A massive decade-long study of Western Equatorial Africa's gorillas and chimpanzees has uncovered both good news and bad about our nearest relatives. The good news: there are one third more western lowland gorillas and one tenth more central chimpanzees than previously thought. 
 
 The bad news: the vast majority of these great apes (80 percent) exist outside of protected areas, and gorilla populations are declining by 2.7 percent annually. 
 
 The WCS-led study titled "Guns, germs and trees determine density and distribution of gorillas and chimpanzees in Western Equatorial Africa" appears in the latest edition of the journal Science Advances. 
 
 The newly published paper was written by 54 co-authors from several organizations and government agencies, including WCS (Wildlife Conservation Society), WWF (World Wide Fund for Nature), Max Planck Institute for Evolutionary Anthropology, Jane Goodall Institute, Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) -- Monitoring the Illegal Killing of Elephants (MIKE), Lincoln Park Zoo, and the Universities of Stirling and Washington, and involved the protected area authorities of five countries. Researchers collected field data during foot surveys carried out over a 10-year period across the range of both western lowland gorillas (Gorilla gorilla gorilla) and central chimpanzees (Pan troglodytes troglodytes) -- surveying an area of 192,000 square kilometers (72,000 square miles -- equivalent to the size of the state of Washington) and including some of the most remote forests on the African continent. 
 
 The authors of the study report an estimated abundance of over 360,000 gorillas and nearly 130,000 chimpanzees across the combined ranges of both subspecies, both of which were higher than previously believed. The gorilla estimate is approximately one-third higher and the chimpanzee estimate is about one-tenth higher. These revised numbers come largely from refinements to the survey methodology, new data from areas not previously included in range-wide estimates, as well as predictions of numbers in the areas between survey sites. 
 
 "It's great news that the forests of Western Equatorial Africa still contain hundreds of thousands of gorillas and chimpanzees, but we're also concerned that so many of these primates are outside of protected areas and vulnerable to poachers, disease, and habitat degradation and loss," said lead author Samantha Strindberg of WCS. "These findings can help inform national and regional management strategies that safeguard the remaining habitat, increase anti-poaching efforts, and curtail the effects of development on great apes and other wildlife." 
 
 Although the majority of great apes were found outside of protected areas, they were still in large forested landscapes close to or bordering existing national parks and reserves and away from centers of human activity. This suggests that protecting large and intact forested areas, with protected areas at their core, is critical to conserving gorillas and chimpanzees in this region. 
 
 The data analysis also revealed a 2.7 percent annual decline in gorilla numbers, a finding that supports the continued status of the species as "Critically Endangered" on the IUCN Red List of Threatened Species. Chimpanzees are listed as "Endangered." 
 
 The combined field time spent by researchers collecting data for the study totaled approximately 61,000 days (or 167 person-years) of time. Researchers walked more than 8,700 kilometers (5,400 miles) -- a distance longer than the north-south axis of the African continent, or from New York to London -- while collecting data on great ape nests that was used to generate population estimates and trends. 
 
 Said co-author Dave Morgan of the Lincoln Park Zoo and Goualougo Triangle Ape Project: "The boots on the ground research teams and partnerships are crucial to the success of these programs and the conservation of gorillas and chimpanzees. These long-term studies enable us to make informed recommendations regarding protected lands and management to help great apes." 
 
 The main factors responsible for the decline of gorillas and chimpanzees are illegal hunting, habitat degradation, and disease. At the same time, it was clear that where wildlife guards were present, above all in protected areas with intact forests, both gorillas and chimpanzees can thrive. 
 
 David Greer of WWF said: "All great apes, whether in Africa or Asia, are threatened by poaching, especially for the bushmeat trade. Our study found that apes could live in safety, and thus in higher numbers, at guarded sites than if there was no protection." 
 
 Said Fiona Maisels of WCS: "Our study underscores the huge importance of intact forests to gorillas and chimpanzees, and of preventing illegal felling of good quality forests." 
 
 Other conservation recommendations made by the authors include land-use planning at national scales to keep ecologically-harmful activities, such as agriculture and new road construction, away from intact forests and the protected areas that serve as important gorilla and chimpanzee refuges. 
 
 Another priority is the implementation of careful logging practices in existing logging concessions that follow Forest Stewardship Council (FSC) Standards for reducing impacts on wildlife and habitats. These standards require that access to forests is controlled, old logging roads are effectively decommissioned and effective patrol systems are put in place to prevent illegal hunting. Ensuring strong implementation is critical. 
 
 An additional threat to great apes -- as well as human health -- is the Ebola virus disease. Continued research into developing a vaccine and the means to deliver it are priorities, as are educational efforts on how to avoid spreading the disease and transmission between humans and great apes. 
 
 Of all the 14 living great ape taxa, western lowland gorillas and central chimpanzees have the largest remaining populations. This is certainly good news. However, their future preservation cannot be taken for granted, given the fact that their dependence on suitable habitat collides with local to global demand for natural resources from their habitat, particularly outside of protected areas, where most of them occur. 
 
 Said Hjalmar Kühl of the Max Planck Institute for Evolutionary Anthropology: "Protecting our gorillas and chimpanzees will therefore require a major increase in political will at all levels -- national, regional, and global. Financial commitments from governments, international agencies for endangered species conservation and the private sector, are also critical for conserving our closest relatives and their habitats." 
 
