Assessing the Status of Great Apes in the Dja Faunal Reserve Using Distance Sampling and Camera-trapping

Central chimpanzee (Pan troglodytes troglodytes) and western lowland gorilla (Gorilla gorilla gorilla) populations are rapidly declining due to habitat loss, poaching, and disease epidemics. We estimate the abundance and distribution of both species in the 5,260-km2 Dja Faunal Reserve, a World Heritage Site in Cameroon. We compare with previous site estimates and with other great ape population estimates from the region. We also document illegal activities in the reserve. A total of 298.2 km of line transects (283) were completed using the standing-crop nest counts method, with a further 1,681.4 km of recces recording human signs. We estimated a chimpanzee nest mean decay rate of 95.4 days (SE = 4.45) and a combined great ape nest mean decay rate of 96.6 days (SE = 2.87). Gorilla population estimates of 0.38 (95% CI = 0.28–0.53) individuals/ km2 and 2,004 (95% CI = 1,447–2,774) individuals confirmed a significant decline since the 1995 survey in the north-central part of the reserve (a 57% decline for the area) and the reserve-wide survey in 2015 (a 70% decline). The population was also much lower than in most other protected areas in the region. The chimpanzee population with an estimated 0.53 (95% CI = 0.38–0.73) individuals/km2 and 2,785 (95% CI = 2,020–3,839) individuals also revealed a marked decline of 34% and 23% compared to the 1995 and 2015 surveys, respectively. Human activity occurred throughout, with the highest levels encountered in the northwest of the reserve. Occupancy estimates from four 40 camera-trap grid surveys showed great apes persisting mainly in the north-eastern part of the reserve where Cameroon's Ministry of Forestry and Wildlife (MINFOF) is considering a community support partnership agreement on sustainable access to forest resources, along with community surveillance networks. The reserve management is also increasing law-enforcement patrols across the reserve. Our findings also inform conservation strategies for great apes across the TRIDOM landscape across Cameroon, Gabon and the Republic of Congo.