Abundance and spatial distribution of the main food species for mountain gorillas in the Virunga Massif, Rwanda

Abstract

The conservation of endangered species can benefit from a clear understanding of the quantity and distribution of their main foods. The population of mountain gorillas (Gorilla beringei beringei) living in the Virunga Massif of Rwanda, Uganda, and Democratic Republic of Congo has doubled in size since the 1980s, due to success in conservation efforts in and around their habitat. However, this increase in population size along with pressures on gorilla habitat raises concerns about spatial-temporal changes in the gorillas’ food plants. This study modelled the abundance and distribution of gorilla food species in the Virunga Massif. A total of 1050 vegetation recordings were collected on five plant species that are known to be frequently consumed by gorillas in one region of the Virungas, the Karisoke area. Two types of datasets collected along vegetation zones were combined: one with plant abundance expressed with Braun-Blanquet scores; and the other with abundance expressed as biomass. Moreover, ecological characteristics of locations where these species occur were extracted from satellite imagery. Analysis of variance and linear regression models were used to examine relationships between food species abundances and predictor variables. Subsequently, maps for the food species were created using boosted regression trees (BRTs). The abundance of species differed across vegetation zones, and the differences were statistically significant among vegetation zones with enough species observations. The accuracy of the BRTs indicated greater than random predictions (AUC > 0.65). This study shows the suitable areas for these gorilla food species and relevant ecological variables determining their distribution. The results provide insights into habitat occupancy by mountain gorillas, and help to design a baseline for monitoring changes in the abundance of gorilla food species under changing climate and anthropogenic pressure.