A Comparison of Chimpanzee Density Estimators in Southern Congo

Chimpanzee populations have declined severely in recent decades because of habitat loss and illegal hunting, emphasizing the need to monitor population density to support conservation. Since 1992, the Jane Goodall Institute has led chimpanzee conservation efforts in the Republic of Congo through the Tchimpounga Chimpanzee Rehabilitation Center and, more recently, the Tchimpounga Nature Reserve (TNR), in partnership with the Ministry of Forest Economy. The abundance of central chimpanzees (Pan troglodytes troglodytes) in the country is largely unknown, with existing studies restricted to conservation projects in Nouabale-Ndoki, Odzala-Kokua, and Conkouati-Douli National Parks. In 2019, we conducted a study in the TNR to estimate chimpanzee density using Spatially Explicit Capture-Recapture (SECR) analysis based on camera trap data. We compared SECR estimates with non-spatial capture-recapture (CR) and the distance sampling standing-crop nest count method. Our results show that SECR provided the most reliable estimates in terms of precision and cost-effectiveness. Over 6 months, 116 chimpanzees were identified across 17 of 28 camera traps deployed in a 252 km² grid. SECR estimated a density of 0.87 chimpanzees/km² (95% CI: 0.58-1.30), while distance sampling produced a similar estimate of 0.81 chimpanzees/km² (95% CI: 0.43-1.53). This study is the first to apply SECR to great apes in the Republic of Congo and provides a foundation for future research and improved conservation management in the TNR.