A decade-long study of Western Equatorial Africa's gorillas and chimpanzees has uncovered both good and bad news.
The good news is that there are one-third more western lowland gorillas and one-10th more central chimpanzees than previously thought.
The bad news is that the vast majority of these great apes (80 per cent) exist outside of protected areas, and gorilla populations are declining by 2.7 per cent annually.
The Wildlife Conservation Society (WCS)-led study titled "Guns, germs and trees determine density and distribution of gorillas and chimpanzees in Western Equatorial Africa" appears in the latest edition of the journal Science Advances.
This paper was written by 54 co-authors from several organisations and government agencies, including WCS, World Wide Fund for Nature, Max Planck Institute for Evolutionary Anthropology, Jane Goodall Institute and Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).
Researchers collected field data during foot surveys carried out over a 10-year period across the range of both western lowland gorillas and central chimpanzees --surveying an area of 192,000 sq.km and including some of the most remote forests on the African continent.
The authors estimated an abundance of over 360,000 gorillas and nearly 130,000 chimpanzees across the combined ranges of both subspecies, both of which were higher than previously believed.
The gorilla estimate is approximately one-third higher and the chimpanzee estimate is about one-10th higher.
These revised numbers come largely from refinements to the survey methodology, new data from areas not previously included in range-wide estimates, as well as predictions of numbers in the areas between survey sites.
"It's great news that the forests of Western Equatorial Africa still contain hundreds of thousands of gorillas and chimpanzees, but we're also concerned that so many of these primates are outside of protected areas and vulnerable to poachers, disease and habitat degradation and loss," lead author Samantha Strindberg of the WCS said.
"These findings can help inform national and regional management strategies that safeguard the remaining habitat, increase anti-poaching efforts, and curtail the effects of development on great apes and other wildlife."
Although the majority of great apes were found outside of protected areas, they were still in large forested landscapes close to or bordering existing national parks and reserves and away from centres of human activity.
This suggests that protecting large and intact forested areas, with protected areas at their core, is critical to conserving gorillas and chimpanzees in this region.
The main factors responsible for the decline of gorillas and chimpanzees are illegal hunting, habitat degradation, and disease.
At the same time, it was clear that where wildlife guards were present, above all in protected areas with intact forests, both gorillas and chimpanzees can thrive.
An additional threat to great apes - as well as human health - is the Ebola virus disease. Continued research into developing a vaccine and the means to deliver it are priorities, as are educational efforts on how to avoid spreading the disease and transmission between humans and great apes.
Of all the 14 living great apes, western lowland gorillas and central chimpanzees have the largest remaining populations.
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