Western Chimpanzee numbers declined by more than 80 percent over the past quarter century

Research published in the American Journal of Primatology earlier this month finds that the overall Western Chimpanzee population has declined by more than 80 percent over the past quarter century.

In order to arrive at an estimate of the chimp’s population numbers, an international team of scientists led by Hjalmar Kühl of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany used transect count data from 20 different sites that encompassed the nesting grounds for some 25,000 of the estimated 35,000 Western Chimpanzees remaining in the wild. The team writes that they “detected a significant negative trend” at 12 of the 20 sites.

“The estimated change in the subspecies abundance, as approximated by nest encounter rate, yielded a 6% annual decline and a total decline of 80.2% over the study period from 1990 to 2014,” the researchers write. “This also resulted in a reduced geographic range of 20% (657,600 vs. 524,100 km2).”

Western Chimpanzees (Pan troglodytes verus) are one of four commonly recognized subspecies of the great ape, the others being the Nigeria-Cameroon Chimpanzee (P. t. ellioti), the Central Chimpanzee (P. t. troglodytes), and the Eastern Chimpanzee (P. t. schweinfurthii). Each population faces different threats, thus a regional approach to the conservation of each subspecies is considered crucial.

The main threats to the Western Chimpanzee are almost all man-made. Habitat loss and fragmentation driven by slash-and-burn agriculture, industrial agriculture (including deforestation for oil palm plantations as well as eucalyptus, rubber, and sugar cane developments), and extractive industries like logging, mining, and oil top the list.

Poaching for bushmeat and the pet trade are also taking a toll on the chimps, while human-wildlife conflict is an issue even in or near protected areas. Women living near a national park in Guinea-Bissau, for instance, consider the chimpanzees themselves unfit for consumption, and blame the animals for malnutrition in their villages because of the damage they do to crops.

Even infectious disease outbreaks that decimate the chimp’s numbers may be more frequent thanks to human activities. Homo sapiens and P. t. verus are, of course, closely related species, and as increasing human populations expand into Western Chimpanzee territory, they bring with them higher risks of disease transmission because the animals are coming into more frequent contact with humans and human waste.

In response to the finding that the Western Chimpanzee population has dropped so precipitously in less than three decades, the International Union for the Conservation of Nature (IUCN) elevated the subspecies’ status to Critically Endangered on its Red List of Threatened Species.

According to the IUCN, “Chimpanzees are completely protected by national and international laws in all countries of their range, and it is, therefore, illegal to kill, capture or trade in live Chimpanzees or their body parts.” But enforcement of these laws is generally weak, the IUCN adds.

Some Western Chimpanzees are known to occur in national parks, but the majority — greater than 70 percent, the IUCN estimates — occur outside protected areas. That could mean that, in addition to the often intractable threats they’re already facing, the chimps could be facing even larger threats in the near future.

The IUCN reports that there is significant overlap between Western Chimpanzee terrain and areas suitable for oil palm development in Western Africa, which is “likely to exacerbate population declines in coming years.” That’s especially true in Liberia, where 94.3 percent of areas of Western Chimp occurrence overlap with areas that might be targeted for oil palm plantations, and Sierra Leone, where there is 84.2 percent overlap. Those two countries, together with Guinea, are considered strongholds for the subspecies, which is already believed to be extinct in the wild in Benin, Burkina-Faso, and Togo. The chimp’s numbers are “in the low hundreds” in Ghana, Guinea-Bissau, and Senegal, the IUCN notes, while “Côte d’Ivoire has seen a catastrophic decline of about 90% of its Chimpanzee population.”

The authors of the American Journal of Primatology study write that the IUCN plans to start updating a 2003 conservation action plan for Western Chimpanzees this year, with the intent of providing “a consensus blueprint for what is needed to save this subspecies.”

The authors also included “a plea for greater commitment to conservation in West Africa across sectors” in their paper: “Needed especially is more robust engagement by national governments, integration of conservation priorities into the private sector and development planning across the region and sustained financial support from donors,” they wrote.

CITATION

• Humle, T., Boesch, C., Campbell, G., Junker, J., Koops, K., Kuehl, H. & Sop, T. (2016). Pan troglodytes ssp. verus (errata version published in 2016). The IUCN Red List of Threatened Species: e.T15935A102327574. http://dx.doi.org/10.2305/IUCN.UK.2016-2.RLTS.T15935A17989872.en. Downloaded on 31 July 2017.
• Kühl, H. S., Sop, T., Williamson, E. A., Mundry, R., Brugière, D., Campbell, G., … & Jones, S. (2017). The Critically Endangered western chimpanzee declines by 80%. American Journal of Primatology. doi:10.1002/ajp.22681

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