Bad news from Uganda: the mountain gorilla population in the Bwindi Impenetrable National Park is smaller than previously estimated. Until recently, environmentalists believed 336 gorillas resided in the park. Now it looks like the number has dropped to 302.
Why the change? The population numbers are usually collected by counting nests and examining the dung left outside each site. Every gorilla builds a nest and before leaving home in the morning, defecates outside. It seemed like a good way to count the animals with minimal human disturbance. But a new genetic method of counting yields different numbers. A team from the Max Planck Institute for Evolutionary Anthropology in Liepzig, Germany tested DNA samples from each of the dung piles and found the number of gorillas dropped by ten percent.
"We assumed that each individual constructs a single nest, but genetic analysis shows that several individuals construct more than one nest," says Katerina Guschanski, head of the German research team. Like lowland gorillas, the mountain variety will make new nests when the original becomes damaged by weather or will simply move on when the nest becomes uncomfortable.
Gorillas survive in only two places in the world: the Bwindi Impenetrable National Park and the Virunga National Park in the Democratic Republic of Congo. Population numbers from the Congo are not expected to change because the gorillas are accustomed to human contact and are counted directly.
Still, the news is discouraging. The mountain gorilla population may not be increasing like once believed. According to census data based on the old counting method, the population had grown from 300 in 1997 to 320 in 2003. However, the accuracy of the data is questionable. "Now we don't really know what is happening with this population," says Guschanski. "Probably the safest thing is to assume that the population is stable, but we will need to wait for another four to five years to assess how it is changing."
[Via New Scientist]
A measure of increase by the old method from 97-03 should still be valid. This new method, if more accurate and lower would only change the totals, not the change.
300 to 320 might actually reflect an increase of 280 to 300, but would still measure an increase unless multiple nest building was a new behavior introduced during that time period (a highly unlikely option).
Actually, the increase based on the old method is not valid. The new study, in addition to revealing that there are less gorillas in the population, also revealed that the old method had an error rate of 7.8%, the rate of double counts. Because that number exceeds 5%, the formerly estimated rate of population increase, the new data have shown that there has been no significant population increase based on data collected using nest counts, and it is possible that the gorilla numbers have actually declined during that time.