 Liz Williamson from the University of Stirling and the IUCN Red List Authority Coordinator for great apes said: "A combination of responsible industrial practices, conservation policies, and a network of well-managed parks and corridors would provide wildlife managers with a winning formula for conserving great apes in Central Africa. Our study has revealed that it is not too late to secure a future for gorillas and chimpanzees." ||||| There are far more gorillas left in the world than previously thought, according to a landmark new survey, with numbers as much as double earlier estimates. 
 
 However, their populations are continuing to fall fast, down 20% in just eight years, leaving them critically endangered. Furthermore, 80% of the remaining gorilla troops do not live in protected areas, leaving them vulnerable to the threats the researchers summarise as “guns, germs and [felled] trees”. 
 
 
 
 The decade-long survey in western equatorial Africa involved almost 9,000km of foot patrols and used the nests that gorillas make every night to assess the population. The scientists covered the entire range of the western lowland gorilla, which accounts for 99% of all living gorillas, now thought to number around 360,000 animals. 
 
 Gorillas are vital for the health of the entire forest, the scientists said, because they spread large seeds and their loss is disastrous in the long term. The larger-than-expected population gives hope for gorilla survival, they said, if poaching and forest destruction for palm oil can be halted. 
 
 “The population could be double,” said Prof Fiona Maisels, from the Wildlife Conservation Society and the one of 50-strong international experts who conducted the survey. “But that is not the big story. Just because there are rather a lot of them does not mean they are not very, very vulnerable.” Gorillas breed very slowly, with females taking 11-12 years to reach maturity and only giving birth every four years. “It takes a very long time to build populations back up,” Maisels said. 
 
 The survey, published in the journal Science Advances, covered about a quarter of the gorilla’s total range - an area the size of France - which is naturally bounded by the Atlantic Ocean and the Congo and Ubangi rivers. They then used the nest counts and data on important factors including the presence of people, roads and intact forest to produce a sophisticated statistical model. 
 
 This allowed them to fill in the gaps between the surveyed areas, unlike previous research. The resulting total was 361,900 gorillas, far higher than the earlier estimates of 150,000 to 250,000. The researchers also assessed the population of the central chimpanzee sub-species, which lives in the same range and represents about a third of all chimps. Their population was 10-80% higher than thought. 
 
 Facebook Twitter Pinterest The western lowland gorilla is by far the most populous subspecies. Photograph: Forrest Hogg 
 
 It is illegal to kill or capture any great ape in all the nations where they live, but poaching to supply bushmeat to fast-growing cities is rife. Outbreaks of Ebola virus have also decimated populations in recent decades, with “catastrophic” impact on populations, the scientists said. “If Ebola wipes through an area, they all go – 90% of them in six months,” said Maisels. 
 
 The most important single factor in determining the survival of gorillas was the presence of park guards to deter poachers, the analysis found. Dominant males are easy to shoot because they attempt to stand up to hunters, trying to protect their group. The males replacing them often kill the babies they have not fathered, compounding the losses. 
 
 In contrast, chimpanzees, which spend much more time in trees, usually flee when hunters approach. “They are not as heroic, if you like,” said Maisels. “Chimps are cleverer and sneak away very quietly.” Local cultural taboos against eating chimp meat also helped protect them in a few locations. The chimp population did not show a significant decline in the study period from 2008 to 2013. 
 
 “The advice we would like to offer is keep up the anti-poaching effort,” said Maisels. “That gets rid of the immediate worry. But the future worry is the removal of forest for agricultural development. If a big area of forest is turned into an oil palm or rubber plantation, it is not habitat for anything anymore, except weeds.” 
 
 Survival of Africa's great apes requires palm industry support, says report Read more 
 
 Only 20% of the gorilla’s range is in protected areas and the researchers said the vast majority of unprotected forests are being opened up to logging and deforestation: “It is vital that we step up our efforts to conserve great apes.” 
 
 Facebook Twitter Pinterest The study produced a total population estimate for the central chimpanzee of nearly 130,000 individuals. Photograph: Emma Stokes/WCS 
 
 The situation varies widely by country, said Maisel, with Gabon having good national policies while the war-torn Central African Republic is struggling to govern at all. Congo is a hotspot for gorillas, she said, and could play a vital role in conservation. 
 
 Liz Williamson, great apes coordinator for the International Union for Conservation of Nature’s red list, said a combination of responsible development and a network of well-managed parks would provide a winning formula: “Our study has revealed that it is not too late to secure a future for gorillas.” 
 
 The western lowland gorilla is by far the most populous subspecies. There are about 3,600 eastern lowland or Grauer’s gorillas – a fall of 75% in last 20 years - 800 mountain gorillas and just 200-300 Cross River gorillas. ||||| We present a range-wide assessment of sympatric western lowland gorillas Gorilla gorilla gorilla and central chimpanzees Pan troglodytes troglodytes using 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 gorillas and 128,700 chimpanzees in Western Equatorial Africa—substantially higher than previous estimates. These two subspecies represent close to 99% of all gorillas and one-third of all chimpanzees. Annual population decline of gorillas was estimated at 2.7%, maintaining them as Critically Endangered on 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. Gorillas and 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 gorillas and chimpanzees. With around 80% of both subspecies occurring outside protected areas, their conservation requires reinforcement of anti-poaching efforts 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 virus disease), and the preservation of high-quality habitat through integrated land-use planning and implementation of best practices by the extractive and agricultural industries. 
 
