The two mountain gorilla habitats are essentially ecological islands, separated and surrounded by a sea of people. The 15 miles (24 kilometers) that separate them might as well be 15,000 miles (21,000 kilometers).
"There is no possibility of gorillas [moving] back and forth; there are too many people in between," Robbins says.
With human pressure from all sides, protecting the apes' habitat has become key.
In the Congolese Virungas, habitat is disappearing to the illegal charcoal trade—a $30 million industry that requires forest clearing. (See also "Congo Gorilla Killings Fueled by Illegal Charcoal Trade" [August 16, 2007].)
"The recent upsurge in demand for charcoal, together with a recent incident of illegal tree felling, indicates that loss of mountain gorilla habitat will be one of the major conservation challenges in the years ahead," says Emmanuel de Merode, an expert on Virunga National Park and chief executive of the Africa Conservation Fund.
In Uganda's Bwindi park, where the political situation is much more stable, researchers are conducting habitat studies that explore what the gorillas eat and how far they must travel to find food.
Covering More Ground
With funding from the National Geographic Society, Robbins is trying to figure out how much the gorilla population in Bwindi could increase given the limited size of its jungle habitat—nearly 81,500 acres (33,000 hectares), or twice the size of the District of Columbia. (The National Geographic Society owns National Geographic News.)
To ensure the survival of a species that ranks as one of our closest relatives, "it's essential that we know how many mountain gorillas their home can sustain," says Robbins, who is employed by the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
Gorillas in both locations—Bwindi and Virunga—consume nearly 50 pounds (20 kilograms) of food a day each, but since they live at different elevations, they have different diets and therefore different space requirements.
Bwindi gorillas live at elevations of 5,000 to 9,000 feet (1,500 to 2,700 meters), while Virunga gorillas make their homes much higher up, at more than 10,000 feet (3,000 meters).
Both populations feed primarily on abundant and readily available herbaceous vegetation, such as wild celery, nettles, and thistles, but the diet of the Bwindi gorillas also includes a healthy portion of fruits, which don't grow at the higher, cooler elevations in the Virungas.
That important food source tends to be spread out over larger areas, a possible reason why gorillas in Bwindi travel farther to feed every day than do the apes in the Virungas—just over half a mile (0.8 kilometers) a day, versus just over a quarter mile (0.4 kilometers).
Another reason may be that other vegetation is less dense in Bwindi than in the Virungas, which encompass three parks totaling 111,200 acres (45,000 hectares).
"This may help explain why the [Bwindi] gorillas use larger areas," Robbins says.
She estimates that each Bwindi gorilla needs up to 7,400 acres (3,000 hectares), while Virunga gorillas need up to 3,700 (1,500 hectares).
The findings so far suggest that the Bwindi population may not be able to grow as large as its neighboring population in the Virungas. That said, Robbins adds: "I think it will be some time before the gorilla populations outgrow their habitat."
Gorillas in the Rwandan Virungas have had their ups and downs, but tourism and conservation are again on the rise.
Glenn Bush, interim director of the Dian Fossey Gorilla Fund International's Karisoke Research Center in Rwanda, is doing work similar to Robbins's.
He also emphasizes the need to do more research on vegetation change. "We're seeing some alarming things," Bush says.
"There is a forest halfway between Virunga and Bwindi seeing die-offs of bamboo. We can't explain what it is. If bamboo [for example] disappears in the parks, what are the implications for gorillas? There's a lot of work that still needs to be done to understand how to conserve these animals for the future," he says.
Despite decades of gorilla research, scientists still lack some basic information about the great apes, including their life expectancy.
"The gorillas that were born when Dian Fossey came [to Rwanda] 40 years ago are only now starting to die," Bush says. "We still haven't followed enough individuals from birth to death to know with any great precision how long the mountain gorillas live."
What researchers do know is how fragile populations are, even without the complications of war and poverty. In Bwindi, researchers do not know if the population has gotten bigger. "But we have no reason to believe that it is declining in size," Robbins says.
Today, in a relatively stable Uganda, Bwindi's gorilla tourism industry is flourishing.
Accompanied by armed park rangers, tourists in Bwindi can observe any of the four groups of gorillas that have been habituated to human contact, for a fee of U.S. $500. Two additional groups are being habituated now.
"Ecotourism has been very successful in Bwindi, as it has been in [the Rwandan] Virunga[s]," Robbins says. "Revenues generated by the gorilla permits in Bwindi cover about 65 percent of operating costs for all of Uganda's [wildlife protection]."
Promising ecological conditions and successful conservation efforts in both Bwindi and the Virungas have inspired Robbins to have hope. "We are optimistic that both populations will continue to increase," she says.
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