Population dynamics of the Bwindi mountain gorillas

Mountain gorillas are critically endangered, with just a few hundred animals remaining in each of two populations: in Bwindi Impenetrable National Park in south-western Uganda and the nearby Virunga Volcanoes on the borders of Uganda, Rwanda and the Democratic Republic of Congo. While the life-history and population dynamics of the Virunga gorillas have been studied extensively, comparable information from Bwindi has not been reported. Such studies are difficult to conduct because gorillas are long-lived, have delayed reproduction, and monitoring known individuals requires habituation of social groups. Bwindi has experienced lower levels of human disturbance than the Virungas, yet its gorilla population has shown little or no growth over the past 20 years, while a subpopulation of study groups in the Virungas have grown by 3–4% per year. Here we show that the lower growth rate at Bwindi may arise mainly from lower fertility than the Virunga study groups, rather than higher mortality. This difference may indicate that the more frugivorous Bwindi gorillas have a slower life-history, or that they are closer to the carrying capacity of their habitat. The study groups at Bwindi had a higher growth rate than the broader population, possibly because they receive veterinary care and better protection from poachers, but further analysis is necessary to understand the complex interactions among human disturbance, ecology, and the gorillas’ population dynamics. Meanwhile, efforts to increase the Bwindi population should place emphasis on reducing human disturbances, improving our understanding of the impact of habitat quality, and ensuring that the gorillas can expand their home ranges into under-utilized areas of the park.