 Our extensive data set, collated through the collaborative efforts of numerous organizations, comprises great ape nest counts from 59 sites in five countries surveyed over 11 years. A preliminary examination of these data provided the science to underpin a regional conservation planning process. The resulting International Union for Conservation of Nature (IUCN) action plan ( 1 ) identified several large landscapes in WEA that are key for the conservation of great apes. From the outset, this planning process involved decision makers and other government representatives in producing a set of detailed, budgeted, time-specific actions to be implemented at each site in each conservation landscape. This paper presents our analysis of the region-wide data set and discusses the implications of our results for conservation planning and the maintenance of intact forests in WEA. 
 
 Wildlife monitoring provides empirical data on population trends, distribution, and the key ecological and human factors affecting them. These data are used to inform conservation actions and evaluate their success, for both effective site management and to identify the best sites to designate as new PAs. Large-mammal population monitoring in African forests involves conducting extensive line-transect and reconnaissance surveys on foot ( 6 , 7 ), covering a wide range of habitats under differing degrees of human impact. Data on large mammal sightings and signs are collected, including night nests built by great apes and the dung of elephants and ungulates. Up to this point, the nest data have been used to estimate the abundance of great apes at local scales, highlighting some large tracts of forest where gorillas and chimpanzees are abundant. 
 
 WEA’s protected areas (PAs) cover over 650,000 km 2 or 12.2% of the terrestrial surface area ( 5 ). This is below the 2020 Aichi Target 11 of at least 17% per country ( www.cbd.int/sp/targets/ ). Instead, large conservation landscapes with PAs at their core, usually surrounded by logging concessions, swamp forests, or community lands, maximize the area of habitat available to wildlife ( 1 , 5 ). 
 
 All great apes are protected by national laws and international conventions; thus, it is illegal to kill, capture, or trade in live individuals or their body parts wherever they occur. Despite this legal protection, the combination of poaching with Ebola virus disease (EVD) has been catastrophic for gorillas and chimpanzees ( 2 ). Another emerging threat is industrial-scale forest conversion for agricultural plantations, especially oil palm plantations—a type of land use inimical to great apes ( 3 ) and other forest-dwelling wildlife. Massive development corridors planned for the continent are proceeding apace, fragmenting and making accessible large areas of previously remote forest, which will, in turn, draw many more people into these sensitive ecosystems ( 4 ). 
 
 To conserve great apes effectively, we need to have reliable estimates of their distribution, density, and abundance, and also to understand the factors driving their population trends in space and time. The objective of this study was to undertake a range-wide, data-driven assessment of these parameters of population status for sympatric African great apes, western lowland gorillas Gorilla gorilla gorilla, and central chimpanzees Pan troglodytes troglodytes. The geographic range of central chimpanzees overlaps with that of western lowland gorillas by 97% ( 1 ). Western lowland gorillas occur in six countries of Western Equatorial Africa (WEA): Angola (Cabinda enclave), Cameroon, Central African Republic (CAR), mainland Equatorial Guinea, Gabon, and Republic of Congo (Congo). Chimpanzees (P. troglodytes) inhabit 21 countries and occupy a much wider ecological niche than the gorillas (G. gorilla and G. beringei), which are found in 10 countries. Henceforth, “gorilla” refers to the western lowland gorilla subspecies and “chimpanzee” refers to the central chimpanzee subspecies. 
 
 The presence or absence of guards, the degree of human influence, and cultural taboos against eating chimpanzees were critical determinants of chimpanzee nest density. First, to isolate the effects of human influence, we compared an area in Congo with low human influence to a similar area in CAR with low-to-medium human influence under the following set of conditions: The areas were equivalent in terms of Guards (present), Slope (flat), Elevation (low), consumption (humans eat chimpanzees), and Ebola (none documented). The model predicted that chimpanzee nest density would be 22% lower at the CAR site. Second, the positive effect of guards became clearer with a similar comparison between the same low human influence area in Congo with guards, and an area in Equatorial Guinea with medium human influence without guards. This time, predicted chimpanzee nest density at the Equatorial Guinea site was 38% lower than at the Congo site. Third, if we compare the same area in Congo with an area along the Gabon coast where chimpanzees are not eaten, then the predicted chimpanzee nest density in the Gabon site is higher than in the Congo site: 88% at low and 61% at medium-to-high human influence. 
 
 Second, to examine the influence of Ebola on predicted gorilla density, we compared an area with guards 0.5 km from a road in Congo to an area in Gabon under the same set of conditions, except that an EVD outbreak had occurred. The gorilla nest density predicted was 75% lower in the areas affected by Ebola, which comprise about 8% of the taxon’s total range. 
 
 Presence or absence of guards, proximity to roads, and Ebola were critical determinants of gorilla nest density. First, to isolate the effect of roads and examine the interaction between roads and guards, we contrasted scenarios in Congo at two distances from a road where guards were either absent or present under the following set of conditions: low Elevation, relatively low HPD (~1.5 people/km 2 ), gorillas are eaten, and no Ebola. When there were no guards, gorilla nest density was predicted to be 55% higher far from a road (25 km) than near to a road (0.5 km). The effect of protection was clear: In areas without guards, even far from a road (25 km), gorilla nest density was predicted to be 52% lower than nest density in areas with guards, but only 0.5 km from a road. 
 
