Beetle breakdown

Little haloes of sawdust, gathering round the feet of certain old and well-loved pieces of furniture (the hoovering is my job), serve as intimations of mortality, if I am in that sort of mood. Anobium punctatum was well established in the house as we found it, its larvae tunnelling the ceiling boards, if not the rafters, and will be here long after we have gone.

But the furniture beetle also has a life outside the old country cottage and city auction room. Its family is among the hundreds of invertebrate animals that spend their lives recycling the fibres of dead trees, and, in the process, nourish the ecosystems of our woods. This is the season, as wayside copses and hedgerows bend old, mossy bones to the light, when we become aware of how much - or how little - they have to work with.

In Europe generally, the loss of natural forest has meant that almost half of the so-called saproxylic species - insects and fungi involved in recycling the woody tissues of trees - are under threat of extinction. In Ireland, their survival is even more tenuous, given the scarcity of ancient broadleaf woodland and the arbitrariness with which dead or dying trees are offered to the chainsaw.

The readiness to sentence old trees to destruction and to "tidy up" woodland often seems strongest in those who know least about the true life-span of trees and the role of their slow transmutation into energy. We no longer have forest, in its true, wild sense, but we still have the shreds of its network of living organisms. And we need them, if our efforts to restore broadleaf woodland are to engage with stable ecological cycles.

To the naturalist, a big dead tree or a rotting log is anything but inanimate. For all the stillness and the silence of the wood, there is an almost tangible buzz of life and activity. Whole teams of specialised beetles, mites, woodlice, flies, spiders, fungi and micro-organisms are working to decompose the woody tissues into nutrients, unlocking their energy and returning it to the soil. What could seem simply a destructive "rotting" is a dynamic force in the life of the wood.

Wood-boring beetles are the pioneers. They do the travelling and chew the holes through which fungi can enter. Some saproxylic fungi have developed sticky spores that the beetles carry with them, and then pay for their transport by helping to break down the wood as beetle-food.

Relays of different species and organisms succeed each other in a process which may take up to half a century, and this measured pace extends even into the insects' lives: beetle larvae can take four or five years to develop as adults. But the whole army of saproxylic organisms - perhaps 200 different species of invertebrate - can decompose as much as 10 tonnes of dead wood per hectare of woodland in a year.

A rotting tree does not have to be dead, as countless hollow specimens show. An old tree does not need its heartwood: it lives and feeds its growth through the outer layer of active sapwood. Rotting often starts when a tree is pollarded and fungi get in through the scars. Then the heart of the trunk becomes home to an army of wood-decomposers, together with bats, owls and tree-creepers.

Where woodland has had a succession of many old trees, linking back, perhaps, to a vestige of native forest, it can be a valuable stronghold of a saproxylic species. Among the insects given conservation priority in the UK's Biodiversity Plan is the violet click beetle, Limoniscus violaceus, which breeds in hollow trunks of ancient beeches in Windsor Forest and ash trees at Bredon Hill: these are among its last refuges in Europe.

In many protected British woodlands, dead wood is given proper respect as an essential stage in the forest ecosystem. Fallen trees are left where they lie, or moved aside to rot down undisturbed.

IN Ireland, the options for conservation are far fewer, and so the more precious. Our last shreds of old woodland are often very small, and have no succession of ageing and regenerating trees. Even in the great oakwood of Killarney, some of it native forest since the Ice Age, overgrazing by sika deer has opened up a disastrous gap in regeneration. About a fifth of the woodland's insects are dependent on dead wood: when the present generation of old oaks finally falls to dust, this vital community of organisms will find it hard to survive.

Ireland has a champion of saproxylic insects in Dr Martin Speight, entomologist with the National Parks and Wildlife Service. He makes them a prime example in what amounts to a personal crusade for insects in European programmes for nature protection.

Until recent years, the idea that insects might merit special conservation measures seemed eccentric, even among scientists, such is the power of humanity's disdain - even though insects make up most of Europe's wildlife and genetic diversity.

At a Council of Europe scientists' conference on the state of the environment in 1991, Dr Speight stressed, typically, how few insect specialists are employed in nature conservation organisations: "The awareness of invertebrates hardly extends beyond a vague understanding that they are useful as bird food and help to pollinate flowers."

But with "biodiversity" now the catchword, conservationists are having to catch up on the insects, making new efforts to assess distribution and to agree on threatened species and how to protect them. Within the European Invertebrate Survey, which has been running for 25 years or more, Martin Speight is helping with techniques to answer difficult questions, such as: how much habitat do you need to keep an invertebrate species safe?

In a woodland, mere hundreds of hectares are not necessarily the answer. Continuity and succession, the rise and fall of trees, are necessary and are the work that the beetles are made for.