When ecologist Hjalmar Kühl first visited the Republic of Congo in 2003, deep in the forest, he met chimpanzees whose curiosity gave away that they had never seen a human before. “You’d try to move away, and they’d come closer,” he says. “They’d just sit there watching us.”

Today it’s “basically impossible” to observe such behaviors at most field sites, Kühl says. The reason seems obvious to him. Unlike 20 years ago, Kühl now rarely finds himself far from a village, road, oil pipeline, logging area or mine. He usually has cell phone service and frequently runs into people, no matter how remote the location may seem. As the wilderness has opened up, hunting has increased; great apes have either disappeared or learned to fear and avoid humans. As Kühl says, “It tells you how much the world has changed in just that time.”

Kühl’s local observations turn out to apply across most of Africa’s great ape range. In many parts of the continent, human presence does not necessarily equate to habitat loss. Forests can still be intact, but animals, including apes, often are gone or occur only at very low density—a phenomenon called “empty forest syndrome.” New research published in the American Journal of Primatology now confirms that, rather than simple habitat loss, the degree of human influence on a landscape is the primary, negatively correlated predictor of whether chimpanzees, bonobos and gorillas (all of which are either endangered or critically endangered) will be found there. This conclusion has serious implications for determining what strategies can best protect great apes.

“When we look at where apes occur in the 21st century, it’s no longer the ecology or the habitat that determines where they are,” says Kühl, who is senior author of the new paper and jointly works at the German Center for Integrative Biodiversity Research and the Max Planck Institute for Evolutionary Anthropology, both in Leipzig, Germany. “Instead it’s human activity that’s driving it.”

Scientists have been working for 20 years to estimate great ape populations in Africa, with hundreds of researchers contributing local-level statistics to a database set up to manage the task (the database also includes Southeast Asia’s orangutans, the only great apes that live outside of Africa). But until now, “there were these here-and-there efforts or maybe something across a region but not across the entire range of apes,” Kühl says. “There’s added value if you put all of this together.”

According to co-author Zacharie Nzooh, biomonitoring and wildlife management coordinator at the WWF Cameroon Country Program Office, the new study is a testament to the importance of using computer models to make dependable inferences about animals that cannot easily be counted. “Conducting systematic surveys for key species populations throughout their range is nearly impossible, as it requires significant financial and human resources that are difficult to mobilize,” Nzooh says. But the new study shows that, with enough local-level data, computer models can help fill that gap.

Kühl, Nzooh and their colleagues synthesized data from studies representing 156 sites with a confirmed ape presence and 134 sites without those animals. The model they built included seven environmental and socioeconomic factors: precipitation, elevation, the total area of intact habitat, a location’s “human footprint,” the country’s gross domestic product, corruption levels (as estimated by Transparency International) and local food taboos. Based on the relationships among those variables and the site-level population data, the researchers were able to predict the abundance of great apes across their entire range.

Human footprint—a measure that includes population density, levels of infrastructure and development, light pollution and the presence of roads—was the factor that inversely correlated most significantly with the number of great apes present. This held true even when these influences occurred alongside or within large areas of still intact habitat. The problem in this case is not the loss of large areas of forest, Kühl says, but the fact that infrastructure offers access to previously remote areas, and thus hunting increases.

In general, the model revealed that the highest densities of great apes now occur in Central Africa, while the lowest occur in relatively more developed West Africa. Additionally, only 11 percent of great apes live within national parks or reserves. This latter finding is important for informing management decisions, and although it may seem counterintuitive, the researchers say it is not entirely surprising. As Kühl points out, most animals globally live outside of formal protected areas.

There were some outliers in the data collected in Uganda, Guinea and Rwanda, despite a heavy human presence in those countries’ ape habitats. In Uganda and Guinea, for example, the apes seem to coexist better with people—in part because local customs forbid hunting them. Another factor might also play a role in Uganda, as well as Rwanda: stringent conservation efforts funded by an influx of tourists.

The researchers also found that countries with larger economies tend to have fewer apes, as do countries with higher levels of corruption. Whereas the latter finding makes sense (corruption can lead to uncontrolled resource exploitation, including illegal logging and wildlife trafficking), Kühl says the former finding raises an important question: Can we develop economies and at the same time protect apes?

For now, there is no clear answer. But the new findings are an important first step toward a deeper understanding of the complex realities of protecting wildlife, says conservation biologist Célestin Kouakou of Jean Lorougnon Guédé University in Ivory Coast, who was not involved in the research. “We can even go beyond apes to apply this method to other mammals,” he says. “This is very important for conservation research.”

Elizabeth Bennett, vice president for species conservation at the Wildlife Conservation Society, who also was not involved in the research, agrees that the paper has major practical implications for conservation. For starters, she says, the boundaries of protected areas should be expanded to include more great ape populations that currently live outside of parks and reserves. Hunting is another area of concern: though most protected areas ban it, more emphasis should be put on preventing poaching outside park enforcement agencies’ jurisdiction. Likewise, infrastructures, such as roads and pipelines, are very commonly constructed through ape territory that is not officially protected—and at times through protected areas themselves. These projects, Bennett says, should be stopped.

Perhaps most critical of all, she says, is the continued creation and support of long-term, locally tailored community-outreach programs. “When so many of Africa’s great apes are outside protected areas and so vulnerable to human activities, ultimately they will only be conserved if local communities are involved in—and supportive of—their conservation,” Bennett says.

Protection and outreach require funding—something that is chronically in short supply, not just in Africa but for conservation worldwide. There are some promising solutions, though. The European Union, for example, has launched a new program to provide ongoing long-term funding for key protected areas globally, including four announced so far in Africa. Some countries and companies are looking at carbon credits, payment for ecosystem services or programs for offsetting environmental damage. In 2017, for example, two mining companies in Guinea financed the creation of a new national park to compensate for the negative impacts on critically endangered Western chimpanzees they caused in other parts of the territory.

Ultimately, though, a lot more is needed—and quickly—to save great apes and the biodiversity they share a landscape with. Great ape populations are declining by 2–6 percent annually, so not acting immediately, Kühl says, means losing at least several thousand of these animals per year. Rather than pointing fingers at governments and communities in Africa, he continues, wealthier countries should acknowledge that they are the predominant drivers and beneficiaries of a lot of the resource extraction underway throughout much of the continent. And as such, they should take more responsibility for mediating the impacts.

“If we’re willing, we could achieve a lot in five years,” Kühl says. “The knowledge is there, and the methods are available—so it’s really just about ‘Do we want to get this done or not?’”