 In terms of covariates relevant to conservation management, guard presence or absence had the greatest predictive power on gorilla abundance, whereas the taboo against eating chimpanzees had the most explanatory power in the chimpanzee model ( Fig. 4 ). In addition, both Canopy height and Ebola were important covariates in the top-ranked models for both taxa. The explanatory power of Canopy height was similar to that of Distance to road and HII for the gorilla and chimpanzee models, respectively, with less variability in the data explained by HPD for the former model. The explanatory power of Elevation and consumption of gorillas for the gorilla model, and of Elevation and Slope for the chimpanzee model, were much lower. Geographic location (Latitude and Longitude) acted as a proxy for other explanatory variables of great ape distribution that were not available at the scale of this analysis. Part of the variance in the data for both taxa was significantly explained by geographic location. 
 
 For both gorillas and chimpanzees, the models selected included the variables Guards, Canopy height, Elevation, Eat gorilla/Eat chimpanzee, Ebola, and geographic location (Latitude, Longitude). Country was always a better predictor than Transparency. For gorillas, human influence was best captured by Distance to road and HPD ( Fig. 3 ), whereas for chimpanzees, it was best captured by HII. The final chimpanzee model selected included Slope and an interaction term between HII and Country; a single smooth function was fitted for Cameroon, Congo, and Gabon combined, because the relationship was similar across these three countries and the combination improved the AIC value; separate smooth functions were fitted for CAR and Equatorial Guinea ( Fig. 4 ). All of these variables, detailed in Table 2 , influenced great ape distribution and densities in the way predicted a priori. 
 
 For each taxon, the final model selected (see table S1) had the largest Akaike’s information criterion (AIC) weight in its set ( 8 ). The final model for gorillas had an AIC weight of just over 0.60 and was 1.7 times more likely to be the best model compared to the next-best model in terms of AIC weight ranking. For chimpanzees, the final model had an AIC weight of 0.59 and was 3.1 times more likely to be the best model. On the basis of predictions from these models, great ape abundance estimates for each of the sites surveyed closely resembled those generated by design-based analyses of the 82 surveys at 59 sites. 
 
 Natural variables. Habitat was an important predictor of both gorilla and chimpanzee densities. Intact forest landscapes (IFLs) are nested in hinterland forests (see Materials and Methods for definitions); just over 30% of the combined range of gorillas and chimpanzees is classified as IFLs and 48% as hinterland forest. More than half (52%) of all gorillas occurred in IFLs, and 67% in hinterland forests. Gorillas were found at higher densities in areas where the canopy height was >25 m (table S1), but were absent or occurred at low densities where canopy height was <15 m. Chimpanzee densities were strongly correlated with canopy height, and no nests were recorded in forests with canopy heights <15 m. Indeed, 41% of all chimpanzees were in IFLs and 60% in hinterland forests. Conversely, chimpanzees occurred at low densities in swamp forests, where canopy height tends to be lower than in terra firma forests. Gorilla and chimpanzee densities were significantly higher in areas not affected by EVD in the previous 20 years; they were only marginally influenced by Slope and Elevation. 
 
 Gorilla and chimpanzee densities were significantly higher in areas where local communities have taboos against eating their meat. At the time of survey, most people at 10% of the 59 sites did not eat chimpanzees, and at 5% of sites, most people did not eat gorillas. 
 
 Gorilla density declined as human population density increased ( Fig. 3C ), except in Equatorial Guinea, where we found no significant relationship. In Cameroon, Congo, and Gabon, chimpanzees tended to occur in areas with low or moderate human influence, and their density dropped rapidly after a medium degree of human influence was reached ( Fig. 4A ). In CAR ( Fig. 4B ) and Equatorial Guinea ( Fig. 4C ), there was an almost linear decline in chimpanzee density with increasing human influence, although this relationship was not significant for Equatorial Guinea (where there are no areas of low human influence). 
 
 The y axis (nest density) is on the scale of the linear predictor. Estimates (solid lines) with CIs (dashed lines) are shown for ( A ) proximity to roads without guards, ( B ) proximity to roads if guards are present, and ( C ) human population density. 
 
 Human-related variables. At the time of survey, guards were absent from 37% of the 59 sites. Over half of the surveys (51%) were carried out in PAs; the rest were in logging concessions or in relatively undisturbed areas with few human settlements, such as the vast swamps of northern Congo. Densities of both subspecies were significantly higher in areas where guards were present compared with those where they were not. In areas without guards, gorilla density declined almost linearly as proximity to roads increased ( Fig. 3A ), but if guards were present, gorilla density gradually rose with increasing proximity to roads ( Fig. 3B ). Chimpanzee density declined sharply with increased proximity to roads where there were no guards, but if guards were present, Distance to road had no significant influence (fig. S2). 
 
