THE world's greatest TV naturalist started with a mischievous put-down when he came north last week to help the Yorkshire Wildlife Trust celebrate its 70th anniversary.

"Thank you very much for inviting me to join your birthday party," Sir David Attenborough, aged 90, told an audience at the University of York's Central Hall. "Seventy years is a very important landmark. Of course, some of us have exceeded that!"

The audience erupted in delighted laughter.

Joking aside, the great naturalist was in York for a very serious purpose: to underline the vital importance of the work that organisations such as the Yorkshire Wildlife Trust do.

The natural world was in greater danger than ever before as a result of our own activities, Sir David said; from deforestation, from intensive agriculture, from the burning of fossil fuels which is warming the atmosphere. And the more we endanger the natural world around us, the more we endanger ourselves.

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Wild nature: Wheldrake Ings

The importance of the natural world goes way beyond the simple delight of hearing a bird sing or watching a peacock butterfly alight on a leaf, Sir David said. Those things are important. "But the natural world is much more important than that. We depend upon it for every breath we breathe, and for every mouthful we eat. We are part of the natural world, and because the natural world today is in greater danger than it has ever been, then we too are in some degree in danger."

The only way that politicians will take seriously the threat to the natural world is if the people they represent make it clear that they take it seriously, and that they believe it is worth making the investment needed to save it, Sir David said.

And that's where organisations such as the Yorkshire Wildlife Trust (YWT) came in, he told his audience. In conservation circles, he said, the YWT is legendary: just the second wildlife or naturalists' trust ever established in Britain. It does vital work, he said, not only in maintaining and preserving the nature reserves under its control, but also in educating countless thousands of ordinary people about the beauty and importance of nature.

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Nature's bounty: field bindweed at Wharram Quarry, a Yorkshire Wildlife Trust reserve

"People nationwide look to the Yorkshire trust for leadership," Sir David said.

Between them, the 47 wildlife trusts that now cover Britain form a formidable lobby, he added.

"Politicians now actually care about what you say. And the more vociferous you are, the more they will take notice.

"So on your 70th birthday I congratulate you for what you have done, and I urge you to go on to even greater things."

The Yorkshire Wildlife Trust, whose birthday the broadcaster had come to York to help celebrate, was founded 70 years ago: although its name back then was the Yorkshire Naturalists' Trust.

It was set up by two great York philanthropists, Sir Francis Terry and Arnold Rowntree, in order to take over and look after Askham Bog.

The bog - which Sir David visited last week - is not particularly big: less than 110 acres in size. But it is unique.

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Sir David Attenborough at Askham Bog

It formed on the site of an ancient lake left behind by a retreating glacier 15,000 years ago.

The lake was colonised by sphagnum mosses. Over thousands of years layer upon layer of moss grew up, eventually forming a dome of peat up to 15 metres thick.

Once, there would have been a lot more peat than there is now. But the Romans began cutting the peat to burn, and by medieval times monks were also using it as fuel, reducing the dome of peat to its present level.

The result is an extraordinary mosaic of habitats - including peat bog, ponds and the remains of drainage ditches - in one small nature reserve.

More than 300 plant species are to be found there - not to mention a host of insects, birds, frogs and newts, and small mammals.

"Askham Bog is a small site that punches way above its weight," says Jono Leadley, the Yorkshire Wildlife Trust's development director.

Some of the plants and animals here are not found anywhere else in Yorkshire - or even in the British Isles, Jono says.

It is this unique habitat that Sir Francis Terry and Arnold Rowntree decided must be saved. They bought two parcels of land at the bog, and the Yorkshire Naturalists' Trust was set up to look after them. The new organisation was legally incorporated on May 2, 1946 - and the YWT dates its history to then.

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Early days: Joyce Payne ice skating on a frozen Askham Bog in the 1940s

It was just the second wildlife or naturalists' trust to have been set up in Britain, after the Norfolk Naturalists' Trust.

The Trust grew slowly at first. In 1955, it launched its first fundraising appeal, to raise money to buy Moorlands, just to the north of York, which became its second reserve. By 1959, membership of the Trust had reached 1,000, and it was able to afford the £1,500 needed to buy Spurn Point from the War Office.

