July 31, 2013
A group of animals separated from their kin by miles of roads, crop fields and other human developments might as well be living on an isolated island in the middle of the ocean. Cut off from their neighbors, that population of animals will no longer mix with others. If the isolated group is small, this can lead to something called a genetic bottleneck, or a reduced amount of genetic variability. In difficult times or changing circumstances–climate change, natural disasters, increased hunting pressure–the lack of genetic diversity may lessen the population’s ability to adapt to external pressures. Therefore, researchers working with endangered species are not only concerned about getting those animals’ numbers up, but also with making sure their populations maintain a healthy dose of genetic diversity.
Tigers, researchers fear, may be undergoing dangerous genetic bottlenecks. Today, these charismatic felines only occupy only seven percent of their historical range, and the species is divided into 76 separated groups throughout their historic range. Many of these groups’ forest patches are connected by thin corridors of trees, but whether or not tigers are actually using these corridors to travel from patch to patch and interact with one another was a question that needed answering.
“For tigers specifically, there [was] no published information available about landscape-level gene flow, meaning we [did] not know enough about how habitat connectivity may influence genetic diversity and population persistence in a metapopulation context,” writes an international team of researchers led by the Smithsonian Conservation Biology Institute, in the journal Proceedings of the Royal Society B.
The team decided to home in on one fragmented tiger population in central India to try and figure out how tigers are or aren’t mingling by using forest corridors. Though tigers once roamed an expanse of jungle covering this pocket of the world, now they exist in four isolated, distinct groups, which make up 17 percent of India’s total tiger population.
The team focused on five tiger reserves (two of which are directly connected) in central India. They surveyed around 9,000 miles of forest and trails, including within the reserves and in the forest corridors linking the reserves. They collected anything they found that the tigers left behind, including feces, hair and claws.
From the roughly 1,500 samples collected, the researchers isolated microsatellite markers–short, repeating patterns in DNA that can be traced down to an individual animal or population of animals. Using these genetic clues, the team to identified 273 individual tigers. Quantifying the degree of genetic variation between samples found in the reserves allowed scientists estimate the current rate of gene flow between the different populations. Then, using a mathematical model that attempts to trace a population back to a most recent common ancestor, they could infer the rate at which tigers have moved through India over the past 10,000 years.
The tigers, they found, continue to mate and exchange genes with those from different reserves even though some of the protected regions are separated by 70 to 230 miles. The better maintained the forest corridor, the higher the rate of gene flow between populations.
Not surprisingly, however, the level of gene flow was significantly higher in the past. Between the populations with the most degraded forest corridors, the rate of gene flow has declined up to 70 percent compared to historic levels. This population fragmentation started as early as 1,000 years ago, the authors calculated, but it really picked up pace in the 18th and 19th centuries when development, agriculture and logging in the area intensified under British rule. This time also marked a period of increased hunting pressure on tigers.
So the good news is that some tigers are still managing to find one another, even in a patchy, fragmented landscape. But the bad news is that these meetings are much less frequent than they were in the past–especially in those places lacking well-defined forest corridors. Still, the authors are generally optimistic about their results, writing: “The genetic diversity of tigers in India has remained high even after a recent (approx. 150 years) 10-fold decline in the effective population size.”
Maintaining gene flow requires maintaining forest reserves and corridors, however, which is not guaranteed for the future. Tiger population fragmentation and separation, the authors write, is still an ongoing process. The team’s results “should not diminish the importance of maintaining and preserving corridors for the future persistence of this metapopulation.” In other words, while it’s great to find that some tigers are still managing to cross between populations, wildlife managers shouldn’t take that as a sign to kick up their feet. The team concludes:
We have presented compelling evidence to suggest that these corridors are effective and functional in maintaining gene flow. These corridors play an important role in maintaining genetic variation and persistence of tigers in this landscape. Reconnecting broken corridors and maintaining existing ones in a politically sensitive and logistically feasible way is a big challenge for conservation biologists and policy makers.
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