Cascading effects of social dynamics on the reproduction, survival, and population growth of mountain gorillas

Abstract

Whilst the conservation impacts of density‐dependent population effects have been well studied, the impact that socially‐driven changes (e.g. group fissions that increase group density) can have on population growth in social species have only recently come to light. An increase in group density and intergroup encounters in a subpopulation of mountain gorillas Gorilla beringei beringei was shown to have driven a more than fourfold increase in infanticide, slowing population growth. In this study, we delve deeper into the consequences of these socially‐driven changes in group density, examining how patterns of reproduction and survival have changed in this subpopulation across more than 50 years. We show that adult female survival increased with strengthening conservation efforts early in the study, then remained stable, while age at first birth remained stable across the entire study. Interbirth intervals and female reproductive success did not vary significantly across time overall. However, socially‐driven increases in group density since the mid‐2000s appear to have had a cascade of effects. In addition to the increase in infant and adult male mortality from violence during intergroup encounters, higher encounter rates, and mortality were likely responsible for an increase in female transfers, which delayed reproduction. Females faced a 7.5‐month delay when transferring once and a 1.5‐year delay when transferring twice or more between births. We show these changes correspond with a 26.63% reduction in female reproductive success in 2007 to 2018, compared to 1994 to 2006, with females taking more than 2 years longer to produce an offspring that survived infancy. These findings demonstrate that social dynamics can have consequences for population growth beyond the immediate mortality effects of violent encounters. This further highlights the critical importance of social dynamics in the conservation of social species, particularly when populations are restricted to small or fragmented regions of habitat.

Effects of environmental change on population growth: monitoring time-varying carrying capacity in free-ranging spotted hyenas

“…For long-lived species, the collection of such data necessarily entails the long-term study of individuals in the wild. Such data are difficult and costly to collect (Sheldon et al, 2022) and thus are most likely to be available for large flagship conservation or research programs (e.g., Culina et al, 2021; Morrison et al, 2022). Where possible, missing information may be drawn from other populations of the same species or closely related species with similar life-history characteristics (Coulson and Hudson, 2002; Heppell et al, 2000; Hernández-Camacho et al, 2015).…”

Anthropogenic Pressures on Gorillas: A Case of Grauer’s Gorillas in Maiko National Park, the Democratic Republic of Congo

“…Over the last 25 years, diseases and poaching have contributed to more than a 60% decline in Grauer's gorillas population [12,13]. The Grauer's gorillas are sensitive to hunting pressure even at low levels because they have an extremely low reproductive rate, with females giving first birth at a mean age of 9.99 years and continuing to reproduce at an interbirth interval mean of 4.09 years [14]. At the same time, half of the protected habitats have been severely affected by Ebola epidemics [15].…”