Penguins in Antarctica. Dolphins in Scotland. Dingoes in Australia. They all face the same danger: Ecotourism

Thanks to cheap flights and a fascination with the environment, ecotourism is one of the fastest-growing sectors of the travel industry. But scientists have now discovered that it may harm the very species it is meant to help. Charles Arthur reports on the dangers behind a growing trend

The silence is nerve-tingling. The lioness is poised to spring on her prey, in the dusty African savannah. Then comes the attack: a struggle, a bite, and the antelope is downed, and dying.

And that is followed by a new sound: walkie-talkies crackling to life with urgent messages, the gunning of Land Rover engines as another score of tourists are driven to the site of the kill to gawp at one of the world's fastest-dwindling species living out its life. To the bewildered lioness, the clicking of camera shutters and the sight of rhinoceros-sized metal beasts is just something she has to deal with - even, or especially, when she is eating. And these days, many companies will be happy to label such a trip "ecotourism".

"That's the worst kind of ecotourism," said Dr Nigel Dunstone, lecturer in zoology at the University of Durham, who has been investigating the effects of ecotourism on animal species since 1985. "But the real problem is that it has become too big a category. You can even find some white-water rafting holidays being called 'ecotourism'. It can cover everything from holidays where everything you do is sustainable, including the way you get to the destination and what you do there, right through to places that just concrete over a huge area for a landing strip and put up a great big concrete hotel."

There are two other things about ecotourism: it is big - no, huge - business, involving as many as 20 per cent of tourists; and in some cases it is harming the very species that it is meant to be helping. Like the "green" supermarket products launched in the environmental fever of the 1980s, its meaning has dissipated amid the eagerness by entrepreneurs in western and developing countries to take advantage of the huge profits on offer.

There is plenty of argument about who the first ecotourist was. One plausible suggestion is that the title should be bestowed on Henry Thoreau, who used to go to the American state of Maine in the 1840s and ponder on its vast wilderness while exploring it in detail, often guided by native trackers. He would trek, hike, camp, climb mountains, fish, canoe and study plants - studies recorded in his book The Maine Woods. Apart from the book-writing bit, it is the sort of itinerary that would not sound out of place in a modern travel agent's brochure attached to somewhere verdant.

But through the turn of the 20th century and on to the Second World War, animals and environments tended to be treated as infinite resources; and the absence of the jet plane meant that few people had access to far-flung places with rare species. The tourism that went on was not environmentally friendly, but it was, thankfully, limited. That has changed with the coming of cheap air travel, allied to the advent of television with big-name botanists who have opened our eyes to the wildlife to be found in distant parts, which means that most western people over the age of five know about exotic species and that you can go and see them, somehow.

That would be all right - but scientists have discovered that, above a certain local level, ecotourism and animals simply do not mix. A report in New Scientist magazine today cites a number of cases, and increasing expert unease, about the way that "nature tourism" is harming the very species it is meant to help.

Wildlife can become infected with human disease: mud and dirt from clothing and vehicles, and the outflow of sewage carry pathogens that can infect native species. Gorillas in East Africa have picked up intestinal parasites following the arrival of tourism. Mongooses and meerkats in Botswana acquired human tuberculosis - which killed an entire group of meerkats in the Kalahari desert. In Antarctica, an especially pristine wilderness, the risk of diseases being passed to wildlife that has been isolated from the germs of the world is a particular concern. And it may already have happened: there have been deaths of thousands of animals including Adélie penguin chicks, sea lions and crabeater seals, but no cause has been isolated. "We need to be vigilant, to limit the possibility of human activity introducing disease into Antarctica's wildlife," Knowles Kerry, of the Australian Antarctic division, told the magazine.

And even if they are not passing on their illnesses, ecotourists can upset animals simply by being there. Rochelle Constantine of the University of Auckland in New Zealand, who has led a team monitoring the behaviour of bottlenose dolphins since 1996, reports in a paper to be published in the journal Biological Conservation that the dolphins become "increasingly frenetic" where tourists' boats are present: when three or more boats are close by, the animals rest for as little as 0.5 per cent of the time, compared with 68 per cent of the time when there is just a single research boat.

