The mountain gorillas of the Virunga Volcano Massif have been the subject of intense research and conservation efforts by the Karisoke Research Center spanning more than 40 years, but many questions remain concerning the relationship between ecological conditions and the population dynamics of the gorillas. Despite two decades of political instability, the mountain gorilla population of the Virunga Volcanoes has received intense conservation efforts such as ranger-based monitoring and veterinary interventions and increased in size over the past decades, from 250 gorillas in the mid-1980s to 480 in 2010. However, this increase has not been uniformly distributed across the Virunga Massif. The gorilla groups studied by the Karisoke Research Center now live in much larger social groups than average and at a density 2-3 times that from the 1970s. The Karisoke area, i.e. the area between Mount Visoke and Mount Karisimbi, also has a higher population density than other areas in the Greater Virunga Landscape such as the eastern volcanoes. This unequal distribution has been attributed to either differences in the intensity of anthropogenic disturbance or differences in habitat structure/composition. Plant species composition and the biomass and density of foods consumed by gorillas are heterogeneous across the Virungas. The Karisoke study area is characterized by a large proportion of open herbaceous vegetation zones where food biomass and nutritional quality (e.g. protein content) is highest.

While the recovery of the Virunga gorilla population is certainly a success story, we do not know what impact this dramatic increase in gorilla numbers/density in the Karisoke area is having on the habitat. The habitat available to the gorillas is limited because of the high human density in the surrounding areas and extensive encroachment in the past. The population is confined to an island in a sea of farmland, and it is both the growing population and the compressed habitat that may eventually push them to the carrying capacity, i.e. the natural limit of a population set by resources in a particular environment.
A collaborative project between the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany (MPI) and the Dian Fossey Gorilla Fund International (DFGFI) has been launched to provide answers to some pressing questions such as: Has food availability changed over time in the Karisoke study area? Has this increase in group sizes and population size had an impact on group dynamics (daily travel distance and energy expenditure) and female social relationships (dominance interactions and feeding competition)?
The first objective of this study is to compare food availability in the historical Karisoke area between the volcanoes of Visoke and Karisimbi from the late 1980s to the present day. This will ultimately allow us to understand if foods consumed by the gorillas are declining in abundance as the gorilla density increases. Intense vegetation sampling is being done using the same methods at the same locations as was done in 1989 by Andrew Plumptre (Wildlife Conservation Society, WCS). Plant samples are also collected and the nutritional and energetic content of the major food plant parts will be determined in the laboratory.
The second objective is to measure how extremely large gorilla group size influences food intake and feeding competition. This is important from a conservation point of view since enhanced aggression and lowered food intake may negatively influence female reproductive performance and ultimately lead to a reduction in the growth rate of the gorilla population. This study focuses on groups of different sizes, viz. Pablo with 43 individuals (11 adult females), Bwenge with 11 individuals (6 adult females), and Ntambara with 11 individuals (3 adult females). This involves detailed observations of food intake and dominance/aggressive interactions of females. We estimate how much food a gorilla female consumes in a certain time period and how many times she gets displaced by other females. One hypothesis that we are testing is that females in larger groups exhibit lower food and nutrient intake relative to females in smaller groups due to competition (more competitors feeding on the same resources/more mouths to feed). While Bwenge's group spends most of its time in the lower Hagenia woodland and in the bamboo zone and is usually within easy reach, Ntambara's group and especially Pablo's group tend to range high up in the subalpine zone at altitudes of up to 3800 m. Locating Pablo's group is often challenging and takes several hours of hiking through muddy terrain and dense shrubby vegetation. We also collect data on the distance travelled per day which we can use to make inferences about feeding competition. We would expect individuals in larger groups to travel further to obtain sufficient food.
This study also involves capacity building as several Rwandan research assistants (BSc level) have been trained in ecological methods and data collection and are an integral part of the project. Field work will continue through the end of 2010 and results should be made available once the analysis is completed. We hope that this study will be a major contribution to gorilla conservation by providing a deeper understanding of the relationship between ecology and population dynamics.

Cyril C. Grueter, Katie A. Fawcett and Martha M. Robbins