Bridging "Islands" of Wilderness a Boon to Animals
Copyright 2001 National Geographic
September 21, 2001
By Bijal P. Trivedi for National Geographic News
When humans chop up areas of wilderness—with roads, farms or other development—species marooned on small islands of forest experience a decrease in genetic diversity. This makes them particularly vulnerable to disease and local environmental stresses—a matter of increased concern if the species is threatened or endangered. Linking these wilderness islands was thought to be the solution. Now scientists in England have proven it.
While the theory, called habitat fragmentation, seems fairly straightforward, it has been difficult to prove, said Kirsten Wolff, of the University of Newcastle-upon-Tyne, United Kingdom, and an author of the new study which appears in the September 21 issue of the journal Science.
The problem has been finding a species whose habitat was fragmented and then restored and obtaining genetic samples of that species both before and after.
Dried red squirrel hides from collections in a couple of British museums provided the key. The hides, which were collected between 1918 and 2000, provided a source of genetic material that Wolff's team used to analyze squirrel populations over most of the 20th century.
"Britain used to be just one big forest but then humans altered the landscape and created scattered pockets of woodland throughout the country," said Wolff.
Many of these pockets of woodland harbored a genetically distinct population of red squirrels—although they were still all the same species.
Replanted Forest Becomes "Bridge" for Wildlife
In the 1920s Kielder Forest was planted and by the 1960s the new woodland had bridged fragments of forest in the north of England with some in southern Scotland. A DNA analysis of squirrel hair from hides harvested in 1980 revealed that squirrel genes from the English region of Cumbria were found more than 100 miles (160 kilometers) away in Scotland.
Kielder Forest had transformed several of the fragments into a single habitat that allowed isolated populations of squirrels to mix and interbreed. The result was that squirrel population in this area was more genetically diverse than before the 1980s.
A more diverse population is particularly important when a species is threatened because it is more resistant to disease and fluctuations in food supplies.
Although wilderness corridors like the Kielder forest expand habitat for the red squirrel, it has also extended the roaming range of the North American gray squirrel that is a particularly detrimental invasive species in Britain. The gray squirrel carries the Parapox virus that is lethal to the red squirrel and heavily responsible for its demise. "There are pros and cons to rejoining fragmented habitats," says Wollf. "On the one hand it allows the free flow of genes among a species. On the other, it increases avenues for disease transmission."
Even if the habitats are not contiguous, says Wollf, "stepping stone" habitats can provide sanctuaries where members of different populations can mingle. For woodland patches to serve as stepping stones for red squirrels they must be within a mile (1.5 kilometers) of one another.
Wollf's work has broad implications for land management. It lends considerable support to attempts to resurrect the habitat of endangered species by rejoining their fragmented territories. For example, one goal of the Wildlife Conservation Society of the United States is to link jaguar habitat from Arizona to Argentina.