Walking through the ancient Atlantic Forest of southern Bahia in Brazil is like taking a pilgrimage through a living version of a Gothic cathedral. Immense columns of trees surge up towards the brilliant flecks of sunlight filtering through the forest canopy high overhead - giving the visual effect of a stained-glass ceiling held by timber pillars. Just as startling are the massive natural supports jutting out from the trunks of some of the bigger trees to stop them from toppling over on the steep slopes - mimicking the flying buttresses of medieval cathedrals.

The Atlantic Forest is the forgotten forest. It does not get the publicity of its Amazonian counterpart yet its place in the scheme of life is just as important because of the sheer density of species that are found here and nowhere else. Parts of the forest are considered to be richer in life than any other rainforest on the planet. Scientists have documented super-high levels of endemic species which live solely within the forest's varied and unique habitats.

My guide was Kevin Flesher, a 6ft 6inch American biologist with a ponytail who has studied the incredible biodiversity of the Atlantic Forest for more than a decade - and nowhere is more diverse than southern Bahia on Brazil's north-east coast. "Within the Atlantic Forest the richest areas identified so far in terms of floral diversity are the forests of southern Bahia. And we are right in that zone. So we are pretty much in the richest zone of tree species anywhere in the world," Dr Flesher said.

And yet this unique habitat has virtually disappeared and what little is left has become the most threatened rainforest on the planet. Many specialists, including Dr Flesher, feel that not enough is being done to preserve the few substantial forest fragments that remain.

The Atlantic Forest once stretched along 1,500 miles of the Brazilian coast - longer than the eastern seaboard of the United States - and, until 500 years ago when the first Europeans arrived, it covered an area of 500,000 square miles, or about 15 per cent of Brazil's territory. Less than 40,000 square miles, or about 8 per cent of this once immense forest survives. What remains has suffered repeated tree felling and the effects of being divided into isolated fragments that prevent the free flow of animals and plants from one part of the forest to another.

For more than 30 years scientists and conservationists have been fighting to preserve these remaining islands of primordial forest with increasingly desperate initiatives. Officially, the Brazilian government has given high priority to the protection of the Atlantic Forest and only last December it passed yet another law designed to protect the Mata Atlantica, as it is known in Brazil.

In 1988 it was designated a Brazilian National Heritage site and then, in 1992, a Unesco World Biosphere Reserve.

In reality, however, such initiatives have failed to stop the logging. Indeed, one Atlantic Forest organisation, an environmental group called Iracambi, believes the deforestation has accelerated since the protection measures were first assigned in 1988. Another group called SOS Mata Atlantica, a Brazilian non-governmental organisation, is also concerned about the loss of the forest, citing "lack of monitoring" and inefficient inspection by public agencies charged with protecting the forest.

Dr Flesher said the problem was not the lack of legislation, but its enforcement. "I'm visiting all the major Atlantic Forest areas in Brazil and what I am seeing is that the federal and state reserves are mostly abandoned. There's no money," he said.

The Brazilian government's Institute for the Environment and Renewable Natural Resources (Ibama) enforces the laws on illegal logging and hunting within the national reserves, but the organisation is suffering a severe financial crisis, Dr Flesher said.

"The money is not there and that is the problem. I'm dealing with Ibama officials all over the country - they help me a lot with my research - and they are telling me that they are broke."

One of the most important regions of southern Bahia is a national park called the Monte Pascoal yet it has been effectively deserted by the authorities, according to Dr Flesher.

"It's completely abandoned ... You need one person to cover about 800 hectares. In southern Bahia you have four people covering 30,000 hectares, How can you control it with just four people? The situation is terrible," Dr Flesher said.

Unlike the hot tropical rainforest of the Amazon, the Atlantic Forest is temperate. However, given that it once stretched 1,500 miles from the north-east coast of Brazil to its southern borders, average temperatures can vary considerably. Here in Bahia, however, it never strays much from a humid 24C.

