Abrupt demographic change affects projected population size: Implications for an endangered species in a protected area

Understanding how demographic parameters change with density is essential for predicting the resilience of small populations. We use long-term, individual-based life history data from an isolated population of the Critically Endangered Northern Muriqui (Brachyteles hypoxanthus) inhabiting a 1000-ha protected forest to evaluate density-dependent demographic rates before and after an abrupt population decline. We found no effect of density on fertility or birth sex ratio, but mortality rates increased linearly with log density over the 33 years of population growth (1983-2015) and the subsequent 7 years of population decline (2016-2022). We used an age- and sex-structured logistic growth model to project population sizes to 2060. Under the 1983-2015 demographic profile, the projected size was 500 individuals, but this dropped to 200 when including the abrupt change. Although the abrupt decline coincided with the end of a 2-year drought and a yellow fever outbreak, we found no statistical effects of climate or disease on the continued population decline after 2016. However, the lower projected carrying capacity for muriquis is consistent with reduced forest productivity and increased predator pressures. These findings demonstrate the value of long-term monitoring for identifying demographic changes that affect the sustainability of wildlife populations in small protected areas.