 Several human-related variables [Guards, Human Influence Index (HII), Distance to road, Human population density (HPD), Eat gorilla, and Eat chimpanzee] and two natural variables (Canopy height and Ebola) were important predictors of great ape density and distribution (fig. S1 and table S1). For gorillas, the most important predictors—in descending order—were Guards, Ebola, Distance to road, Canopy height, Time (year of survey), HPD, Eat gorilla, and Elevation. For chimpanzees, the predictors—again in descending order of importance—were Eat chimpanzee, Canopy height, Ebola, HII, Guards, Slope, and Elevation (see Materials and Methods for definitions of variables). 
 
 Chimpanzees have a different pattern of distribution. They occur at low densities in swamp forests and at their highest densities along the Monts de Cristal-Monts du Chaillu mountain range that runs northwest to southeast from Equatorial Guinea through central Gabon to southern Congo. They are relatively abundant along the southern coast of Gabon and contiguous coast of Congo. A third key area is the forests of south-central Cameroon and northern Congo. Only about 19.3% of chimpanzees in WEA are found in PAs, but of these, 16.3% are in PAs with no guards. A further 14.1% of chimpanzees inhabit FSC-certified concessions and are protected by guards, and about 3.7% are in guarded, noncertified concessions. Therefore, the majority (65.8%) of chimpanzees are not protected by guards. 
 
 Gorillas occur at their highest densities in PAs, FSC-certified logging concessions, and swamp forests, and their stronghold is northern Congo. Other areas of importance are southeast Cameroon, north and central Gabon, and parts of southern Congo. Only 22.6% of gorillas in WEA live in PAs, but 20.3% of these are not guarded. A further 21.4% are in FSC-certified logging concessions and another 1.8% in noncertified concessions protected by guards. Overall, 58.7% of gorillas in WEA are not protected by guards. 
 
 Around 13 and 51% of the combined ranges of both subspecies are in PAs or timber concessions, respectively. Sites staffed with wildlife guards—most PAs and all Forest Stewardship Council (FSC)–certified logging concessions—cover only 12.7 and 7.4%, respectively, of the great apes’ range. An additional 3.1% of their geographic range is noncertified concessions protected by guards. Thus, a total of 23.2% of the gorilla and chimpanzee range is protected by guards. The remaining 76.8% is unprotected, leaving the majority of great apes highly vulnerable to poaching. 
 
 Gorilla populations in WEA declined by 19.4% between 2005 and 2013, with an average annual loss of ca. 2.7%. Nationally, with the exception of Angola (where no survey data were collected), estimated gorilla losses ranged from 18.5 to 21.7% ( Table 1 ). We did not detect a statistically significant change in chimpanzee numbers over the same period. 
 
 Most of these great apes reside in two countries—Congo and Gabon ( Fig. 2 and Table 1 ). Congo harbors 60% of all gorillas and 43% of all chimpanzees in 32.7% of the total forest domain in WEA, whereas Gabon, with the highest percentage of the region’s forests (34.4%), hosts 27% of the gorilla population and 34% of the chimpanzees. Both taxa occur at particularly high densities in northern Congo and southern Cameroon. 
 
 Regional and country-specific abundance estimates for western lowland gorillas and central chimpanzees (with 95% CIs), plus areas of forest, PAs, and FSC-certified concessions. Also shown by country: percentage of regional gorilla and chimpanzee populations; percentage loss of gorillas 2005–2013; area of forest as given by Verhegghen et al. ( 53 ) apart from Angola, for which Global Forest Watch data ( http://www.globalforestwatch.org/country/AGO/4 ) were used; and percentage of total regional forest cover per country. Information on PAs and FSC certification was downloaded from the WRI Interactive Congo Basin Forest Atlases ( http://www.wri.org/our-work/project/congo-basin-forest-atlases ), apart from Angola (Cabinda), for which we used the legal document for the creation of Mayombe National Park ( http://extwprlegs1.fao.org/docs/pdf/ang118430.pdf ). For Cameroon, the area south of the Sanaga River only is included, which is within the range of western lowland gorillas and central chimpanzees. 
 
 We estimate that, in 2013, gorillas numbered 361,919 weaned individuals [95% confidence interval (CI), 302,973 to 460,093], and chimpanzees numbered 128,760 weaned individuals (95% CI, 114,208 to 317,039). We used the top-ranked models to map gorilla and chimpanzee distribution as density surfaces ( Fig. 1 ) and to estimate range-wide and country-specific population sizes ( Table 1 ). 
 
