Population of mountain gorillas in Bwindi determined by census
A census of mountain gorillas, Gorilla beringei beringei, conducted in 2011 in Bwindi Impenetrable National Park, Uganda, confirms a minimum population of 400 gorillas, raising the total world population of mountain gorillas to 880. The official result was released today by the Uganda Minister for Tourism, Wildlife, and Antiquities Hon. Maria Mutagamba alongside representatives of the Uganda Wildlife Authority.
The increase in the Bwindi population since the last census, from 302 in 2006 to 400 in 2011, is attributed to improved censusing techniques of these rare and elusive apes as well as actual population growth.
In this latest census, teams systematically moved through Bwindi not once, but twice, looking for and documenting mountain gorilla night nests and faeces, and collecting faecal samples for genetic analysis. The first sweep was conducted with a small team from February 28 to September 2, 2011 and the second sweep conducted with multiple teams from September 10 to November 3, 2011. With the genetic analysis, scientists were able to determine how many unique groups and individuals were found by the field census teams through both sweeps, in what is referred to as a modified mark recapture method.
In short, the two sweeps of Bwindi allowed census teams to find more gorillas than a single sweep would have. Further, it is likely that some gorillas were missed by field census teams in the 2006 census of Bwindi’s mountain gorillas. But all signs are that this population of mountain gorillas is indeed growing.
“This method gives us the clearest picture of the status of mountain gorillas in Bwindi that we have yet had,” adds Maryke Gray, Technical Advisor to the International Gorilla Conservation Programme (IGCP; a coalition of the African Wildlife Foundation, Fauna & Flora International, and WWF). “Even with evolving census methods, this result indicates that this population has indeed increased over the last five years, and that is very encouraging for this critically endangered species.”
Mountain gorillas live in social groups and the census results indicate that the 400 mountain gorillas in Bwindi Impenetrable National Park form 36 distinct social groups and 16 solitary males. Ten of these social groups are habituated to human presence for either tourism or research and included, at the time of the census, 168 mountain gorillas or 42% of the Bwindi population.
While it was initially planned to include Sarambwe Nature Reserve in the Democratic Republic of Congo, a protected area continuous with Bwindi and therefore potential habitat for the Bwindi population of mountain gorillas, it was not possible to do so due to insecurity in the Sarambwe area at the time of the census.
The estimate of the total world population of mountain gorillas now stands at 880, representing the 400 individuals in Bwindi confirmed in this 2011 census and 480 mountain gorillas in the Virunga Massif confirmed by a census in 2010. Both populations of mountain gorillas have had positive trends in population growth over the last decade.
“In fact, the mountain gorilla is the only great ape whose population is increasing despite continuous pressure on its habitat. This positive trend is due to the strong collaboration among the three countries where mountain gorillas live and the collective efforts on the ground by park staff, surrounding communities and local government, and non-governmental organizations,” adds Dr. Augustin Basabose, Interim Director of IGCP.
The 2011 Bwindi mountain gorilla census was conducted by the Uganda Wildlife Authority with support from l’Institut Congolais pour la Conservation de la Nature and the Rwanda Development Board. The census was also supported by the International Gorilla Conservation Programme (a coalition of the African Wildlife Foundation, Fauna & Flora International, and WWF), the Max Planck Institute for Evolutionary Anthropology, Conservation Through Public Health, the Mountain Gorilla Veterinary Project, the Institute of Tropical Forest Conservation, and the Dian Fossey Gorilla Fund International.
This census was funded by WWF-Sweden with supplemental support from Berggorilla & Regenwald Direkthilfe e.V., the Wildlife Conservation Society, and the Max Planck Institute for Evolutionary Anthropology.
Note: While the Uganda Wildlife Authority uses a figure of 340 mountain gorillas from the 2006 census of mountain gorillas in Bwindi, the International Gorilla Conservation Programme uses the final minimum estimate of 302 mountain gorillas from the same census. The estimate of 302 mountain gorillas in Bwindi in 2006 incorporated the results from the genetic analysis, the first time it was used to verify results from the field census of mountain gorillas, ensuring that each individual was counted once. Source: Guschanski et al. 2009. Counting elusive animals: Comparing field and genetic census of the entire mountain gorilla population of Bwindi Impenetrable National Park, Uganda. Biological Conservation 142: 290-300.