for National Geographic News
When humans chop up areas of wildernesswith roads, farms, or other
developmentspecies marooned on small islands of forest decrease in genetic diversity. This makes them particularly vulnerable to disease and local environmental stresses, a matter of much concern especially 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. She is 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.
Hides from dried red squirrel 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 squirrelsalthough they were still all the same species.
Kielder Forest was planted in the 1920s, and by the 1960s the new woodland 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 populations in that area were more genetically diverse than before the 1980s.
Greater diversity of a population is particularly important when a species is threatened because the diversity makes it more resistant to disease and fluctuations in food supplies.
Although the Kielder forest expanded habitat for the red squirrel, it also extended the roaming range of the North American gray squirrel, which is a particularly detrimental invasive species in Britain. The gray squirrel carries a Parapox virus, which is lethal to the red squirrel and has contributed heavily to its demise.
"There are pros and cons to rejoining fragmented habitats," said Wollf. "On the one hand, it allows the free flow of genes among a species. On the other, it increases avenues for disease transmission."
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