When David Streeter and his fellow conservationists got together in 1961, prospects for wildlife and countryside in Sussex was bleak.

To check the decline, he helped form what was to become, after a few name changes, the Sussex Wildlife Trust.

At the weekend, the group marked its 40th anniversary.

One of the big things founder members like David Streeter set out to change was the ethos of conservation.

There was air pollution, which is nowadays much more under control, although the invisible gases which contribute to global warming are still with us.

Agriculture had entered its industrial phase, spraying chemicals on the soil and grubbing out hedgerows. Woodland was becoming little more than ranks of conifers.

Old chalk grassland on the South Downs, one of the great contributions from Sussex to Britain's and Europe's biodiversity, was rapidly being destroyed.

Mr Streeter, one of the trust's deputy presidents, said: "That kind of assault on the countryside was very much on the agenda.

"Part of what changed is countryside ethos and now, as we know, they are thinking about turning the South Downs into a national park."

The trust was one of a wave of similar groups set up as concern about the vanishing countryside mounted.

Botanist and TV presenter Dr David Bellamy, president of the group which represents all the county wildlife trusts, remembers his own experience of the countryside as a boy in London.

He said: "I used to cycle down to Brighton through a landscape which was buzzing with wildlife. Now it has almost gone and I blame the Common Agricultural Policy. We are putting it back together.

"The green revolution is here and right the way across the world there are similar groups, many of them founded on the greenprint of the wildlife trusts."

That greenprint is typified in the trust's experience.

Among the first things it lobbied for was councils to set up local nature reserves. Pagham Harbour, North Common, Chailey and Rye Harbour were among the first and now Sussex has as many as anywhere.

It started work on the Sussex biodiversity record, which details plants and animals in the county. Today, it is one of the most comprehensive in Britain.

In 1963, the trust bought its first reserve, two fields of wetland at Amberley Wildbrooks. Twenty years later it won a key battle to stop the area being drained, a turning point in how wetlands are managed.

More reserves followed in the Seventies, among them the Mens, near Petworth, an area of wild forest which is still the trust's largest reserve.

New reserves bought in the Eighties included Pevensey Marshes, part of the vast area of wetland between Eastbourne and Bexhill, and Ebernoe Common, near Petworth.

In the Nineties, one of the main additions was Eridge Rocks, near Crowborough, an outcrop of sandstone rare in Sussex.

In all there are 39 reserves covering more than 3,000 acres. Trust membership today stands at more than 12,000.

Two of the big projects on the horizon are doubling the size of the Ebernoe reserve and the trust's Malling reserve at Lewes.

Farmland next to Ebernoe has been bought and is to be turned into a large area of open wild forest. Ebernoe itself is recognised by the EU as a special area of conservation.

The trust's head of conservation, Tony Whitbread, said: "Of course wildlife does not stay within boundaries. For example, Ebernoe is one of the most important places in the country for bat species."

At Malling Down, land next to the existing reserve has been bought.

Dr Whitbread said: "We are aiming to double the area of that and restore chalk grassland in an area which was arable land.

"These two sites are a good indication of where the trust is going. We are not just creating nature reserves, we are working with others as well to look to enhance the landscape as a whole."

The trust has spent about £700,000 on the two projects. At the weekend, it launched the Restoring Lost Landscapes appeal to raise the £93,000 still needed.

Wetlands, such as Amberley Brooks, the Pevensey Levels and river valleys, are another focus for the 21st Century.

Dr Whitbread said: "That is going to be good for biodiversity but also for flood water storage and stopping rivers going into spate. If the Ouse valley was restored to a more natural state, Lewes would be at less risk from floods."

Things may have got better but the trust's work has not finished. If there is a theme running through the history of the trust, it is the damage agriculture has done and, to a lesser extent, continues to do.

Dr Whitbread said: "Much of the environmental quality we have in Sussex has been provided by farmers over generations and we are strong supporters of a viable farming industry.

"We want to see farmers paid for providing social and environmental goods as well as producing food.

"A good indicator of a healthy environment is healthy wildlife. We want to get that through to people."