Absence of evidence is not evidence of absence: Discovery of a large, continuous population of Pan troglodytes schweinfurthii in the Central Uele region of northern DRC

With great ape populations in decline across much of their range, it is crucial to obtain a global picture of their distribution and abundance, in order to guide conservation activities and to provide baseline data against which to monitor their trends. Although great apes are popular, charismatic species, we still do not possess a complete understanding of their distribution and abundance, which hinders their long-term protection. We highlight this problem by providing information on the distribution and abundance of the Eastern chimpanzee (Pan troglodytes schweinfurthii) in the northern Democratic Republic of the Congo (DRC), a region which has until now received little attention. We conducted a standing crop nest survey in the Bili area in 2005 and exploratory reconnaissance walks (recces) across the Bas-Uele region between 2004 and 2009. At Bili, the nest encounter rate in the remote forest was 4.84 nests per km (CI = 2.78–8.55) and in the area closer to the road it was 1.92 nests per km (CI = 1.08–3.43). In 2012, we repeated a part of the original transect survey and found that the nest encounter rate had remained stable over that period. On our recce walks across the region, we encountered chimpanzee nests in all forests surveyed, and within 13 km of the largest population centers. Our results suggest that the Central Uele landscape and neighboring regions are home to one of the largest remaining continuous populations of Eastern chimpanzees, that extends across at least 50,000 km2, likely representing thousands of individuals, but which is falling under increasing pressure from habitat destruction, mining and the bushmeat trade. This population has until now remained hidden from researchers and is not protected. Our results reflect gaps in our current understanding of ape distribution and abundance, and highlight the importance of obtaining more sound and complete data before assessing species status and making recommendations to guide conservation efforts.