In a study of protected Californian forest, scientists have shown that hiking, wildlife-watching and similar low-impact activities are linked to a sharp drop in numbers of carnivores such as bobcats and coyotes.
"We saw dramatic, fivefold reductions in the native species," said Adina Merenlender of the University of California, Berkeley, who ran the study with Sarah Reed of the San Francisco-based Wilderness Society.
In 2004, ecotourism grew three times as fast as the tourist industry as a whole. Now, one in five tourists go on eco-holidays. It has been shown to have an impact on a range of species, from dolphins and dingoes to penguins and polar bears.
However, the upside of ecotourism is that its revenue provides one of the best incentives for local communities to protect endangered animals instead of hunting them.
Philip Seddon, a wildlife management specialist at the University of Otago in Dunedin, New Zealand, said that the finding that such apparently harmless activities may alter the make-up of wildlife communities challenges the main concept of ecotourism, i.e. it minimises impacts and maximises benefits.
Now reserve managers may in future have to make more areas off-limits to tourists.
Focussing their study on 14 protected zones of oak woodland in northern California, at each site the researchers collected faeces left by the target species along a series of 500-metre sampling paths. Later, they compared the quantity found in areas out of bounds to humans with that found along sampling paths in similar "paired" areas nearby where access was allowed.
Banning of people from one area showed that native species such as bobcats, coyotes and grey foxes thrived and were typically five times as abundant as in more heavily trafficked areas. Similarly, faeces of domestic animals, particularly dogs, were only found in the areas visited by humans.
While it's known that human activity can alarm animals, but Merenlender said that this is the first time a consistent effect has been demonstrated across entire communities.
"We see it over the whole park, not just a single trail," Merenlender said.
However, this doesn't mean that low-key ecotourism is always harmful, said David Sheppard, head of the programme on protected areas run by the International Union for Conservation of Nature (IUCN).
He said: "It's hard to make generalisations. It can depend heavily on species, as some are more affected by recreation than others."
"We're not in any way advocating that people stop seeing nature. But we're trying to heighten the awareness of site managers to these unexpected impacts on wildlife," said Merenlender.
The team has reported their findings in Conservation Letters.