Guns, germs, and trees determine density and distribution of gorillas and chimpanzees in Western Equatorial Africa

We present a range-wide assessment of sympatric western lowland gorillas Gorilla gorilla gorillaand central chimpanzeesPan troglodytes troglodytesusing the largest survey data set ever assembled for these taxa: 59 sites in five countries surveyed between 2003 and 2013, totaling 61,000 person-days of fieldwork. We used spatial modeling to investigate major drivers of great ape distribution and population trends. We predicted density across each taxon’s geographic range, allowing us to estimate overall abundance: 361,900 gorillasand 128,700 chimpanzees in Western Equatorial Africa—substantially higher than previous estimates. These two subspecies represent close to 99% of all gorillasand one-third of all chimpanzees. Annual population declineof gorillaswas estimated at 2.7%, maintaining them as Critically Endangeredon the International Union for Conservation of Nature and Natural Resources ( IUCN) Red List. We quantified the threats to each taxon, of which the three greatest were poaching, disease, and habitat degradation. Gorillasand chimpanzees are found at higher densities where forest is intact, wildlife laws are enforced, human influence is low, and disease impacts have been low. Strategic use of the results of these analyses could conserve the majority of gorillasand chimpanzees. With around 80% of both subspecies occurring outside protected areas, their conservation requires reinforcement of anti- poachingefforts both inside and outside protected areas (particularly where habitat quality is high and human impact is low), diligent disease control measures (including training, advocacy, and research into Ebola virusdisease), and the preservation of high-quality habitat through integrated land-use planning and implementation of best practices by the extractive and agricultural industries.