July 8, 2009 | 2
It’s been a long century for the Amur, or Siberian, tiger (Panthera tigris altaica), the largest of the six remaining tiger subspecies. Once hunted nearly to extinction, just 50 tigers remained when Russia protected the species in 1947. Despite that protection, illegal poaching soon dropped that number to as few as 20. But enforcement and careful conservation over the ensuring decades have done wonders, and today the worldwide Amur tiger population stands at more than 900—421 in captivity and an estimated 500 in the wild.
But despite that success, future danger could still lurk in the Amur tigers’ very genes. New research led by a team from the University of British Columbia, in Kelowna, Canada, has shown that Amur tigers in the wild have an “effective population” of just 27 to 35 individual tigers, meaning they show a very low level of genetic diversity—the lowest, in fact, ever shown in wild tigers.
The study, published this month in the journal Molecular Ecology, examined genetic material found in the scat of 95 tigers, representing approximately 20 percent of the wild population.
Lead author Michael Russello, acting director for the Centre for Species at Risk and Habitat Studies at U.B.C., says he expected to find a “genetic bottleneck” in the Amur tiger population, but this study did not turn up any specific evidence of inbreeding problems. Still, he says, “we actually detected evidence for a historical bottleneck, likely linked to post–ice age colonization of [the tiger's] range some 10,000 years ago.” As a result, he says, “Amur tigers have likely been surviving with low levels of variation for some time, potentially precluding further severe loss in the 20th century decline.” (For more about the spread of big cats, see the Scientific American feature article, “The Evolution of Cats.”)
And despite the low level of genetic diversity, Amur tigers still appear to be genomically sound for now. “Regarding genetic health,” says Russello, “no physical symptoms of inbreeding depression have been reported in wild Amur tigers as has been for other large felids suffering from severe population bottlenecks,” for example, kinked tails in Florida panther (Puma concolor) or sperm abnormalities in African cheetahs (Acinonyx jubatus).
But this lack of inbreeding symptoms might not last forever, thanks to continued pressure on the tigers by human development. Russello’s research found that Amur tigers mostly exist in just two population groups, which have become separated. The majority of the tigers live in Russia’s Sikhote–Alin Mountains, but a smaller population of just 20 tigers lives in Southwest Primorye, near Russia’s border with China. This isolated population, which almost never interacts with the larger group, showed an effective population of just 2.8 to 11 individuals.
Connecting these two populations may be essential to preserve the Amur tiger’s genetic diversity, especially for the second population. “An immediate conservation implication of our work is to facilitate connectivity and gene exchange between these areas,” says Russello. “This is especially important for the Southwest Primorye population, whose small size will promote continued loss of genetic variation without influx from Sikhote-Alin. In addition, it is thought that Southwest Primorye may act as a potential source for re-colonizing extirpated populations in China, so it is quite important to maintain a healthy population in that region.” Studies in the 1990s showed fewer than 15 Amur tigers still in China, with no evidence of breeding females. But “the majority of habitat available to Amur tigers continues to exist on the Chinese side of the border,” says Russello. “This fact combined with growing political will in China to promote tiger conservation in the northeast is encouraging for the future of tigers in that region.”
Interestingly enough, Russello’s research found that captive Amur tigers may represent a genetic bounty for the species. His team sampled 20 captive tigers in North America and found genetic variations that no longer exist in the wild. This difference doesn’t mean the captive tigers should necessarily be mated with wild tigers to extend those genes, though. “At this time, reintroduction of Amur tigers is likely not warranted,” says Russello. “The costs associated with such efforts, such as disease transmission, would likely outweigh any benefits linked to an infusion of genetic variation from captivity. That is not to say that such efforts may not be warranted in the future.”
Image: Amur Tiger, via Wikipedia