By Alex Kirby BBC News Online environment correspondent

The bush fires raging across swathes of New South Wales are terrifying.

For people in the areas ablaze they spell disruption at least, the prospect of losing their homes, and the possibility of death.

For wild species, like kangaroos and koalas, death on a large scale is already a reality. Yet the fires may offer benefits to the forests in the longer term.

No forest has ever existed without having to cope with periodic fires. The State Forests of New South Wales website says: "Fire is very much a natural part of many Australian ecosystems.

"The Aborigines used fire on a regular basis, and had a profound influence on vegetation. Since the exclusion of Aboriginal burning, many areas of forest have undergone a change, particularly in the understorey.

No trees without fire

"Many areas which were once quite open and grassy now contain thickets of shrubs and vines."

Fires are a natural way of clearing old growth, causing organic matter to decompose rapidly into mineral components which fuel rapid plant growth, and recycling essential nutrients, especially nitrogen.

Controlled burning can help

Some trees cannot survive without periodic blazes. Lodgepole and jack pines are serotinous species - their cones open and their seeds germinate only after they have been exposed to fire.

In Australia, the mountain ash, a flowering tree that grows in temperate areas, needs a site to be thoroughly burnt and to be exposed to full sunlight before it can regenerate.

Some eucalyptus species are largely fire-resistant but can help a fire to spread, shedding their bark when they burn and releasing flammable oils from their leaves.

Forests adapt themselves to relatively small intermittent fires. But when policymakers try to suppress fires altogether, they encourage the accumulation of dead growth and allow new species to establish themselves.

When a fire does start, it finds more fuel to sustain itself than would normally be there.

Trapped by the flames

Some critics say opposition by environmental groups to any controlled burning in forests encourages combustible growth to accumulate and intensify any fires that do start.

They argue that forests need active management, including deliberate fires as a form of pruning.

Koalas can starve, even if they survive the fire

But if the forest flora stand to make some gains from the flames, it is a different story for much of the fauna.

The Australian Koala Foundation says that the survival rate of koalas depends on the intensity of the fire.

Even when flames do not reach the canopy of the trees, they may suffer paw burns when they come to ground to change trees and tread on the smouldering, hot ground.

They may also suffer from smoke inhalation and exposure due to the loss of foliage.

They are fussy eaters and if the trees they like are burnt they may find it difficult to find replacements.

Many perish when they climb to the treetops to escape the fire, while others starve when their food supplies are destroyed.

The most common natural cause of forest fires is probably lightning, though globally most fires are started by people.