GALAPAGOS ISLANDS, Ecuador -- A few weeks ago, 19 Ecuadorean citizens detained on these world-renowned islands were marched onto a plane and sent back to the continent under armed guard. Their crime? Illegal migration.

So far this year, the government has expelled 1,000 of its citizens from the Galapagos -- a living laboratory of unique animal and plant species -- who were there without residency and work permits. It has also "normalized" 2,000 others, in effect giving most of them a year to leave.

The migrants are attracted not by the tortoises or blue-footed boobies but by the islands' booming economy, which offers plentiful jobs and good pay. Typical wages run 70 percent higher than on Ecuador's mainland, the public schools are good, and violent crime is nonexistent.

Last year, Ecuador was stung by a United Nations warning that the islands, whose human population has doubled in 10 years to about 30,000, are at risk from overcrowding and mismanaged tourism.

Priming the economy is the apparently insatiable demand by foreign tourists for a close-up look at giant tortoises, elephant seals, flamingos, marine iguanas and other species in their native habitat. As a result, scientists warn, that habitat is becoming increasingly less pristine.

The 2007 report issued by UNESCO, the United Nations' cultural arm, placed the islands on its "in danger" list, a designation upheld in July.

The rising tide of tourists, residents and suppliers has introduced a host of alien species, including rats, goats, cats and, more recently, mosquitoes and fire ants, UNESCO's Marc Petry said by telephone from Paris. Such intrusions, as well as sewage and oil discharged from boats, threaten the islands' plant and animal life, he said.

The expulsion of Ecuadorean nationals has sparked a debate about whether the government should be more concerned with imposing a cap on tourism than culling residents.

Scientists at Galapagos National Park want to see a limit on visitor traffic, which in the last decade has grown 13 percent a year on average. Tourists visiting the park this year are expected to total about 180,000, more than officials say they can keep up with.

"When visitors reached 50,000 a year, we said to ourselves, this really is the limit. We can't handle any more. But now it's triple that figure," said Sixto Naranjo, the park's coordinator and former director.

The government of President Rafael Correa has resisted any move to cap the number of visitors.

Environment Minister Marcela Aguinaga said in a September interview that there was no sign that tourism was "oversaturated." Migration controls, resident training and the development of a new "tourism model" are the answers, she said.

"President Correa declared the islands in danger four months before UNESCO and already had taken several measures to confront the problems," Mr. Aguinaga said.

The management of Galapagos tourism is a sticky issue for Correa, a self-described green president who briefly taught college economics. Galapagos tourism generates an estimated $200 million a year in revenue, about one-fourth of which ends up in the pockets of local ship captains, cooks, guides and other suppliers living on the islands. The rest goes to airlines and tour packagers on the mainland.

Despite rising prices and global economic downturns, the number of visitors has increased tremendously since 1980, with middle-aged Americans and Europeans making up the fastest-growing market segment. These tourists, in contrast to the shoestring-budget backpackers of two decades ago, are increasingly affluent.

Nearly half the Galapagos visitors this year will be from the U.S., and most will spend $2,000 to $3,000 for a four-to-seven-day boat tour of selected islands, on top of the not-inexpensive airfare. Half the islands' visitors have annual incomes of $50,000 or more and one-third are older than 50, said Fabian Zapata, director of INGALA, the regional planning agency.

Most will emerge as amazed as naturalist Charles Darwin, whose 1835 visit inspired his "The Origin of Species," the tract in which he laid out his theory of evolution.

UNESCO declared the archipelago a World Heritage Site in 1978, the inaugural year for the designation.

Mr. Correa's government was the first to strictly enforce laws that require formal "visas" for Ecuadoreans to visit the Galapagos. The papers of all arriving at the islands' two airports are checked. Many with limited-stay tourist visas simply remain to look for work.

Checkpoints and patrols have become routine on Santa Cruz Island, home to the port town of Puerto Ayora, at 20,000 residents the Galapagos' largest city and the embarkation point for most tours.

But the undocumented still slip through security and now represent an estimated 20 percent of the islands' population.

Businesses say they are besieged by undocumented Ecuadoreans looking for work.