Worried biologists have warned that wild animals are manifesting signs of extreme stress when they come into contact with humans.
"Polar bears and penguins, dolphins and dingoes, even birds in the rainforest are becoming stressed. They are losing weight, with some dying as a result," the British journal says.
Heart rates increase, reproduction decreases and hormones go awry with contact, made ever more frequent by the growing numbers of holiday adventurers flocking to remote, biodiverse areas.
Bottlenosed dolphins along New Zealand's coast became frenetic around tourist boats, resting for as little as 0.5 percent of the time when three or more boats are close, compared with 68 percent of the time in the presence of a single research boat, researchers who have studied them since 1996 found.
Polar bears who should be resting during chilly October and November months in Manitoba, Canada, ahead of their seal-hunting season, have also been made anxious by humans.
University of Manitoba, Winnipeg, researchers Markus Dyck and Richard Baydack said signs of vigilance among male bears increased nearly sevenfold when tourists were around, adding that it could use up their energy, thereby reducing essential body fat, with potential long-term consequences on breeding.
One vehicle is all it takes to set the bears on edge, they said.
Among the baby yellow-eyed penguins of New Zealand's Otago peninsula, researchers frequently found more than a 10-percent drop in weight for chicks in areas frequented by tourists.
Philip Seddon of the country's University of Otago said this could be the result of parents taking longer to reach chicks after foraging for food.
"Penguins will run back into the sea if approached on the beach, and will wait beyond the breakers until a beach is clear," he told New Scientist.
Underweight baby penguins were less likely to survive, he said, adding that tourists coming to see the flightless birds could bring about the end of their colony.
From polar caps to steamy jungles, ecotourism has had a major impact on local economies and wildlife populations.
The industry promotes an up-close view of nature and a host of outdoor activities, while encouraging sustainable and environmentally-sensitive development.
It has grown at what New Scientist reported is an annual 10- to 30-percent rate to become a billion-dollar industry which in 1998 attracted nearly nine million people in 87 nations and territories.
Even the United Nations has gotten behind the business, as when it dubbed 2002 the year of ecotourism.
But apart from a few notable exceptions, ecotourist projects are unaudited, unaccredited and promise environmental responsibility without actually having to prove it.
Mostly they follow more basic guidelines concerning land use, cutting down trees and kindly trying not to scare the animals, the British journal says.
Even carefully controlled ecotourism often makes itself felt. As Seddon told New Scientist: "Transmission of disease to wildlife, or subtle changes to wildlife health through disturbance of daily routines or increased stress levels, while not apparent to a casual observer, may translate to lowered survival and breeding."
In the Amazon rainforest, a colorful bird called the hoatzin was studied by Antje Müllner of Germany's Frankfurt Zoological Society and Martin Wikelski of Princeton University.
In tourist-visited areas in the Cuyabeno reserve of Ecuador, only 15 percent of hoatzin nests contained an egg, compared with 50 percent in restricted areas.
Young hoatzins had double the levels of the stress hormone corticosterone than those in tourist-free areas. By placing microphones in their nests, researchers discovered that while the birds did not flee at human contact, their heart rates soared.
Ecotourism is an excellent resource for biodiverse developing nations, but biologists are calling for precise pre-studies such as theses before setting up business, the journal says.
"Pre-tourism data should always be collected, where possible," Rochelle Constantine, who studied New Zealand's dolphins, says.
"The animals' welfare should be paramount because without them there will be no ecotourism."