Spanish explorers called them Las Encantadas, the Enchanted Isles, and Charles Darwin used his studies of the islands as the foundation for his theory of natural selection. The Galapagos are among the world's most important scientific treasures, a group of stark volcanic islands fringed by deserted beaches and inhabited by unique varieties of giant tortoise, lizards and birds.

Yet life on this idyllic United Nations world heritage site has turned sour. Pitched battles have broken out between fishermen, armed with machetes, and conservationists. Ecuador, which owns the islands, has sent a naval patrol to quell disturbances, while police last week began arresting local men for rioting and assault.

The controversial director of the Galapagos National Park - which controls 97 per cent of Galapagos land and the marine reserve extending to 40 miles offshore - has been sacked, while an air of uneasy tension hangs over the archipelago, as the islanders prepare for next month's election when they pick deputies to represent them in Ecuador's national assembly.

'It's a very tense situation,' said Leonor Stjepic, director of the London-based Galapagos Conservation Trust, which raises money to help projects on the islands. 'We are watching it with concern.'

The violence has been triggered by an alarming growth in the islands' population. Puerto Ayora, on Santa Cruz island, housed just 45 inhabitants in the Fifties. Today there are more than 10,000, while the islands' total population is more than 19,000 and growing by 6 per cent a year, despite recently introduced laws to limit waves of immigrants fleeing the slums of Ecuador for a life 'in paradise'. On top of this, more than 100,000 tourists visit the islands every year.

Such numbers have put the islands' special ecology under intense pressure. Conservationists, backed by the Ecuador government, have replied by imposing strict controls to protect the islands' iguanas, blue-footed boobies and giant tortoises. (Variations in these creatures, on different islands, led Darwin to his great, revolutionary theory.)

These moves have infuriated many local people, however. They want to exploit the islands' waters and catch its protected species of sharks, lobsters and sea cucumbers (marine creatures related to sea urchins), which can fetch high prices in Japan and South Korea.

Infuriated fishermen laid siege to the Charles Darwin research station on Santa Cruz last February, threatened to kill Lonesome George - the last surviving member of the Pinta Island subspecies of the Galapagos giant tortoise - and blockaded island ports.

The dispute was defused after the Ecuador government made concessions by increasing fishing quotas, which angered conservationists. 'It is tragic, the short-term gain of a few fishermen versus the long-term survival of the Galapagos,' said John McCosker of the California Academy of Sciences. 'They are killing the golden goose.'

Then, last month, the Ecuador government appointed Fausto Cepeda as the national park's new director, a post that has become a political football for the mainland government. There have been nine directors in the past 18 months.

This appointment was particularly controversial, however. Cepeda was known to have close ties with the fishing industry, and the rangers, who run the national park and marine reserve, rebelled.

More than 300 staged a sit-in at the park's headquarters, barricaded themselves in and prevented Cepeda from taking up his post. After haranguing his staff, Cepeda departed. Later a group of about 100 fishermen, carrying machetes, saws and cutlasses, stormed the park gate and the rangers' barricades. A pitched battle broke out, and at least two people suffered serious injuries.

Eventually, Cepeda - with the fishermen's help - entered the park. 'I am in office. I am in control. And I am trying to lower the tension,' he announced.

The Ecuador government took no chances, and sent a patrol boat to maintain the peace. A few days later, Ecuador Environment Minister Fabian Valdivieso met a delegation of rangers. After discussions, he emerged to tell newspapers that he had decided to remove Cepeda from the post.

In his place, Valdivieso has given the job to biologist Victor Carrion. A local man, and an experienced biologist, Carrion has the support of the rangers, and may be able to defuse the Galapagos crisis.

However, the long-term pressures on the islands, with their continually swelling population, are serious and will not disappear that easily. As the population continues to rise, the pressure on the environment, one of the most important in the world, gets more intense.

'We cannot keep the Galapagos in a bubble,' said Stjepic. 'We have to balance its special environment with the needs of local people. In that sense, it is a microcosm for all the other threatened parts of the world. So getting it right here is going to be a very, very important trick to pull off.'

· Additional research: William Lee