The great raft spider, Britain's largest spider, is an imposing creature. Fewer than 100 of these water loving, fish-eating arachnid monsters are hanging on under intense pressure at Redgrave and Lopham Fen, a nature reserve managed by the Suffolk Wildlife Trust.

Threatened by drought and years of excessive water abstraction, the fen is now at the centre of a rescue operation led by Suffolk Wildlife Trust and Essex and Suffolk Water who will divert enough water to the site to save the species.

The Suffolk Wildlife Trust is just one of 47 local Wildlife Trusts and more than 50 urban wildlife groups that together make up The Wildlife Trusts, and Redgrave and Lopham is just one of more than 2,200 nature reserve managed by The Wildlife Trusts throughout the UK.

"The Wildlife Trusts have a surprisingly low profile," says The Wildlife Trusts' director general, Dr Simon Lyster. "We are probably doing more practical conservation and education work than any other conversation organisation in Britain, and we have been around for over 80 years."

The Wildlife Trusts began life as the Society for the Promotion of Nature Reserves, formed in 1912 by the far-sighted naturalist Charles Rothschild. And as a multitude of county naturalists' Trusts sprang up, they were eventually joined under one umbrella as the national association of The Wildlife Trusts.

The Wildlife Trusts now boast a 260,000-strong membership and a junior branch, Wildlife Watch. The membership is astonishingly active. Some 25,000 volunteers put in well over one million days of work last year and most of the 2,200 nature reserves are run by volunteer wardens, supervised by experienced conservationist. The reserves themselves are very varied - from the seabird paradise of Skomer to a green patch in inner city Birmingham, from flower-rich hay meadows to upland mires, from lowland health to the Hebridean isle of Eigg and an urban oasis behind London's King's Cross Station. Many are Sites of Special Scientific interest and harbour threatened creatures such as the natterjack toad and the marsh fritillary butterfly. The largest reserve is Benmore Coigach - 6,000 hectares of mountains wilderness in the Scottish Highlands. The smallest is Hethel Old Thorn - the oldest hawthorn in the country, planted in Norfolk in the 13th century.

Regular collection of biological data is central to The Wildlife Trusts' work, and their unrivalled grassroots network has enabled them to build up a picture of tens of thousands of sites of local importance to wildlife throughout the country. Information about these "Wildlife Sites" is then fed into local development plans and the local Wildlife Trusts fight hard to ensure the information is taken into account in any proposals for new housing or roads or even water abstraction. Many Wildlife Sites occur on agricultural land, and a major task for Wildlife Trusts is to work with local farmers to help and encourage them to protect these sites and to make them aware of some of the conservation grants available.

The Trusts care about "biodiversity" - that green buzz word which means "the variety of life".

Biodiversity is both the rare and common species and habitats which we value so much in our towns and in the wilder countryside. The red squirrel, the common frog, the harbour porpoise, the military orchid, the medicinal leech and the golden eagle all come under the scrutiny of The Wildlife Trusts. Wildlife on our doorstep isn't forgotten either - the Wildlife Trusts believe the UK's 15 million gardens are valuable havens for wildlife and urge gardeners to adopt greener practices.

"We want a UK richer in all wildlife - common and rare," says Dr Lyster. "It is important that we save our most threatened species, but many people will never see them and we want people to appreciate and value the wildlife they see every day as well."

The sense of personal rewards for The Wildlife Trusts' volunteers is immense. Everyday office work contrasts with a myriad of outdoor reserve tasks such as the traditional art of coppicing woodland, hedge laying, ditch digging and scrub bashing. Survey and monitoring encompasses everything from counting rare orchid blooms - 100,000 in the case of one Manx Wildlife Trust reserve - to looking for signs of the growth in the otter population or checking nestboxes for dormice and bats.

Dr Simon Lyster emphasises how Wildlife Watch, the junior club, sows the seeds by encouraging young people to contribute simple observations to scientific research. The last major survey, Frogwatch, was "The most comprehensive survey of the UK's frogs ever undertaken, primarily by children" and found that spawning respond to temperatures and could be used as measures of global warming. "Education is fundamental," says Dr Lyster, "and the fact that over 500,000 children participate in a Wildlife Trust Education programme each year is arguably our single most important achievement."

Data from The Wildlife Trusts has made a significant contribution to an audit of the UK's declining wildlife - the Government-backed UK Biodiversity Action Plan - rescue plans for 116 threatened species and 14 endangered habitats. This came about as a result of commitments made under the 1992 Rio Earth Summit. The Wildlife Trusts are not only at the forefront of efforts of implement these rescue plans, but the data they provide helps show how well - or badly - we are doing.