Long-term inference of population size and habitat use in a socially dynamic population of wild western lowland gorillas

Abstract

Genetic estimation of population sizes has been critical for monitoring cryptic and rare species; however, population estimates do not inherently reveal the permanence or stability of the population under study. Thus, it is important to monitor not only the number of individuals in a population, but also how they are associated in groups and how those groups are distributed across the landscape. Adding to the challenge of obtaining such information with high precision for endangered and elusive species is the need for long-term collection of such data. In this study we compare sampling approaches and genotype non-invasive genetic samples to estimate the number and distribution of wild western lowland gorillas occupying a ~ 100 km 2 area in Loango National Park, Gabon, for the periods 2005-2007 and 2014-2017. Based on the number of genotyped individuals we inferred a minimum of 83 gorillas during the first and 81 gorillas during the second study period. We also obtained similar capture-recapture population size estimates for the two periods despite variance in social dynamics like group formations, group dissolutions and individual dispersal. We furthermore found area fidelity for two groups that were sampled for 10-12 years, despite variation in group membership. Our results revealed how individual movements link groups in a 'network' and show that western lowland gorilla populations can show a high degree of temporal and geographic stability concurrent with substantial social dynamics.

Western gorilla space use suggests territoriality

“…The flexible system of defending and sharing space suggested here, combined with growing evidence for long term social bonds between gorilla groups 26,53,54 and a far more dynamic social system than previously thought 46,55 , implies the presence of a complex social structure in gorillas where between-group interactions are influenced by many factors including social affiliations, kinship and the defence of space. Gorillas may therefore represent a valuable model system for investigating how elements of territoriality can occur simultaneously with affiliative between-group interactions.…”

From groups to communities in western lowland gorillas

“…Other studies have considered WLG group dynamics in the longer term, showing social units that appear, split, or disappear [42][43][44]. However, group dynamics here do not merely result from individual birth, death, or migration, but reflect an ever-changing society over a short time.…”

Lethal coalitionary attacks of chimpanzees (Pan troglodytes troglodytes) on gorillas (Gorilla gorilla gorilla) in the wild

“…Recent studies 59 were able to distinguish genetically distinct gorilla groups within the study area; when overlaid against the Rekambo chimpanzee community home range there was clear overlap with seven distinct gorilla groups (see Supplementary Fig. S1 in the Supplementary information).…”