New Turf for the Rarest Gorilla

February 1, 2012

A high-tech study of Cross River gorilla habitat finds that the critically endangered ape’s range is more than 50 percent bigger than previously documented. By protecting habitat corridors between the gorilla’s populations, conservationists may be able to help their numbers grow.

Fewer than 300 Cross River gorillas survive on our planet, restricted to the forested mountainous terrain on the border region of Nigeria and Cameroon. And they don’t live there alone—in fact, this rarest of the four gorilla subspecies occupies a region of high human population density and heavy natural resource exploitation. As a result, Cross River gorillas are listed as Critically Endangered, threatened by both habitat disturbance and hunting.

Now, conservationists working in Central Africa to save them have some welcome news: there’s more suitable habitat than previously thought, including some vital corridors. If protected, these corridors can help the great apes move between sites in search of mates, according to the North Carolina Zoo, WCS, and other groups.

The newly published habitat analysis, which used a combination of satellite imagery and on-the-ground survey work, will help guide future management decisions for Cross River gorillas living in the mountainous border region between Nigeria and Cameroon.

The study appears in the online edition of the journal Oryx. According to WCS conservationist and co-author Andrew Dunn, “The good news for Cross River gorillas is that they still have plenty of habitat in which to expand, provided that steps are taken to minimize threats to the population.”

Using high-resolution satellite images, the research team mapped the distribution of forest and other terrain in the Cross River region, then traveled to more than 400 control points to confirm the map’s accuracy. The land-cover map was combined with other environmental data to determine the extent of the gorilla’s habitat. The team then ranked the habitat’s suitability, with steep, forested areas of low human activity receiving a high rating, and lowland areas more significantly impacted by people receiving a low rating.

With the new habitat suitability map to guide them, the team selected 12 locations considered optimal gorilla turf for field surveys. Most of these areas had no previous record of gorillas, but to their surprise, the team found signs of the great apes (in the form of gorilla dung and nests) in 10 of the 12 sites. Their discoveries confirmed the value of using satellite image analysis to predict suitable habitat and to prioritize areas in which to conduct further surveys.

Overall, the study’s findings represent a significant expansion—more than 50 percent—of known Cross River gorilla range. The results also support recent genetic analyses that suggest a high degree of connectivity between the 11 known locations where gorillas occur.

The study also located parts of the population facing isolation due to habitat fragmentation. For example, a significant number of Cross River gorillas live in the Afi Mountain Wildlife Sanctuary in Nigeria, but they are largely separated from a neighboring population by farmland and other forms of habitat degradation.

“For small populations such as this one, the maintenance of connective corridors is crucial for their long term survival,” said WCS researcher Inaoyom Imong. “The analysis is the first step in devising ways to rehabilitate degraded pathways.”

Authors of the study will use their findings at the upcoming Cross River gorilla workshop to help formulate a five-year regional plan for the subspecies. “This latest research has greatly expanded our knowledge on Cross River gorilla distribution, which will lead to more effective management decisions,” said WCS conservationist and co-author Aaron Nicholas.

Conservation work on Cross River gorillas in this region is a priority for several U.S. government agencies, including the U.S. Agency for International Development, U.S. Fish and Wildlife Service, and U.S. Forest Service.

The study was made possible through the generous support of the Arcus Foundation; Great Ape Conservation Fund; KfW (German Development Bank); Lincoln Park Zoo; National Geographic Conservation Trust; Primate Conservation Inc.; and U.S. Fish and Wildlife Service.

To learn more, read the press release.

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