 DISCUSSION 
 
 Predicted abundance and temporal trends The larger-than-previous estimates for both gorillas [~361,900 compared to 150,000 to 250,000 individuals (2)] and chimpanzees [~128,700, updating an estimate of 70,000 to 117,000 (2)] result from the fact that our predictions cover the entire geographic ranges of these taxa—not only the areas actually surveyed but also the land in between. In the past, no corrections were made for animals in areas that had not been surveyed. Together, the other gorilla subspecies are estimated to number fewer than 5000 individuals, and they inhabit much smaller geographic ranges than the western lowland gorilla (2). Although our analysis shows that there are many more gorillas remaining in WEA than previously thought, they are declining precipitously, by 2.7% per year over the time frame examined (2005 to 2013). At this rate, the reduction in the gorilla population would be expected to exceed 80% over three generations (2). Therefore, western lowland gorillas are classified as “Critically Endangered” under the IUCN Red List of Threatened Species criteria. The number of central chimpanzees is also somewhat higher than previous estimates, and using population estimates taken from the Red List (2), they comprise roughly one-third of the species’ total. Although it is unlikely that the chimpanzee population remained stable, any change in their numbers between 2005 and 2013 was not statistically significant. Consequently, the subspecies remains “Endangered” on the Red List. Monitoring great apes over large geographic scales is currently limited to surveys of the nests they build, rather than direct observations of the animals. Using estimates of nest density to derive estimates of great ape density, distribution, and abundance has the potential to introduce inaccuracies from the different analysis components. The first is the potentially incorrect attribution of a nest to species (gorilla or chimpanzee) based on its characteristics. However, the models we have used for this attribution have shown a high degree of accuracy (see Materials and Methods for details). Second, the conversion rates used to obtain great ape density from nest density, namely, nest creation and nest decay, have the potential to introduce bias. We used a nest-building rate obtained through a long-term study (9). The model used to obtain spatially explicit nest decay rates came from a long-term study that accounted for habitat and nest characteristics, as well as variation in rainfall (10). Models themselves are also prone to biases due to mis-specification, as well as inaccurate or missing explanatory variables. We used the best explanatory variable layers available at the regional scale. In addition, we had the added advantage of being able to validate our model-based results using the design-based estimates for each survey, which do not rely on fitting an unbiased model. Thus, even with these caveats, we believe that our analysis provides reliable information for long-term conservation planning for these great ape subspecies. Although poaching, disease, and forest loss and degradation are known to be the principal threats to all great ape taxa, this regional analysis demonstrates for the first time the quantitative effects on gorilla and chimpanzee densities in a spatially explicit manner. Our study found that 77.4% of gorillas and 80.7% of chimpanzees in this region live outside the PA network. Clearly, PAs alone are not enough for their long-term survival; additional areas need to be targeted for gorilla and chimpanzee conservation, and the actions necessary to assure their protection must be implemented. We found that the densities of both subspecies are higher where guards are present in suitable high-canopy habitat, and where human influence is low in terms of the density of human settlements, human population size, infrastructure development, industrial agriculture, and extraction activities. 
 
 Impacts of human population density, access, poaching, and presence of guards Over the past 60 years, the proportion of people living in cities in the WEA region has trebled, from around 20% in 1960 to around 60% today (in Gabon, 87% of people live in urban areas) (11). Many urban human populations, living far from wildlife, attribute cultural value to bushmeat, appreciate its taste, and are prepared to pay a premium for it. As the numbers of people residing in the towns and cities increase, demand to supply the commercial bushmeat trade—which includes ape meat—grows, increasing the pressure exerted on wildlife (12). In addition, the synergy of the negative impacts of natural resource exploitation and roads providing easy access to forest for hunters has been well documented (13), and when both are unregulated, great ape populations decline rapidly (14). We have shown that great apes benefit from the presence of guards, with the positive impact for gorillas being greater than for chimpanzees. This difference may be explained by aspects of their socioecology, as synthesized by Williamson et al. (15): Gorillas live in stable groups, averaging 10 individuals with a single adult male. Chimpanzees live in “fission-fusion” communities, which do not move as a group, but are most often found in small parties or mother-infant pairs that can disperse quickly. Because of their larger social units, group cohesion, and being more terrestrial than chimpanzees, gorillas are both more vulnerable and easier to kill in large numbers with guns. Similarly, Grauer’s gorillas are easier to track than eastern chimpanzees, usually traveling shorter distances in a day and leaving denser trails (16). Chimpanzees in heavily hunted areas are unlikely to be aggressive toward human adversaries, whereas adult male gorillas put themselves in danger by defending their females and offspring. Gorilla group stability is disrupted if the dominant male is killed by a poacher, and if a group subsequently disperses, then this will likely lead to further deaths through infanticide by an unrelated adult male (15). In addition, in areas without guards, great ape densities are positively correlated with increasing distance from roads, as hunters’ efforts and travel costs mount (Fig. 3A and fig. S2A). However, where guards are present, gorilla density is high at roadsides, reflecting the gorillas’ liking for the abundant herbs that grow in light gaps created by removal or disturbance of the canopy along roads and for roadside timber storage. Gorilla density remains high up to 25 to 45 km from roads where there are guards, and then decreases (Fig. 3B). Beyond 45 km, only six of our sites were guarded, and an EVD outbreak had occurred in three of those (the maximum distance from a road in this data set was 80 km). There is no significant change in chimpanzee density with increasing proximity to roads when guards are present (fig. S2B), which could be because chimpanzees are able to move stealthily and elude poachers. Gorillas are less abundant in areas with higher human population densities, again pointing to their vulnerability to poaching. Chimpanzee density is negatively affected by human influence, but the exact nature of this relationship varies by country. The exception for both taxa is Equatorial Guinea, where human variables do not significantly influence density of the few great apes remaining: Human population density is high almost throughout the country, and even the most remote areas are no more than ~15 km from a road. The effects of high human population density and proximity to roads are compounded by the lack of patrols in the PAs of Equatorial Guinea (17). Chimpanzee density was not significantly influenced by Slope. In contrast, two other studies in heavily hunted montane areas found that chimpanzee densities were higher on steep slopes (18, 19). Hunters are less likely to use steep slopes, which require more effort to access than level land or medium slopes, but perhaps Slope had less influence than other factors in our analysis, because most of WEA has low human population densities and hunting has been less intense than in West Africa. 
 
