Africa's gorillas and chimpanzees are dying off at a startling rate despite their protected status, even in the more remote parts of the continent that have long been considered their strongholds, according to newly collected survey data.

Surveys conducted from 1998 to 2000 found that ape populations have shrunk by more than half since 1983 in Gabon and the Republic of Congo, two nations where forests remain largely undisturbed and most of Africa's apes live today. The main causes are hunting for bushmeat and an epidemic of Ebola hemorrhagic fever, which has left thousands of dead primates in its bloody wake.

With hunters venturing ever deeper into the forest along newly cut logging roads, and the Ebola virus poised to sweep into parks where many of the world's remaining gorillas and chimpanzees have taken refuge, disaster is close at hand, researchers said.

"The stark truth is that if we do not act decisively our children may live in a world without wild apes," the international team of scientists conclude in their report, posted yesterday on the Web site of the journal Nature.

It remains unclear what it would take to slow or halt the decline, scientists said in interviews. Indeed, differences of opinion have already spilled over into political wrangling that experts hope to sort out next month at an emergency meeting in Washington, where conservationists, infectious disease specialists, international aid workers and public health officials will try to forge a cooperative strategy.

Meanwhile, the researchers want their data used to reclassify gorillas and chimpanzees as "critically endangered," a strictly defined category that would justify greater conservation measures for the animals, now listed as "endangered."

"We need more law enforcement to stop the poaching and we need money for field research to understand how Ebola is spreading," said Peter D. Walsh, a Princeton quantitative ecologist who led the new study with scientists from groups in Gabon, Britain, Spain and the United States.

The new analysis is the most comprehensive comparison of 1980s survey data with surveys conducted in the past few years. It involved labor-intensive searches for nesting sites in Gabon and Congo -- countries that remain relatively unknown to armchair conservationists but are home to about 80 percent of the world's gorillas and most of the world's chimpanzees.

Those countries are considered the most hospitable for apes because they retain so much forest cover -- crucial for chimps, which nest in trees, and also for apes, which build nests on the ground from branches and leaves. The two countries retain between 60 percent and 80 percent of their original forest cover, compared with 7 percent to 14 percent in eastern and western Africa, respectively.

Yet tree cover is no guarantee of survival, the new research shows.

Even where trees have not yet begun to fall in large numbers, new logging roads have given hunters unprecedented access to apes -- and easy truck transport of the meat not only to villagers but also to Africa's growing urban markets. Demand for bushmeat remains high despite ongoing efforts to educate people about the threat of extinctions and the risk to consumers, who can die from handling Ebola-tainted meat.

Ebola hemorrhagic fever -- an incurable disease that causes massive, fatal bleeding and has killed more than 100 Congolese people in recent months -- now rivals hunting as the biggest threat to apes, the researchers said. Thousands -- perhaps even tens of thousands -- of the animals are believed to have succumbed to the rapidly fatal disease.

"All the evidence points to a very dramatic and severe impact of Ebola," said William Karesh, head of the field veterinary program of the Wildlife Conservation Society, one of the groups involved in the latest work. The disease has wreaked havoc in Congo's Lossi sanctuary, Karesh said, and now is on the borders of Odzala National Park, home to the world's highest densities of gorillas and chimpanzees.

In northern Gabon's Minkebe forest, the new survey indicates, ape densities have declined by 99 percent in the past decade, mostly as a result of Ebola.

Scientists are not sure how best to slow the virus's spread, in large part because they don't know whether it is passed only from ape to ape or via other animals.

Scientists also differ in their approaches to reducing the bushmeat trade. Many groups favor eco-tourism and other means of making nature profitable for local people. But these programs take time to set up, and time may be short.

The new study, which documents a 56 percent decline since 1983, predicts that ape populations will decline another 80 percent in the next 33 years -- less than two ape generations. Walsh says that justifies implementing politically unpopular crackdowns on poachers and an emergency infusion of $10 million for field research from the U.S. Fish and Wildlife Service.

That agency currently spends about $250,000 a year on ape conservation in Congo and Gabon, said Richard Ruggiero, Africa program officer for the service's office of international affairs. A preventive health program for gorillas and a great-ape disease study are among the agency's programs.

"We consider the latest Ebola outbreaks to be significant to the point of alarm," Ruggiero said. "We need to pay attention to this."

In the longer term, governments need to consider setting aside more protected areas, said Rebecca Kormos, a research fellow with the Center for Applied Biodiversity Science at Conservation International in Washington. "We're learning that you can't have just one area where you're protecting the apes. Not just because of diseases like Ebola, which can wipe out an area, but also because of things like civil unrest and conflict."

Perhaps most important, and most difficult, said Karesh of the Wildlife Conservation Society, is to forge better relations with local villagers. "They've been neglected and isolated for decades, they have different contexts about what causes disease, and they're scared of outside people and scientists," he said. "It's very hard to do conservation in that climate."

Karesh noted that the most recent outbreak of Ebola in Congo was detected in gorillas before it started killing people -- a warning that could have saved lives.

"Human or livestock or wildlife health can't be discussed in isolation anymore," Karesh said. "There is just one health. And the solutions require everyone working together on all the different levels."