Night falls swiftly in the rainforest. In the gloom, the photographer Thomas Marent is searching for the rare black cayman deep in the Venezuelan rainforest. The South American reptiles are at their most active at night, but it is also when they are at their most dangerous. The intrepid snapper is excited to come across a baby one on the bank of a pond. As he feverishly takes pictures, it starts to make a shrill, rasping sound. "I didn't realise the noise was a distress call to its mother," says Marent. "Suddenly, an adult came crashing out of the darkness. It was so dark I hadn't known she was nearby. I've never run so quickly in my life." It's one of many close shaves Marent, now 40, has had during an epic venture to capture the beauty of the rainforests.

Born in Switzerland in 1966, Marent grew up with a love of the natural world and had taken photos of the butterflies and birds found in the mountains near his home. But he was unprepared for the incredible diversity and abundance of life that he found on that first visit to the tropical rainforest on the north-east coast of Australia in 1990.

"It was during the rainy season when there is lots of life to see," says Marent. "I was fascinated by all the little creatures I could find that I didn't even know existed." His senses overwhelmed, it sparked a passion that has taken him through the rainforests of five continents. His desire to document what he found in each habitat became the driving force in his life for the next 16 years. "I didn't know it would take this long," he says, somewhat ruefully. "I had the idea for a book years ago, but each time I came back from somewhere, I realised that there were some important animals missing from my collection, and also some rainforest that I had not yet been to, and so it went on."

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He financed it all by working in Switzerland, most recently as a biologist. Marent's pursuit of some world's most elusive and camera-shy creatures has impelled him to go to extraordinary lengths and required limitless patience - some pictures took years to get.

At times, it has been alarming and, occasionally, downright dangerous. His most frightening moment came when he was in a part of Colombia where guerrilla fighters were based. Two uniformed, armed men approached his tent, demanding to know what he was doing. "They turned out to be regular army, but I was very scared," says Marent. "The worst thing was, they told me to put my hands up and when I did, all the birds flew away!"

Though he has had many heart-stopping moments during his explorations, Marent says the greatest risk is getting lost. Once, he strayed from a trail and couldn't find his way back. "After just half a day I started to imagine all the terrible things that might happen to me. I began to panic. I was very relieved when I finally found my way back to the trail."

The dedicated photographer lets nothing get in the way of capturing images of the rare creatures under the canopy. In Australia, he came across some bright-blue crayfish after heavy rainfall that had left the forest floor wet enough for them to hunt on land. "This is the only place in the world that these crayfish can be found, so I got busy taking pictures," he says. Unfortunately, he became so preoccupied that he failed to notice the heavy rain had brought out leeches, too. His legs were covered with them. "I ripped the leeches off and headed for the campsite but no matter how fast I walked, they kept appearing. I ended up running out of the forest," he says. "Leeches are not so bad," he adds, stoically. "They look disgusting but they don't carry diseases."

Over the years, Marent has witnessed first-hand the destruction of his beloved rainforests. In Southeast Asia, palm plantations have led to the clearing of one of the world's most biologically diverse regions. Borneo alone has between 10,000 and 15,000 species of flowering plant and Indonesia's rainforests are some of the oldest on Earth. Now, only fragments remain. Marent says that illegal cutting is commonplace and corruption is rife with the police, who are supposed to monitor the forest. "I feel so sad when I fly over areas where forest used to be and there is now nothing, with everything cut down."

The photographer has grave concerns about plans to build a road from Peru all the way to Brazil through the Amazon rainforest. "It goes right through national parks and, of course, animals will be hunted and more logging will happen. Governments say they are concerned about the rainforests, but it seems they care more about economics."

Now that his mission is complete with the publication of his book, Rainforest, Marent's fascination for these precious habitats remains undiminished. "To me, rainforests are the greatest treasure houses on Earth. It is tragic that we are losing them just as we are beginning to appreciate their true value."

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Rainforest: A Photographic Journey, sponsored by Nikon, runs at Proud Camden from 6 to 29 October. www.proud.co.uk. Rainforest: A Photographic Journey, is published by Dorling Kindersley with The Rainforest Foundation, £25; (2 per cent of the royalties will go to the foundation)

The world's disappearing forests

Where there was once 15.5 million square kilometres of tropical rainforest worldwide (14 per cent of the land), deforestation has reduced this to 6.7 million sq km.

Around 150,000 sq km of rainforest, equivalent to the size of England and Wales, is destroyed every year.

In Congo Basin countries such as Cameroon, Congo and Gabon, logging is the principal threat; every year, 137,000ha are logged in these three countries alone. It is estimated that one hectare of rainforest has to be felled to log just one mahogany tree.

Asia lost almost a third of its tropical forest between 1960 and 1980.

Almost 90 per cent of West Africa's rainforest has been destroyed.

Between 1980 and 2000, 14 per cent of Brazil's rainforest was cleared.

Madagascar has 10,000 species of plants, 80 per cent of which are endemic. Almost 90 per cent of its forests have been destroyed.

About 50 per cent of all mammals and 25 per cent of all bird species in peninsular Malaysia will become extinct by 2020 if forest destruction continues.

We lose 50 species every day due to tropical deforestation.

Almost 65 per cent of Central America has been cleared to create pasture for grazing cattle. Rainforest land cleared for pasture or farming degrades quickly and is usually abandoned.

In the 20th century, 90 tribes of native peoples have been wiped out in Brazil alone.

Many scientists argue that current rates of deforestation will see nearly all the world's tropical rainforests destroyed by between 2030 and 2050.

Only 4 per cent of the world's tropical rainforests are protected.