The Galapagos Islands, the first place on the planet officially designated as a World Heritage site, has been declared "in danger" by the UN.
Experts said the 19 islands and surrounding ocean were under threat from "invasive species", increased tourism and growing immigration.
Isolated some 1,000km (620 miles) off of Ecuador's coast, the islands contain much unique plant and animal life.
They were protected by Unesco 1978, with the boundaries extended in 2001.
The UN Environment, Scientific and Cultural Organisation (Unesco), which administers the list of World Heritage sites, added the Galapagos Islands to a comparatively small list of sites facing clear dangers.
In a statement, the organisation said increased international interest in the islands - which are Ecuador's most popular tourist attraction - was effectively contributing to their gradual decline.
"The number of days spent by passengers of cruise ships has increased by 150% over the past 15 years," the organisation said in a statement.
"This increase has fuelled a growth in immigration and the ensuing inter-island traffic has led to the introduction of more invasive species."
Earlier this year Ecuador's President Rafael Correa said the Galapagos were at risk and in need of urgent action to protect their unique ecology.
He said he was considering a range of measures designed to protect the islands' environment.
The wide variety of unusual flora and fauna on the islands, much of it found nowhere else on the planet, inspired naturalist Charles Darwin and helped contribute to his theory of evolution.
Unesco also placed Niokolo-Koba National Park in Senegal on the endangered list because of the threat of poaching and a proposed dam on the Gambia river.
The committee is considering 45 applications to join the World Heritage list from 39 countries. It currently contains 830 sites.