More reserves were to follow: Fen Bog in 1964; Strensall Common in 1965; Potteric Carr near Doncaster in 1968; Wheldrake Ings in 1971; and Flamborough Cliffs in 1999.

Today the organisation owns or manages about 100 nature reserves across Yorkshire. It has 40,000 members, several hundred volunteers, and about 100 paid members of staff.

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Spurn point

Its remit involves far more than simply looking after the nature reserves in its care. It also carries out species surveys; it works with other landowners and businesses across Yorkshire to try to create wildlife 'corridors' that allow species to move from one place to another, so they don't exist cut off in island reserves; it aims to introduce local people to the natural world through volunteering projects; and it runs educational programmes.

Despite its best efforts, over the 70 years since the Trust was first formed, enormous damage has been done to Yorkshire's wildlife and wild places, says the Trust's chief executive, Dr Rob Stoneman.

This destruction matters more to our everyday lives than many of us might realise, he says. Unspoiled countryside is the "natural infrastructure that slows the flow from hilltop rain to over-flowing urban drain; that filters out some of the pollution we put into the atmosphere and into our soils and rivers. (It's) our natural health service, our natural carbon storage system."

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Fighting back: volunteers repairing a boardwalk at Wheldrake Ings

We have the Yorkshire Wildlife Trust, however, to thank for the fact that the destruction of Yorkshire's wild places hasn't been even worse.

Now, 70 years on from the Trust's beginnings at Askham Bog, it is time to move into a new phase, says Dr Stoneman: the fightback.

"We need to be not just clinging on to the last vestiges of a wildlife rich land and sea but restoring wildlife across Yorkshire," he says.

That means removing conifer plantations to allow ancient native woodland to grow back, he says; restoring uplands and rivers so that they run wild with fish; allowing stormwater to spill back onto natural flood plains; and making our towns and cities greener, with "street trees the norm (and) parks and transport corridors acting as ecological superhighways bringing wildlife to where people live."

That may sound like a tall order. But as Sir David Attenborough said, the more noise those who care about the environment make, the more politicians will be inclined to listen...

A CLOSE ENCOUNTER...

The people of Heslington might have been forgiven for thinking there had been an earthquake, so loud was the ovation that greeted naturalist Sir David Attenborough when he arrived to give a talk as part of the Yorkshire Wildlife Trust's 70th birthday celebrations last week.

Afterwards he took part in a question and answer session - and it gave him the chance to regale his audience of 2,000 people at the University of York with some wonderful stories.

Asked what had been his closest shave, the naturalist described a close encounter with a herd of elephants.

As they prepared to film, there was a lot of talk about how wild animals rarely attacked, and that even if they did it was likely to be a dummy charge to scare you away, Sir David said.

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Sir David Attenborough

He was instructed to get out of his Landrover and go as close to the herd as he dared. "I was terrified," he told his audience.

Nevertheless, he and his guide completed filming, and he made it back to his Landrover. Then, as they were driving away, there was a terrific bang. They had been charged - not by an elephant, but by a rhino. "It had put its horn under the grille, lifted up the body, and shook it," he said. The rhino eventually disappeared - and Sir David gave his punchline. He turned to his guide, who was gripping the steering wheel white-knuckled, and said: "That was a hell of a dummy charge!"

Then there was the incident with gorillas.

He'd been in Rwanda, filming mountain gorillas for his series Life on Earth. The reason for filming, he explained, was to talk abut the importance of the evolution of the opposable thumb - a hugely important development in human evolution. The plan was to film gorillas eating, showing the way they used their thumbs.

He crept as close as he dared, before turning to face the cameras.

"Then I felt a hand on my shoulder," he said. "I turned my head and it was an enormous female. She put her finger in my mouth and peered inside. I was delirious. I sat there, and then two little babies came and did up my shoelaces. I just sat there, blissful.

"Afterwards, I said to the director, 'wasn't that unbelievable?' And he said 'I think we got a few seconds of it.' I said 'a few seconds? I was there ten minutes!' And he said 'I know, but we only had a bit of film left and I was waiting for you to talk about the opposable thumb!'"