Introduction

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.

Often the relationship between a butterfly and its larval foodplants only benefits the butterfly, and many plants actively defend themselves against being eaten. Passiflora plants for example produce tiny growths which resemble butterfly eggs, deterring Heliconius butterflies from laying real eggs on them. They will not lay further eggs if others are already present, presumably because the larvae are cannibalistic.

Many plants defend themselves by producing toxins that can kill larvae, but the larvae are often able to evolve methods of combating this defence. Some species for example have learnt to bite though the stem or main leaf vein, to prevent the toxins from reaching the leaf tissue.

Other species have become immune to the toxins, but store them within their bodies so that they become poisonous themselves. These species usually "advertise" their acquired unpalatability to birds, by evolving bright warning colours. Consequently most birds leave these butterflies alone, but some have found ways of dealing with them. Great tits e.g. have learnt that they can decapitate larvae and extract their guts to dispose of the toxic contents, before consuming the edible parts.

In the rainforests of Amazonia many butterfly species are involved in "mimicry complexes" whereby several different species with varying degrees of toxicity share almost identical wing patterns that warn birds about their actual or supposed poisonous nature. The toxic models include orange and black "Tiger" butterflies in the tribes Danaini and Ithomiini, and near-identical mimics from the Heliconiinae, Nymphalinae and Dismorphiinae.

Although relatively little is known about the lifecycles and ecology of the majority of butterflies and moths, it seems likely that a large number of species are involved in mutually beneficial relation-ships with other organisms. In some cases the relationship is obvious and simple - e.g. butterflies pollinating flowers in exchange for nectar, but in many species it is very complex and can involve several species of organism at different stages of the butterfly's lifecycle.

Symbiosis - an inter-dependent relationship between two or more organisms

The caterpillars of many members of the family Lycaenidae have evolved to become carnivorous, feeding on ant grubs, aphids or coccids. One of the best studied examples is larva of the Large Blue which feeds when very small on the flowers of thyme Thymus pulegioides, but after a few weeks, when it 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 larva eats ant grubs !

The larva is tolerated by the ants because they are able to "milk" it, obtaining a sugary substance which is exuded from its dorsal "honey gland". The larva hibernates, and later pupates in the ant's nest. When the butterfly emerges from the pupa, it uses another pheromone to appease the ants, enabling it to crawl unharmed 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 also 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.

Generalists and specialists

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, migrating across the countryside in search of other suitable sites where they can lay their remaining eggs.

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 main 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.

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 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, and ultimately becomes extinct.

The catastrophe threshold

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. Even then the species often continues to decline because the gene pool is reduced. Reduction, fragmentation and isolation of habitats means there is little or no opportunity for fresh genetic material to arrive, and ultimately the species is lost.

If butterflies and other wildlife are to survive, it is vital that conservation and land management are coordinated at national level to ensure that sufficient areas of habitat are maintained in suitable condition throughout the country. It is also essential to ensure that existing areas of habitat are linked together by creating and maintaining a network of natural corridors ( woodlands e.g. can be linked by hedgerows and small copses, while grasslands can be linked by roadside verges ) so that butterflies and other wildlife can 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 many 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 & spiders. Orb spiders catch butterflies in their webs, and crab spiders lie in wait on flowers, ambushing them when they visit in search of 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 detailed information about predators and parasitoids see the Enemies of Butterflies pages.

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 more will be killed before they have time to mate and lay their eggs.

The effects of climate

Theoretically, 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 though - 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 attack 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 :

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 cause the extermination of a small colony of butterflies.

Habitat loss

In the past, when human populations were much 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 sprawl to recolonise sites.

Once a species is lost from a site, it is probably lost for ever, unless man intervenes. New habitats need to be created, and existing sites need to be managed for butterflies, and linked by a network of natural corridors. Consideration also has to be given to artificial population maintenance via reintroduction and genetic diversity management programs.