To maintain healthy levels of genetic diversity, animal populations need to remain sufficiently large. As habitats become fragmented, however, so too do the resident populations, which can result in a loss of genetic variation. That, in turn, puts these populations at risk of extinction. But if the habitat fragments are linked by corridors, or "stepping stone" patches, they can serve as a single large habitat. A new study reveals dramatic evidence of this fact. Reporting today in the journal Science, researchers describe how habitat defragmentation boosted genetic diversity in British red squirrel populations. The authors note that the findings could have important implications for conservation management of species that occupy fragmented landscapes.

Though their numbers have plummeted elsewhere, red squirrels remain relatively abundant in northern England and southern Scotland, inhabiting a patchwork of forest fragments. To assess how the recent planting of a large new forest linking the patches in the two regions might have impacted the local squirrel populations, Marie L. Hale of the University of Newcastle and colleagues studied genetic markers in 102 dried red-squirrel skins collected in Britain over the past century. Squirrels from the Cumbria region held the key, reflecting an increase in genetic diversity in the 1980s. That surge, it turns out, corresponds to the timing of the maturation of a new forest, which the squirrels apparently used to migrate between western and northern forest fragments. Indeed, the researchers report, the landscape defragmentation led to "substantial genetic mixing of Scottish and Cumbrian genes in squirrel populations up to 100 kilometers away from the site of the new forest."

"These findings suggest that where a network of stepping stones is available within a critical dispersal distance, gene flow can be very rapid through highly fragmented landscapes," the authors conclude. Furthermore, they observe, the study indicates that human-made alterations affecting the connectivity of a landscape can bring about changes in genetic makeup "not only in the area of habitat change but in the populations hundreds of kilometers from the site of habitat change.