The researchers were expecting a decline, but not such a dramatic one
The population of the endangered West African chimpanzees in Ivory Coast has fallen by about 90% in less than 20 years, a study has suggested.
Researchers found 90% fewer nests than a similar audit carried out in 1990, which suggested the chimp population had crashed from 12,000 to about 1,200.
Increased levels of deforestation and poaching were likely to be main factors for the decline, they added.
Details of the survey's findings appear in the journal Current Biology.
Ivory Coast, thought to be one of the last strongholds for the species (Pan troglodytes verus), was believed to be home to between 8,000 and 12,000 individuals.
This estimate was primarily based on a nationwide survey carried out in 1989 and 1990.
When scientists carried out the most recent count in 2007, using the same techniques as the 1990 audit, they discovered a very different situation.
"Our results show that there has been an alarming decline in chimpanzee numbers, and that urgent action is required to prevent them disappearing entirely," the team wrote.
The researchers revisited 11 sites that had been surveyed 17 years earlier.
"The dramatic result was that in most areas where we had found chimpanzees (in 1990), there were now none left," said co-author Christophe Boesch, who was also involved in the earlier survey.
"We were expecting a decrease but not such a dramatic one," he told the BBC.
Professor Boesch, a director at the Max Planck Institute for Evolutionary Anthropology, Germany, said poaching and deforestation were on the increase as a result of the nation's rapidly growing human population.
The number of people living in Ivory Coast is now estimated to be 18m, up from 12m in 1990.
"The forest has been cut back in order to grow cash crops and other things," he explained.
"Also, chimpanzees, like many other species, are hunted for their meat. In some regions, including West Africa, something called 'empty forest syndrome' has been recorded.
"This is where the forest itself is still intact but it has been emptied by hunting."
The researchers said that there was a link between increases in human populations and higher rates of poaching and deforestation.
They added that the civil unrest in the nation since 2002 was likely to have exacerbated the problems.
But Professor Boesch said that there was one glimmer of hope in the otherwise bleak findings.
One of the sites was located within the boundaries of Tai National Park, where the local population of chimps had fared much better.
"What differentiated this population from the others is that it is located within a national park, which means it is fully protected from poaching," he explained.
"Secondly, during the country's period of civil unrest, the park was supported by international conservation projects."
He added that the combination of the two pointed to a potential way to protect the long-term survival of the species.
"If we want to be serious about conservation, the international community needs to invest in conservation and has to invest in a sustainable way."
But he added that if a global conservation effort was not forthcoming, then the prognosis was grim.
"Our closest living relative will not survive, and I ask myself about what this means for the future of humans if we let this species disappear."