In just 20 years, poaching and the Ebola virus have cut in half the ape population of western equatorial Africa, long considered the last stronghold of the continent’s gorillas and chimpanzees, according to a new study.

The study, appearing today in the electronic edition of the journal Nature, is only the latest to warn that wild ape populations are vanishing. However, it is the first to use large-scale surveys to demonstrate the immediate threat of extinction.

“The bottom line is that this time the sky really is falling,” said Peter Walsh, a visiting research fellow at Princeton University and co-author of the study.

The forests of Gabon and the Republic of Congo are believed to still hold tens of thousands of apes, or most of the world’s common chimpanzees and roughly 80 percent of its gorillas.

Even though 60 percent to 80 percent of the native forests in those nations remain intact, logging has opened up roads that have greatly facilitated the hunting of apes for food, Walsh said.

Poaching accounts for most of the estimated 56 percent decline in the ape population between 1983 and 2000. Surveys indicate that ape populations have fared worst in the forests closest to cities, where so-called bush meat is sought as an expensive delicacy.

Now, even in the relatively remote areas where gorillas and chimpanzees survive, Ebola has come to rival hunting as a threat.

An ongoing outbreak of the deadly disease has killed hundreds of gorillas and chimpanzees in the area near Congo’s Odzala National Park over the last few months, said Dr. William Karesh, a veterinarian with the Wildlife Conservation Society and contributor to the study.

Ebola has killed about 120 people in Congo during the same period, according to the World Health Organization.

At current rates of decline, ape populations will drop 80 percent over the next 33 years — if not much sooner, Walsh said.

Walsh and his international co-authors said there is a critical need for more of everything: more protected areas for the apes, more work to prevent poaching and more research on Ebola.

They urge that both the gorillas and chimpanzees be reclassified from “endangered” to “critically endangered” by the World Conservation Union.

Russ Mittermeier, president of Conservation International and chairman of the World Conservation Union’s primate specialist group, said he would support the change in status, but only if it didn’t undervalue far rarer species of ape, including the mountain and Cross River gorillas. Their numbers have dwindled to the hundreds.

“Obviously the situation over there is pretty critical,” Mittermeier said of the study’s findings.