The high levels of biodiversity within the Atlantic Forest have astounded scientists. Some 20,000 species of flowering plants can be found in the forest, of which about 6,000 are considered endemic. The forest is also home to some 1,300 vertebrate species - not including fish - of which more than 500 are endemic. Some 38 of the 69 species of mammals in Brazil that are listed as severely endangered are found in the Atlantic Forest - and 25 of them are endemic. The most threatened mammals include the woolly spider monkey, four species of capuchin monkey and the golden lion tamarin.

All of the 16 endangered amphibians are endemic to the forest, as are 118 of the 160 bird species and 13 of the 20 endangered reptiles. Logging, hunting and illegal land occupation leading to forest fragmentation all increase the risk of extinction.

Much of the Atlantic Forest was lost when Portuguese settlers began to colonise Brazil 500 years ago - the most famous tree of the forest, the pau brasil or brazil wood, even gave the country its name. More than 92 per cent of the Atlantic Forest has disappeared since 1500. The biggest trees went for shipbuilding and some were even used in the reconstruction of the Portuguese capital after the Lisbon earthquake of 1755.

But much of the deforestation occurred simply to clear the land for crops and human settlements. Today, some 130 million Brazilians live on land that was once part of this ancient forest, from Sao Paulo and Rio de Janeiro in the south, to Salvadore, the old Portuguese capital, in the north.

The past 50 years has been one of the most destructive periods, largely as a result of land encroachment by a rapidly expanding population mired in poverty. Between 1990 and 1995, for instance, more than 500,000 hectares were destroyed. Between 2000 and 2005 every state where figures were available registered a loss of forest despite laws to prevent illegal logging.

Southern Bahia and the neighbouring state of Espirito Santo are particularly important for the animals and plants of the Atlantic Forest because they constitute a "centre of endemism" - a place where endemic species have evolved into a unique and rich tapestry of biodiversity.

Botanists have identified no less than 454 tree species in a single hectare of forest within southern Bahia - the same level of tree diversity seen in the whole of Germany. It remains the highest record of tree diversity in the world. And a single tree can support up to 3,000 different species of insects.

The Atlantic Forest is created by moist maritime winds which bring copious quantities of rain to irrigate the lush semi-tropical and temperate vegetation growing on the mountainsides. It is home to a bewildering array of ferns, mosses, palms, orchids, lianas, bromeliads and other tree-dwelling plants that live within the dense forest canopy.

"Because of its geographic isolation from other forest types, this forest has one of the highest percentages of endemism in the world: over 50 per cent of tree species and 92 per cent of amphibians are found nowhere else in the world," said Wayt Thomas of the New York Botanical Garden, who has made a study of the region.

Dr Flesher is one of a small army of scientists and conservationists trying to reverse the deforestation trend. He is working as a consultant for the Michelin rubber company which has introduced a radical plan to preserve what remains of the Atlantic Forest on its plantation about 120 miles south of Salvadore, while keeping the jobs that help to prevent local people from falling into the sort of total poverty that only exacerbates forest encroachment.

He is advising Michelin on a pioneering scheme to replant disused parts of its plantation with endemic trees grown from seed to form "ecological corridors". The aim is to re-connect different forest fragments to allow the free movement of animals and plants from one area to another.

Fragmentation is recognised as the final death knell of rainforest deforestation. Many species, from top predators such as the jaguar and puma to small monkeys, need room to roam either to hunt prey or find potential mates with sufficient genetic differences to prevent inbreeding.

Cutting swaths out of the forest prevents this natural movement, which is why Dr Flesher and fellow scientists are trying to establish living corridors to bridge two or more biological islands. "Our reserve has 1,500 hectares of forest and 1,500 hectares of wetlands and old rubber trees. Eventually it will be 3,000 hectares of forest," Dr Flesher said.

It is a step in the right direction, but a depressingly small one. In the Brazilian state of Parana alone, for instance, some 28,000 hectares of Atlantic Forest were cut down between 2000 and 2005 - and Parana is just one of half a dozen states where deforestation has continued despite the many laws designed to stop it. As Dr Flesher explained: "The laws are beautiful, it's just that they are not enforced in many places."

(By Steve Connor, The Independent, 22/05/2007)