Population dynamics of western gorillas at Mbeli Bai

Abstract

Long-term studies of population dynamics can provide insights into life history theory, population ecology, socioecology, conservation biology and wildlife management. Here we examine 25 years of population dynamics of western gorillas at Mbeli Bai, a swampy forest clearing in Nouabalé-Ndoki National Park, the Republic of Congo. The Mbeli population more than doubled from 101 to 226 gorillas during the study. After adjusting for a net influx of gorillas into the study population, the increase represents an inherent growth rate of 0.7% per year, with 95% confidence limits between -0.7% and 2.6%. The influx of gorillas mainly involved immigration of individuals into existing study groups (social dispersal), but it also included the appearance of a few previously unknown groups (locational dispersal). The average group size did not change significantly during the study, which is consistent with the possibility that western gorillas face socioecological constraints on group size, even when the population is increasing. We found no significant evidence of density dependence on female reproductive success or male mating competition. The distribution of gorillas among age/sex categories also did not change significantly, which suggests that the population had a stable age structure. Our results provide evidence of population stability or growth for some western gorillas (albeit within a small area). The results highlight the value of law enforcement, long-term monitoring, and protected areas; but they do not diminish the importance of improving conservation for this critically endangered species.

Socioecological factors influencing intraspecific variation in ranging dynamics of western lowland gorillas (Gorilla gorilla gorilla) in Ndoki Forest

“…In the Virunga mountain gorilla population, increased group density has led to less exclusive spaces within groups' home ranges and increased rates of aggression during IGEs, to the point of impacting infant mortality and the overall population growth rate (Caillaud et al, 2014, 2020). While the overall population density for the Ndoki region is relatively high (Morgan et al, 2006, 2019) and the population growth rate for the region increased in the last 25 years (Robbins, Manguette, et al, 2022), the group density is lower than that of the Virunga mountain gorilla population (Bradley et al, 2004; Caillaud et al, 2020). This could lead one to suggest that density effects on intergroup space use on gorillas in Ndoki are likely less pronounced than those documented in Virunga.…”