 Impacts of forest loss and degradation Although historically low compared to other parts of tropical Africa, rates of deforestation and degradation in Central Africa are increasing (20), as the human population in WEA grows by 2.7% annually and doubles every 28 years (it is currently around 43 million) (11). Old-growth forests are being slowly lost as the shifting agricultural edge around human settlements advances, but recent work in the Democratic Republic of Congo (DRC) has shown that a high proportion of shifting agriculture is on land that has been cleared previously, which is effectively a rotation system (21). Forests are also being degraded where there is selective logging. We used canopy height to reflect two things: selective logging in terra firma forest (overall forest height reduction) and swamp forest (which is naturally shorter than terra firma forest), or both. Gorillas and chimpanzees always require some forested habitat, and they prefer intact forest, with a medium (25 to 35 m) or high (>35 m) canopy. Chimpanzees occur at their highest densities where canopy height is >35 m. Gorilla densities were highest where the canopy was 25 to 35 m high (intact forest), which includes swamp forest, open canopy forest, and forest where the height reduction results from human activity (selective logging, but not agriculture). Chimpanzees depend on fruiting trees as a primary food source; in contrast, gorillas can tolerate and may even prefer more open forest conditions (22), including secondary and swamp forests in which understory herbs are abundant. In addition, chimpanzees nest almost exclusively in trees, usually high in the canopy, whereas gorillas often sleep at or near ground level (23). Finally, chimpanzees are highly territorial, and they are less flexible than gorillas in terms of shifting their home range when they are displaced by human activities. This can cause lethal conflict between neighboring chimpanzee communities (24). Rates of deforestation are lower in than outside the conservation landscapes, especially in the PAs (5), and these landscapes hold the largest remaining populations of gorillas and chimpanzees. Nonetheless, most great ape habitat in the landscapes is in concessions that are logged repeatedly, with rotation times too short (often only 30 to 40 years) to allow regeneration of the tree species that the great apes rely on (25). By the third rotation, forest structure and composition have been radically changed (26). Empirical data have shown that chimpanzees were much less abundant in logged, unhunted forests than in unlogged, unhunted forests (14). This likely explains the higher densities of chimpanzees along the Monts de Cristal-Monts du Chaillu mountain chain, where logging has been less intense than in lower and flatter terrain, because of the comparative difficulty of access and higher costs of timber evacuation. Central African forests are only moderately suitable for industrial agriculture (27). However, if economic incentives shift, then industrial agriculture, such as oil palm production, could expand dramatically (to date, less than 1% of gorilla and chimpanzee range has been allocated to palm oil concessions). In contrast to selective logging, large plantations require clear-cutting and would increase forest loss with catastrophic consequences for great apes (3) and other wildlife. 
 
 Impacts of cultural taboos Taboos against eating gorillas exist in only 1% of their range (~7400 km2 in Mbam-Djerem, Deng Deng, and associated logging concessions in central Cameroon). This variable thus played a limited role in the range-wide analysis. In contrast, more chimpanzees benefit from such taboos, as reflected by their high densities in ~25,700 km2 of coastal Gabon and adjacent Congo (Mayumba and Conkouati; 5% of their range). Along this strip, chimpanzees are found wherever forest remains, although most has been selectively logged multiple times. More people hold a belief or uphold a custom that prohibits them from killing and eating chimpanzees, as they bear a closer physical resemblance to humans than gorillas do. The positive association between great ape densities and taboos against eating them could be reinforced through conservation advocacy to the wider public. 
 
 Impacts of Ebola The densities of gorillas and chimpanzees were significantly reduced where EVD had occurred in the previous 20 years, with little difference in magnitude of EVD impact on each subspecies (fig. S1). Mortality rates have reached 90 to 95% during the worst EVD outbreaks (28, 29), and large populations in Congo and Gabon were heavily affected by EVD before the surveys included in our study were conducted. We found no significant evidence of population recovery in post-EVD areas, either because our model was not sensitive to small upward trends or because poaching was hampering recovery, or both. Without the pressure of poaching, some great ape populations begin to recover, and growth has been detected 10 to 20 years after an EVD outbreak at sites where anti-poaching activities were effective (30). In addition, illegal habitat modification in WEA is at lower levels than elsewhere; therefore, recovery is possible, albeit slow (total population recovery could take 75 to 131 years in the absence of poaching) (31). Where there is no effective anti-poaching or habitat protection, post-disease recovery of great ape populations will be even slower or impossible. At present, EVD in wild great apes cannot be prevented. A review by Leendertz et al. (32) recommends that research into disease ecology and Ebola vaccine delivery should continue. In the meantime, great ape-to-human transmission of EVD can be avoided through appropriate and effective dissemination of information regarding the dangers implicit in the handling and eating of ape meat (33). 
 
