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Survey Provides Most Accurate Count to Date of African Great Apes

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Makumba gorilla group with their infant twins in Central African Republic. / Terence Fuh Neba, WWF Central African Republic

Scientists have compiled the largest survey data set ever assembled to reliably estimate gorilla and chimpanzee population sizes in Western Equatorial Africa, providing statistics crucial to continuing conservation efforts in the region.

The data set, published in the April 25 issue of Science Advances, includes reports from a decade's worth of field research from 2003 to 2013 from the Republic of Congo, Gabon, Cameroon, Central African Republic, mainland Equatorial Guinea and the Cabinda enclave of Angola.

Until now, no dependable information on the numbers of western lowland gorillas (Gorilla gorilla gorilla) and central chimpanzees (Pan troglodytes troglodytes) had been available at a species-wide scale. This is largely because of the logistical challenges of assessing primate populations in remote tropical forests, where surveys are carried out on foot and each one takes weeks or months to complete.

To count the apes, Samantha Strindberg of the Wildlife Conservation Society, lead author of the study, and her colleagues developed spatial models based on ape nest counts that predicted where gorillas and chimpanzees were most abundant. They collected data from 59 sites in five countries, totaling 61,000 person-days of fieldwork.

Their results suggest that there are substantially higher numbers of apes in Western Equatorial Africa compared to prior estimates. They count 361,900 gorillas and 128,700 chimpanzees, roughly one-third more and one-tenth more, respectively, than previous estimates.

Despite these numbers, great ape populations are still in decline and critically endangered. Roughly 80% of the populations of both species live in unprotected areas. The research team estimates the annual population decline of gorillas to be 2.7%, maintaining their status as critically endangered on the International Union for Conservation of Nature Red List. Chimpanzee numbers, in contrast, have remained consistent, which Liz Williamson, study author and senior research fellow at the University of Stirling in the United Kingdom said can be attributed to their stealthy nature. That is, unlike gorillas — which bark, roar and may even charge toward an adversary — they do not put themselves in harm's way by confronting danger and are more able to avoid poachers.

All great apes are protected by national laws and international conventions, making the killing, capturing, and trading of live animals illegal, but a combination of poaching and the spread of Ebola virus disease has been catastrophic for gorillas and chimpanzee populations. A third emerging threat is industrial-scale forest conversion for oil palm plantations, which can ravage nearby forest-dwelling wildlife. Such massive development fragments remote forests and makes them more accessible — drawing human activity into already sensitive ecosystems.

"In the future, there will be a dramatic change in what the region looks like, with the expansion of rubber, oil palm and banana plantations and other crops. We need to act now to ensure that national, integrated land-use planning locates development where it has the least impact on critical biodiversity," said Williamson.

"It is essential that western lowland gorillas and central chimpanzees receive the highest level of protection possible," said Williamson, noting that almost all of the forest in Western Equatorial Africa outside of protected areas has been allocated to logging companies. She said that conservation efforts must include both effective anti-poaching and high-quality habitat where no logging or other activities are permitted. Such efforts also require that logging roads be closed to vehicles when logging activities cease, to prevent poachers from using those roads. "Where there are no guards, we have seen that great ape and elephant numbers are extremely low near roads, as they are more accessible to poachers, and these routes facilitate transportation of the meat or ivory."

The researchers note that it would be impossible to survey the entire range of western lowland gorillas and central chimpanzees, which spans 290,000 square miles. They managed to survey about a quarter of the range — focusing on the national parks, their buffer zones where the harvesting of publicly owned forests for a specific period is allowed and a gigantic swamp forest in northern Congo.

"Not only is it impossible to simply count the animals, but they also flee when they hear approaching humans, so we have had to estimate animal numbers using their signs and trails," said Williamson. "In the case of great apes, they — conveniently for us — build a new nest to sleep in every night. Methods based on nest counts were designed and refined over almost 30 years, so that we can now estimate great ape numbers at individual sites."

The authors said their next steps include fine-scale research on how the factors associated with logging, including road development and canopy loss, impact great ape density and distribution. They also plan to continue to refine methods used to estimate rates of nest decay. "A big improvement that might occur in the near future would be cost-effective technology that would allow researchers to transition to methods that permit observation through the forest canopy — some kind of canopy-penetrating technology, like LIDAR, Light Detection and Ranging, a remote sensing method," said Williamson. "We are not there yet, but it's at the forefront of many peoples' minds. In addition, we are generating future scenarios to design the best possible integrated land-use planning, whereby new economic developments are located where they will do no harm to native forests and the wildlife they harbor."

{Credit for associated image: Terence Fuh Neba, WWF Central African Republic]