Periodic censuses of endangered populations of high-profile species help us to understand their population dynamics, to assess the success of conservation programmes aimed at ensuring their survival, and to ensure that they receive continued attention from the global conservation community. Mountain gorillas (Gorilla beringei beringei) are highly endangered, with just two small populations in Bwindi Impenetrable National Park in southwestern Uganda and the nearby Virunga Volcanoes on the borders with Rwanda and the Democratic Republic of the Congo.
A survey of the Bwindi population was carried out in 2002, and results showed that the population had increased since the previous census in 1997 by approximately 7%, to 320 individuals (McNeilage et al. 2006). The Virunga population currently numbers around 380 gorillas (Gray et al. 2006).
A new census of the Bwindi gorilla population was carried out between April and June 2006 to determine the population’s total size and structure, its distribution across Bwindi, and the potential impact of human disturbance on the population.
To estimate the total population size for the gorillas, the park was intensively surveyed by teams with the goal of locating every single gorilla group (see McNeilage et al. 2006 and Gray et al. 2006 for detailed methodology). To ascertain that we were not double-counting groups, and to ensure that we were able to distinguish and identify each group, fecal samples were collected to create genotypes, or unique genetic identifications, of the gorillas in each group. The genetic analysis is being carried out at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
The results show that the 5 habituated groups in Bwindi contained a total of 76 individuals at the time of the census. In addition to these, 25 unhabituated groups were found, containing 227 individuals along with 11 lone silverback males, giving a total uncorrected population count of 314 individuals.
As in other censuses of mountain gorillas, we then used a correction factor to account for infants not counted (their dung could not be observed in the nests) and the likelihood of not counting some gorillas, to estimate that the total population size is 340 individuals. The current age composition of the population indicates a healthy distribution of individuals in the adult and immature age classes. Approx. 22% of the gorillas (17% of the groups) are habituated to humans. Further details of the population structure, genetic composition, and the relationship between gorilla distribution, change in population size, and human disturbance will be given in forthcoming publications.
These results indicate a continued steady increase in the population of mountain gorillas in Bwindi Impenetrable National Park. The increase to 340 gorillas represents a 6% increase in total population size since 2002 and a 12% increase since 1997. Overall the gorilla population has been increasing at an approximately 1% annual growth rate. While research in the Virunga Volcanoes has shown that gorilla populations are capable of growing at a higher rate than this, a 1% annual growth rate over nearly a decade is still indicative of a reasonably healthy and well protected population.
Alastair McNeilage, Martha M. Robbins, Katerina Gushanski, Maryke Gray and Edwin Kagoda
We would like to stress the value of this exercise as a collaborative effort among Uganda Wildlife Authority staff, researchers, and conservationists as well as among participants from Uganda, Rwanda and the Democratic Republic of the Congo. This was the fourth census carried out in Bwindi and the Virungas in the past decade and many participants had also gained training and experience through the Ranger Based Monitoring Program. As a result, the level of knowledge, skills and motivation of all participants was very high and helped make this census a success.
The census benefited from the support and participation of the USAID PRIME West Project, Institute of Tropical Forest Conservation (ITFC), Uganda Wildlife Authority (UWA), Wildlife Conservation Society (WCS), International Gorilla Conservation Programme (IGCP), Max Planck Institute for Evolutionary Anthropology, John D. and Catherine T. MacArthur Foundation, World Wide Fund for Nature (WWF), Berggorilla & Regenwald Direkthilfe, Karisoke Research Centre, Office Rwandais de Tourisme et Parc Nationaux (ORTPN) and the Institut Congolais pour la Conservation de la Nature (ICCN).
We thank the following people who worked as team leaders: Sarah Sawyer, Nick Parker, Augustin Basabose, James Byamukama, Chrispine Safari, Moses Dhabasadha, Moses Olinga and Emmanuel Tibenda. We also thank the many participants in the census including those from Rwanda, D. R. Congo, and the communities surrounding Bwindi for their extremely hard work and enthusiasm to complete this exercise. Aggrey Rwetsiba of UWA provided valuable comments on a draft of this report.