After 200 years, we still need you, Rev White: Malcolm Smith looks at how Britain's greatest amateur naturalist links an 18th-century Hampshire village with the global concerns of the Earth Summit

A FEW weeks ago, the British Trust for Ornithology, the country's leading bird research organisation, reported that the numbers of our commonest summer migrant, the tiny, somewhat unprepossessing willow warbler, declined by more than one- third between 1989 and 1992. What is remarkable about such a statistic is not so much the decline itself, but the fact that there is enough information to issue such a confident bulletin.

Gilbert White would have been amazed and, no doubt, pleased by all this. Arguably England's first ecologist, he was the author of The Natural History of Selborne, first published in 1789 and the fourth- most-reprinted book in the English language.

On Saturday, Selborne is hosting a Bicentennial Commemoration presented by the Gilbert White Museum and the Institute of Biology at The Wakes, White's home for most of his life and where he died on 26 June 1793.

White knew a thing or two about willow warblers. Comparing them with their lookalikes, the chiffchaffs, another common migrant warbler breeding in Britain, he had rightly spotted the willow's slightly larger size and light-coloured legs. And he had recognised differences in their songs. 'No two birds can differ more in their notes,' he wrote on 18 April 1768, 'for the one has a joyous, easy, laughing note; the other a harsh, loud chirp.'

Whether he found out much more about these tiny leaf warblers, we do not know. Certainly his book, a compilation of letters to Daines Barrington, another amateur naturalist, and Thomas Pennant, the zoologist and traveller, reveals little more on the subject.

His writings may seem avian aeons away from today's sophisticated computer-aided analysis of bird abundance and distribution data. But his work has influenced many eminent naturalists, and the questions he asked about nature and the inter-relationships of plants and animals with their environment are more relevant today, in view of last year's Rio summit, than they were in 18th-century England.

The Rev Gilbert White was born in Selborne, Hampshire, in 1720. He spent most of his life in the village after an education at Basingstoke and Oriel College, Oxford. His study of natural history was based on first-hand field observation, a new concept in his day. 'Unusually,' says Bob May, Royal Society Research Professor at Oxford University, 'White asked analytic questions about the natural world rather than seeing it as a cabinet of curiosities.'

Today's English countryside is a good deal less rich in wildlife than in White's time. But far more people are fascinated by what remains. Or, more accurately, with a small part of what remains. According to the trust's Chris Mead, about 85 per cent of its more than 9,000 members have undertaken surveys on its behalf. Hundreds of keen, trained bird ringers get up before dawn to catch, record and release birds in their chosen study areas. At the other end of the ornithological spectrum, more than 30,000 bird watchers are contributing data for the New Atlas of Breeding Birds in Great Britain and Ireland, due to be published in November.

You may be forgiven, then, for believing that any garden blackbird or waterway kingfisher able to struggle through the breeding season without its most intimate details being recorded by a zealous trust member will be a rare bird.

The Royal Society for the Protection of Birds has about 1 million members. More than 200,000 people belong to county wildlife trusts. Butterfly Conservation, the fastest-growing wildlife charity in Britain, has 10,000 members. But there are no royal societies for nematode protection and few people record what is happing to marine crustaceans. So do roundworms, threadworms, barnacles and crabs matter?

As part of the planet's biodiversity - the buzz- word for our wealth of plant and animal species and varieties - they do, for three reasons, according to Professor May: we have a moral duty of stewardship, to hand on to our descendants a world as rich as we inherited; as a potential source of useful products such as medicines; and because the interactions between biological and physical processes on Earth, many of which we barely understand, keep the place habitable.

Having signed the Biodiversity Convention at Rio, the Government has embarked on a Biodiversity Action Plan. Even though we know more about the natural history of Britain than of any other country, the plan is bound to highlight enormous gaps in our knowledge. These were emphasised by Professor May when he addressed a seminar on biodiversity organised by the Department of the Environment and the Joint Nature Conservation Committee.

Of Britain's nearly 50,000 species of plants and animals - low by world standards - some 30,000 are invertebrates, of which about 22,000 are insects. Birds, by comparison, number about 230 breeding species. We have 72 mammals and roughly 1,200 flowering plants, but more than 2,000 crustaceans.

Most organised recording, however, concentrates on birds, flowering plants and some of the more showy insects. The rest are left to a comparative handful of mainly professional investigators.

We cannot assume that British species, whether poorly or well recorded, will necessarily survive. Roughly 1 per cent of our invertebrates have become extinct this century. According to Plantlife, a conservation charity, 22 flowering plants and at least 44 others, including mosses and lichens, have died out over the past few decades.

The balance needs to be redressed. We should be studying our least understood plants and animals, including many marine creatures and land and freshwater life ranging from algae to nematodes. This will doubtless be the domain of the dedicated professional; committed birders are not likely to substitute gall wasps for gulls.

Nature in the round is what Gilbert White observed. It requires observation more now than it did two centuries ago, if biodiversity is to be more than flavour of the year.

For further information on next weekend's events at Selborne, telephone the Gilbert White Museum, 0420- 50275 (closed Mondays).