 

 

 

 

If populations are to remain stable from year to year, only a single fertilised female needs to survive from that original figure of 500 eggs. Butterfly populations are rarely stable however - poor weather during the flight season greatly reduces mate-locating and egg-laying opportunities, mild winters increase the likelihood of over-wintering larvae and pupae succumbing to fungal attacks or viral diseases, and a succession of warm springs can have a very detrimental affect on species such as the Marsh Fritillary which over-winter as larvae :

If the weather in early spring is cold and sunny the larvae warm themselves up by basking on dead leaves, and are able to feed and develop quickly. Their parasitoid however, the wasp Apanteles bignellii over-winters as a pupa, hidden in the shade amongst grasses, and develops slowly. Consequently when the adult wasps emerge they discover that most of the caterpillars have already pupated. A few weeks later large numbers of Marsh Fritillaries emerge and breed. In warm springs however the wasps are able to emerge early and find large numbers of caterpillars to attack, resulting in a low emergence of Marsh Fritillary adults.

It can be seen from the above that a couple of successive warm springs, combined with dull or wet weather during the butterfly's flight season can easily be enough to wipe out a small colony of butterflies.

In the past, when human populations were lower and more wild habitats existed, butterfly populations could naturally re-establish themselves, as there were always other colonies nearby. Sadly, in the modern world, wild habitats have been fragmented, and butterflies are usually unable to find their way across the vast expanse of sterile farmland and urban landscape to recolonise sites.

Once a species is lost from a site, it is lost for ever, unless man intervenes. New habitats need to be created, existing habitats need to be managed for butterflies, and habitats need to be linked by a network of natural corridors.

 
Related subjects :
 
Survival Strategies  - colour, pattern and behaviour as survival tools.
Habitats in Britain - conservation and management of butterfly habitats.
Species Index - links to articles dealing with the behaviour of individual species.
Conservation organisations in Britain.
Parasitoids and Predators - of eggs, larvae, pupae and adult butterflies.
 
Recommended reading :
 
The Butterflies of North America, Scott.
 
Several introductory chapters dealing very thoroughly with the biology and behaviour of butterflies and skippers, with particular reference to North American species, all of which are described in detail.
 
Butterflies of the World, Preston-Mafham. ( OUT OF PRINT )
 
Despite the misleading title, this is not an identification guide. Instead it is a fascinating and detailed introduction to the biology, behaviour and ecology of the world's butterflies. It is probably the most interesting and informative butterfly book that you'll ever read and deserves to be on the bookshelf of every butterfly enthusiast. Highly recommended.
 
Text and photographs protected by Copyright © Adrian Hoskins 2007-2008, and must not be reproduced or published in part or in whole elsewhere in any form without written permission from Adrian Hoskins. Breach of copyright will be pursued by litigation.
 
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