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Rising Numbers May Not Be Enough to Save Tigers and Kiwis
15 May 2013 1:10 pm
The little spotted kiwi is a shy worm-eater so small it can be cradled in a child's arms. The Bengal tiger is a 220-kilogram predator that shouldn't be cradled in anyone's arms. But new research shows the cuddly bird and the powerful feline share an unfortunate fate: Though their numbers have stabilized or are even rising, some populations are suffering from profound genetic isolation or loss of genetic diversity—enough in some cases to leave them deeply vulnerable to new diseases and other threats.
Taken together, the findings demonstrate that "just because population sizes of threatened species have recovered doesn't mean that they are okay," writes Richard Frankham, a professor emeritus at Macquarie University in Australia and an author of several conservation-genetics textbooks who was not involved with the work, in an e-mail. "Genetic management of fragmented animal and plant populations is one of the most important, largely ignored issues in conservation biology."
Populations don't come much more fragmented than those of the little spotted kiwi, Apteryx owenii, the daintiest of New Zealand's five kiwi species and the one that seems, based on population growth, to be faring best. In 1912, officials transplanted five little spots to a refuge called Kapiti Island, where the birds would be safe from the predators and plume hunters that nearly finished them off. The birds flourished. They've since been drafted to establish populations in seven more locations, all of them isolated to ensure that the birds stay out of the jaws of stoats, dogs, and cats.
Now, the little spot, at 1600 to 1700 strong, is "considered to be quite safe," says conservation geneticist Kristina Ramstad of Victoria University of Wellington. "They're an example of a kiwi that recovered from very small numbers."
Populations that arise from just a few individuals, however, tend to have members that share a relatively high percentage of DNA—a lack of genetic diversity that can strip them of the evolutionary resources that they may someday need to respond to a deadly new disease, a change in climate, or some other challenge. Ramstad wanted to find out whether today's little spots bore any of those ill effects. She and colleagues collected DNA from more than 180 of the animals—many nabbed with the help of "kiwi dogs" that can sniff out birds hidden in their burrows—living in four different refuges. They analyzed 15 different locations in the birds' DNA and found that it's likely that the Kapiti Island population, which at some 1200 birds is by far the biggest, is losing some of its precious genetic variation with every generation, for reasons that are unclear.
The three other populations that the researchers studied are also losing genetic variation—and they didn't have much to begin with, because each was founded by a small number of Kapiti birds, the scientists report online today in the Proceedings of the Royal Society B.
Making matters worse, Ramstad's team found that the little spot's strongest hope for an infusion of genetic diversity was an illusion. In the 1980s, the last two birds from a lineage distinct from the Kapiti group were transplanted to Long Island, where they and a few Kapiti birds were supposed to hit it off and raise families. But the DNA of birds now living on Long Island shows no sign of the alternate lineage. Those two birds apparently died without leaving descendants.
"Yes, we have eight populations, and yes, they are all growing in size in terms of number of birds," Ramstad says. "But they are all incredibly low in genetic diversity. … If the right disease comes along, it could wipe all of them out."
Compared with the kiwi, tigers are in relatively good shape. In 2011, there were an estimated 1700 Bengal tigers, Panthera tigris tigris, in India alone, compared with 1400 in 2006, and the Bengal also ranges through Nepal, Bhutan, and Bangladesh. Even so, Bengals now occupy only 7% of their historic range, prompting researchers to wonder whether the animals had lost genetic variation as they retreated to their current holdouts.
To calculate the genetic diversity of the tigers of a century ago, the scientists turned to a serendipitous gene bank: stacks of tiger pelts and bones in museums. The team extracted DNA from 53 museum specimens, all collected between 1836 and 1955 and from the scat of 123 living tigers.
A comparison showed that historic and modern tigers don't differ much in their nuclear DNA, the type passed on to offspring in both egg and sperm. But that's not the case for mitochondrial DNA, which is passed only from mother tigers to their cubs. Today's tigers show only 7% of the same genetic variants that their ancestors did, meaning that 93% of the genetic variation of the past has been lost, the researchers report online today, also in the Proceedings of the Royal Society B.
Why the difference between nuclear and mitochondrial variation? The answer lies in tiger behavior, the scientists say. Male tigers roam widely. But female tigers are homebodies who pass their territories to their daughters. As a result, when a patch of forest is destroyed, so, too, are the lineages of any female tigers living there.
The researchers also found that historically, Bengal tigers clustered into two distinct groups based on their genes. Now there's a third group -- the 100 or so tigers living in the semiarid stretches of western India. That additional genetic separation arose when habitat loss cut the western tigers off from their eastern brethren.
If tigers in the western group remain isolated, "that population, if you ask me, looks like it's headed toward extinction," says population geneticist Uma Ramakrishnan of the National Centre for Biological Sciences in Bangalore. She says her results show that conservation efforts need to focus on maintaining connections between populations, not just on the number of tigers living inside a few protected areas. "We're already seeing very detrimental effects of this loss of connectivity," she says. "I feel [this] is probably a conservation red flag."
Other scientists say that the new findings are important because they reveal the hidden dangers to high-profile species. The news is not all bad, says L. Scott Mills, a wildlife population ecologist at the University of Montana in Missoula, pointing out that without heroic transplantation efforts, the little spotted kiwi would now be extinct. But the new research "is also saying that if we as a society want wild species like this to stay around, we're going to have to … put some real effort into it," he says. "That's just a stark reality, and biologists can't do it."