 Industry, infrastructure development, integrated land-use planning, and conservation management Often, the conditions where great apes can thrive are met only in well-managed PAs and in FSC-certified concessions with few, if any, people living in them. Around three-quarters of the combined range of gorillas and chimpanzees fall into neither of these categories, and although Equatorial Guinea, CAR, and Angola have PAs, at present there are no FSC-certified concessions in the great ape range. The unprotected range includes conservation areas, noncertified logging concessions, extractive industries (timber, minerals, and petroleum), large- and small-scale agriculture, and infrastructure (for example, transport and power generation). Proactive national and regional integrated land-use planning that includes conservation as a vital element is essential to maintaining Central Africa’s biodiversity, ecological integrity, and functionality (5). One way to flag areas for economic development while minimizing negative impacts on great apes and other wildlife would be to choose sites that are far from PAs or other intact forests, where forest has already been lost or degraded, and human density is relatively high. Many of the factors highlighted in this study as detrimental to great apes also result in declines of other species (12, 13). A good example of national land-use planning taking wildlife conservation into account is that being pioneered in Gabon. The Gabonese government is developing national strategic planning documents (www.pnatgabon.ga/) whereby, for example, oil palm and rubber plantations will be developed not only where agricultural and social conditions are appropriate but also where conservation values will not be compromised (34). In this particular case, the areas most suitable for oil palm are along a major road that crosses central and southwest Gabon—not only are the biophysical characteristics of the land appropriate for the crop (and transportation facilitated by the well-maintained road) but also the forests on either side of this national access route have already been defaunated by heavy hunting over decades. We have shown that where no guards are present, proximity to roads negatively affects great apes; therefore, regional and national spatial road planning should minimize proximity to PAs and to large remote tracts of forest (25). It is unlikely that areas outside PAs and responsibly managed concessions will be protected by guards, so at a national and regional level, avoidance of road creation in areas suitable for great apes and other large wildlife will be key. At the concession scale, to ensure that the best possible conditions for great apes are maintained within individual priority landscapes, timber companies should implement reduced-impact logging practices and adopt strict wildlife protection policies in their concessions. These practices are crucial in areas with the largest populations of great apes, such as a concession in northern Congo, shown by our analysis to harbor one-fifth of all western lowland gorillas. Our study underlines that two of the most important variables determining great ape density are the presence of guards (as a proxy for effective law enforcement) and maintenance of habitat quality. These are critical factors for industry to undertake not only during prospection and exploitation but also after they cease activities. Adoption of species-specific approaches would ensure that both gorillas and chimpanzees are conserved in timber concessions. Here, land-use planning at the concession scale should take into account the maintenance of particular habitat types. A precautionary approach should be adopted, because most extractive industries and other land uses in this region reduce canopy cover in mixed-species forest, which is the habitat preferred by chimpanzees. It is also important to consider the extent of particular habitat types within a specified area, because great apes have seasonal preferences for some vegetation types (for example, monodominant forests and swamp habitats). Negative impacts on chimpanzees could be avoided through controlled harvesting of particular tree species and mitigation of the disturbance associated with exploitation. If logging companies follow the High Conservation Value (HCV) approach (an FSC forest management designation), this requires strict control over harvesting levels of tree species known to be important for great apes. HCV also calls for reforestation along abandoned roads. Road building in this region causes high levels of initial deforestation. However, most of these roads are temporary (35), so logging companies should replant important great ape food trees along closed routes. Further detailed recommendations that take into account the socioecology of gorillas and chimpanzees can be found in the studies of Morgan and Sanz (25) and Morgan et al. (36, 37). We recommend that all industrial development projects, whether public or private sector, adopt international best practice for identifying, planning, and mitigating impacts on priority biodiversity. By any definition, Critically Endangered gorillas and Endangered chimpanzees are biodiversity priorities. International best practice for development projects and their investment partners, such as international financial institutions, are based on the International Finance Corporation’s Performance Standard 6 and associated Guidance Notes (and similar standards such as the World Bank’s Environmental and Social Safeguards) and the Equator Principles. These are relevant for all sectors, including linear infrastructure, forestry and other extractive industries, agro-industry, and hydropower. Identifying risks to wildlife and prioritizing actions to avoid negative impacts will ensure that conservation is an integral part of national development.
– Although still highly vulnerable, there are more western lowland gorillas than previously thought, a new study finds. Over a 10-year period, scientists scoured 72,000 square miles of western equatorial Africa on foot patrols studying nests and other clues, reports the Guardian. Earlier estimates placed the gorilla population at 150,000 to 250,000. The new data puts the number of gorillas at 361,900, more than doubling the low end of the previous estimate. Scientists also tailed central chimpanzees in the same region and found them more plentiful than expected. They estimate the central chimp population to be 130,000, which is 10% to 80% higher than previously thought. The animals surveyed represent 99% of all gorillas in the world and about one-third of all chimpanzees. Researchers found poachers to be one of the biggest threats to gorillas. Dominant males will stand up to hunters in an effort to protect their group and get shot in the process; the surviving males then kill the dominant male’s offspring, furthering the loss. Chimpanzees, on the other hand, "are cleverer and sneak away very quietly," says Fiona Maisels, a professor at the University of Stirling in Scotland and one of the 50 researchers on the project. They are also helped by cultural taboos against eating chimpanzee meat. Gorillas are still endangered—in fact, despite the revised population estimates, their numbers are trending downward by 2.7% annually, according to a press release, and most live in unprotected areas, leaving them vulnerable to hunters and illness. “Just because there are rather a lot of them does not mean they are not very, very vulnerable,” says Maisels. (A funky, green-haired turtle is in trouble.)