Nest counts reveal a stable chimpanzee population in Sapo National Park, Liberia

Surveying and monitoring primate populations is key for conservation decision-making. The western chimpanzee (Pan troglodytes verus) has recently been up-listed to “Critically Endangered” on the IUCN Red List. Population status and trends of the remaining populations of chimpanzees almost always rely on counts of sleeping nests along line transects. In tropical forests, permanent line transects can facilitate survey work but may also be avoided by animals, complicating data analysis and possibly resulting in erroneous estimates. We conducted surveys in Liberia’s Sapo National Park and its buffer zone along clear-cut (‘permanent’) and uncut (‘temporary’) line transects to estimate chimpanzee abundance and compare chimpanzee densities inside and outside the park. We recorded all indirect signs of chimpanzee presence and human activities on 16 permanent transects in Sapo and six temporary transects in the surrounding buffer zone. Our analysis revealed a population density of 0.83 individuals/km2 across the park and its buffer zone. Compared to previous estimates, this suggests a relatively stable population over the past eight years of roughly 1,000 chimpanzees. It appears that poaching and habitat encroachment, which has persisted in the area since the first chimpanzee survey in 2009, did not have a significant negative affect on population growth over time. Possible reasons for this include local taboos against killing chimpanzees, poachers not targeting chimpanzees directly, and the chimpanzees’ behavioral flexibility. Nest encounter rate inside the park was considerably lower than in the buffer zone and significantly fewer nests were found on or near permanent transect lines (19%) than temporary transects (44%), indicating an avoidance effect of chimpanzees towards the former. The study also shows that permanent transects were frequently used by illegal miners and poachers, providing direct evidence of the impact that humans have on chimpanzee habitat use and the potential negative implications of cutting permanent transect lines on wildlife in the park. We call for an increase in conservation and law enforcement efforts to protect depletion of wildlife resources from the park. We also stress the need for legal enforcement of the park’s buffer zone, in order to effectively protect wildlife against poaching, habitat destruction and disease, to ensure the continued survival of chimpanzees in Liberia’s oldest national park.