That is unusual behaviour which is "potentially serious for the population", according to Gordon Hastie, an expert in marine mammals from the University of British Columbia in Vancouver. His team's observations of dolphins in the Moray Firth in Scotland found that they spent more time surfacing together when there were boats around than otherwise: that could lead to them needing to rest more at night, and hence cut the time they have to forage for food and to socialise.

As New Scientist notes, the list goes on: polar bears waiting for the ice to freeze at the Hudson Bay in Canada, so they can start hunting seals, are much more alert when there are vehicles around - as has been the case since the early 1980s, when ecotourist visits first started in the region. That extra alertness burns up energy that the animal might need later. In Australia, nearly 350,000 tourists visit Fraser Island off the Queensland coast every year, often hoping to see its native dog-like dingoes. Then in April 2001, two dingoes attacked and killed a nine-year-old boy; the authorities culled 31 of the animals to prevent more attacks.

In fact, Dr Constantine argues that the effects of such ecotourism on animals' breeding could take years to appear, by which it might be too late to reverse. Her PhD study - into the effect of "swim with dolphins" boat trips on the dolphins themselves - suggested that less than 20 per cent of dolphins want to take part; and that it can actually be detrimental to a mother and calf to have a human leap into the water with them.

But the reality is that ecotourism is not going away. Conservation International, a organisation based in Washington DC, estimates that "nature tourism" - of which ecotourism is a subcategory - is growing at between 10 and 30 per cent annually, compared with just 4 to 5 per cent for tourism; and it is reckoned to account for one in every five tourists worldwide. The UN declared 2002 the "International Year of Ecotourism". But it could have done that every year this century and not have been wrong.

But can the destinations bear it? According to Dr Dunstone, only if there are fewer than 30 people in a location. Above that, it is impossible to do it sustainably. "If you have anything bigger, up to 50 or 100 people, then you are always going to have an adverse effect." His own studies in the 1980s, and investigations into the impact of ecotourism on the distribution and behaviour of mammals in the Manu National Park, Peru, found (by radio tagging of the animals the tourists wanted to view) that above that 30-person ceiling the human trails become too wide, the need for clean water and fresh food and sewage disposal too great, and the animals too disturbed by the intrusion for anyone to benefit.

Dr Chris Southgate, who teaches the ecotourism course at the University of Central Lancashire and has worked with projects in Kenya, thinks that what is really needed is a kitemark - rather like the "Fairtrade" symbol - that will tell western consumers who might be paying between £200 and £300 per night that the visit they are going to make will not leave harm behind.

"Ecotourism should actually distance tourists from wildlife so that there isn't this impact on feeding, behaviour, migration and so on," said Dr Southgate. "The problem is that the label is used too broadly - just as all 'ecological' labels have been, not only in tourism. We need tourists to know what is genuine ecotourism, and what's exploitative."

Yet Dr Dunstone thinks that ecotourism, done correctly, is a wonderful idea. "At the very least it means people are putting money into the local economy, coming to view animals rather than hunt, shoot and eat them," said Dr Dunstone. "I'm absolutely for it, so long as it's done under controlled circumstances. Surprisingly, it works best when it's high-end: Belize at one stage banned backpackers, because it wanted the tourists who would pay top dollar. I think it was rather clever - it meant a lot of income for the local people."

The advantage to local communities cannot be underestimated. "The phrase 'poacher turned gamekeeper' is really true; the best guides are the ones who used to be poachers and now are showing people where to go to view the animals," Dr Dunstone said.

There is another way to do as much ecotourism as you want, and in the process add to the sum of knowledge about those animals. It's called scientific research. "Yes, it's true - and in effect you get paid to do it," said Dr Dunstone. "But don't tell anyone. We don't want them all to find out."