March 6, 2006
Bighorns Benefitted from New Blood
Science reports on a study by Hogg et al., the "first detailed evidence" that endangered species still present in the wild (unlike the Kihansi toads) may benefit from introducing outsiders to rescue isolated populations. As I recall, some time ago, scientists reported the same strategy was showing success with Florida panthers that had Texas panthers introduced to provide genetic variation.
For a population of animals spiraling towards extinction, things get bad before they get worse. Small numbers means fewer mate choices, more inbreeding, and less-healthy offspring. Scientists have "genetically rescued" such populations in captivity by introducing outsiders to freshen up the gene pool. Now, researchers report the first detailed evidence of a successful application of this strategy in the wild. The beneficiaries: a historically isolated flock of bighorn sheep in Montana.
The western United States was once swathed in herds of bighorns. But by 1922, domestic sheep diseases, hunting, and habitat loss had eliminated all sheep from places such as the National Bison Range (NBR) in northwestern Montana. In that year, wildlife managers hoping to nurse the NBR's bighorn population back to health transplanted 12 sheep from Banff National Park in Alberta, Canada. The herd waxed and waned in isolation until 1985, when scientists introduced new blood in the form of 5 rams from other herds in Montana and Wyoming. Over the next decade, 10 more sheep were introduced.
Conservation biologist John Hogg of the Montana Conservation Science Institute in Missoula and colleagues set out to evaluate the strategy's success... Their findings were dramatic: The most outbred rams--descendants of introduced sheep--fathered 2.6 times as many healthy lambs as did the most inbred rams, and the most outbred ewes gave birth to 2.2 times as many healthy lambs as their inbred counterparts did. Outbred females also produced lambs nearly a kilogram heavier than did inbred moms. "I was surprised at the magnitude of the effect," Hogg says.
The findings may influence the way wildlife managers look after small populations, says Hogg, whose team reported its findings online 28 February in Proceedings of the Royal Society: B. Managers often like to keep animals away from other populations to minimize the spread of disease, he says, but the study shows "it makes sense to manage with both disease and genetics in mind."
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