Great-ape habitat in Africa has shrunk precipitously in the past two decades, according to the first continent-wide survey of the state of environmental conditions suitable for the animals.

Gorilla habitat has been hit particularly hard, researchers have concluded. Since 1995, Cross River gorillas have lost 59% of their habitat; eastern gorillas have lost 52%; and western gorillas have faced a 31% loss.

Bonobos have suffered a 29% loss in their habitat; central chimpanzees have experienced a 17% shrinkage, and western chimpanzees, 11%.

According to a paper published this week in the biogeography journal Diversity and Distributions, the areas that underwent the greatest habitat loss were found in the centre and east of the Democratic Republic of the Congo, western Equatorial Africa and the upper Guinean forest in Liberia.

The one apparent bright spot for humankind’s closest relatives came in habitats suitable for Nigeria-Cameroon chimpanzees and eastern chimpanzees, which saw almost no change. But even here, the researchers point out, this lack of decline may be attributable to habitat loss in east and west Africa having already been substantial by the 1990s.

Great-ape watchers may have become accustomed to reading about habitat loss and population declines, and indeed, researchers have previously engaged in a range of site-specific efforts and landscape surveys aimed at assessing the decline of suitable environmental conditions for African great apes. But what makes the new survey all the more unnerving is that until now, very few studies have combined the considerable body of existing geographic-information-system and remote-sensing information and ape population data on such a scale.

To provide a new assessment that went beyond the patchwork of local efforts, 47 scientists from 31 institutions analyzed data compiled in the Ape Populations, Environments and Surveys database — a depository launched in 2007 by the International Union for the Conservation of Nature that contains all existing ape survey information — isolating some 15,000 confirmed great-ape sites in 68 different areas. Then, comparing the environmental conditions such as climate, vegetation and human impact at these locations to sites across tropical Africa where there was an absence of apes, the researchers were able to more precisely calculate the ‘suitable environmental conditions’ — or habitat — necessary for apes to live.

The authors then fed this habitat assessment into a statistical model to estimate what remained of these environmental conditions across Africa in 1995, and again in 2010.

The authors say that the rapid losses over the past two decades are a result of the sharp socio-economic changes the region experienced in this period, as previously untouched forests became criss-crossed by logging roads.

Much of what had remained of previously intact African ape range is now being rapidly converted to human-dominated spaces. There remain pockets of prime ape habitat, but they are often in protected areas. It is unknown whether these protected zones are large enough for the apes or even sufficiently protected.

Drawing on existing research and pointing to the low density of chimpanzees in Sierra Leone, Guinea, Guinea-Bissau and Uganda, the investigators add that as long as poaching pressure is limited, there are some groups that can maintain themselves in mixed-use areas. At the same time, chimpanzees enjoy much greater behavioral flexibility than gorillas, making it easier for them to survive in human-modified landscapes.

On the basis of these findings, the authors conclude that conservation efforts need to be “radically scaled up”.

This article is reproduced with permission from the magazine Nature. The article was first published on September 28, 2012.