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Tropical forests where many primates live are quickly disappearing, mainly to make room for agriculture, but also for logging, mining and other industrial activities.

Researchers find three-quarters of the world's 504 primate species are in decline

Primates are now so threatened by human activity that the group is heading for "a major extinction event" by the middle of this century, scientists warn.

The grim forecast comes from a global assessment of all known species of primates, the mammalian order to which humans belong and whose members are most like us in their biology and behaviour. Most primates are tree dwellers that live in tropical forests, and those environments are disappearing at an alarming pace, mainly to make room for agriculture, but also for logging, mining and other industrial activities.

In several countries, primates face additional pressure from hunting and capture for food and trade, both legal and illegal, taking place at unsustainable rates.

The result is that three-quarters of the world's 504 primate species are in decline and about 60 per cent now face extinction, according to a report published Wednesday in the journal Science Advances.

"We really are at a tipping point," said Paul Garber, a professor of anthropology at the University of Illinois at Urbana-Champaign and co-leader of a team of 31 experts from a dozen countries that combined forces to produce the report.

Speaking from Brazil, where he leads a field study, Dr. Garber described the imminent loss of so many primate species as an environmental disaster that will require action from international agencies, governments and companies to avert. "It's everyone's problem," he said.

Estimated global distribution of primate species today (top) and by the end of the 21st century (bottom) under a worst-case scenario projection of agricultural expansion into primate habitat.

If nothing is done, he added, it's not just the primates who will lose. In many ecosystems, fruit-eating primates are prolific dispersers of seeds that help regenerate forests and create habitat for other species. And because of their close kinship to humans, all primates – including apes, monkeys and prosimians such as the lemurs of Madagascar – are part of an invaluable repository of information about the underpinnings of human biology and social interaction.

"They give us a window into the factors that played a role in our own history and evolution," Dr. Garber said.

To create their assessment, Dr. Garber and his colleagues used data from the so-called "red list" of threatened species maintained by the International Union for Conservation of Nature, combined with other peer-reviewed sources and United Nations databases on population growth and economic development, to chart the expansion of human activity into primate habitat. The results suggest that over the course of the 21st century, 75 per cent of primate habitat will be impacted.

And while it's no surprise to find that species are at risk, it's the breadth and speed of the transformation now under way that has scientists raising the alarm.

In Southeast Asia and in the Amazon, "we're seeing enormous areas destroyed, which you never saw 20 years ago," said Anthony Rylands, a co-author of the report and senior research scientist with Conservation International, a Washington-based environmental not-for-profit.

This includes large-scale clearing of forests for palm oil, soy and other crops, as well as for cattle. Areas once so remote it would take primatologists days to reach them are now crisscrossed by roads and powerlines.

Dr. Garber said that a once-undisturbed forest study site where he conducts primate research in Costa Rica has become little more than a tiny island amid pineapple plantations in little more than a decade's time.

He added that working on the report and taking a measure of the global challenges primates face has proved a sobering process. "I was more of an optimist before I started this," he said.

The report also notes that additional threats posed by climate change and emerging diseases are expected to take a further toll on dwindling primate numbers in the coming decades. And as human populations continue to grow in poor rural areas adjacent to primate habitat, particularly in Africa, the threat to primates from hunting will continue to pose a threat.

Colin Chapman, a primatologist at McGill University who was not involved in writing the report, agreed with its conclusions.

"It really does paint a pretty dire picture and it stresses the need that we're going to have to stop business as usual if we want to continue to have primates in our world," he said.

Dr. Chapman, who has worked extensively in Uganda, among other field locations, added that there are many examples where conservation efforts have succeeded at a local level once communities are on board.

"We need to scale that up to a global level," he said.