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OUT OF BOUNDS 50 Outdoors
Papa Bear, Mama Bear, and Baby Bear
Researchers tracing grizzly bear genealogy in Northern Continental Divide Ecosystem with intricate family tree nd ‘rapid restoration of genetic diversity’
TBY CLARE MENZEL OF THE BEACON
ABITHA GRAVES WAS AN ECOL- ogist with a conundrum. She had a big question and no way to
nd answers. The West Glacier-based U.S. Geological Survey scientist, who is best known for her local huckleberry research, wanted to understand the genetic diversity of grizzly bears in the Northern Continental Divide Ecosys- tem. But traditional methods of scienti c investigation — such as mark-recapture, tracking technology, and observational assessments — don’t typically produce su cient or comprehensive results for solitary and wide-ranging large carni- vores like grizzlies.
Many sleepless nights later, Graves found her answer. There existed an “incredible, rare” data set of grizzly bear genetic information collected by Kath- erine Kendall, Graves’ USGS predeces- sor. Kendall’s work spans 14 years, from 1988 to 2012; utilizes two independent sampling methods, hair traps and bear rubs; and was collected from 1,115 indi- vidual bears across the 7.9 million-acre ecosystem, a designated grizzly bear recovery zone that stretches from Can- ada to Missoula.
Graves, who is interested in statistics and the development of new analytical tools, realized she could use the “highly rigorous spatial and temporal grizzly bear genetic data” to identify kin rela- tions between bears — in other words, she would build a family tree with organized pedigrees, or records of descent, across the ecosystem. It could illustrate trends in genetic diversity, a measure of the vari- ation in a group’s genes that lends adapt- ability to challenging conditions such as the introduction of a new disease or cli- mate changes.
Pedigrees have long been used in conservation, mostly with the goal of increasing diversity in captive popula- tions, Graves said, but her recent work shows that the same method can be used in conjunction with wide-ranging, long- term data to advise the management of wild populations, as well. The results of the investigation, led by Graves and USGS biologist Nate Mikle, were published this fall in a paper titled, “Demographic mechanisms underpinning genetic assimilation of remnant groups of a large carnivore.” The paper outlines bear pedi- grees in four regions of the ecosystem and o ers one explanation for varying, but
A bear in Glacier National Park. COURTESY TABITHA GRAVES
improving, diversity in di erent regions. “We were very excited that the results show this population moving such a posi- tive direction,” Graves said. “Our results inform conservation and management of grizzly bears, and this family tree is the rst step in understanding the ecology of recovering species, particularly bears in
this system.”
Graves and Mikle rst used Kendall’s
data set to evaluate the baseline genetic diversity of bears in 2004, the year when Kendall expanded her sampling system- atically across the entire ecosystem. Using six measures of diversity, Graves found that the ecosystem’s highest diver- sity existed in the north, with less diver- sity toward the south.
Geography o ers some explanation for this pattern. Glacier National Park has long been a “stronghold” for the pop- ulation, which was listed as threatened in 1975, because it o ers unique protections from humans. Accordingly, the park and adjacent lands have become known as the ecosystem’s population “core,” hosting a high density of grizzly bears. Land to the south, further from the “core,” has sup- ported lower densities of bears that were “probably semi-isolated” from the others, according to the paper.
Such isolation often spurs interbreed- ing and can be “recipe for decline in genetic diversity,” Graves said.
When the researchers began compar- ing 2004 data to later sets from 2011 and 2012, they found not only that genetic diversity in the “core” Glacier Park region had remained strong, but also that diver- sity in the southern regions had increased “really fast,” as Graves said, and by statis- tically meaningful amounts.
It was time to turn to the bear gene- alogy for answers. Statistical analysis of a family tree could elucidate the under- lying genetic demographic processes responsible for the uptick in genetic diversity. Graves used a computer pro- gram that places individuals into clusters of family groups by genotype to draw up a tree con guration based on the scienti c principle that the simplest explanation is often the best one.
With this family tree in hand, the sci- entists found what seemed to be a very clear explanation for lower genetic diver- sity. The small, isolated populations of the south were home to dominant males with high reproductive success rates, meaning that a few males were respon- sible for producing a large percentage of the next generation.
Graves and Mikle also found a straight- forward explanation for recent increasing genetic diversity in the southern regions. As the isolated populations grew due to increasing arrivals of “immigrant” bears from the more diverse “core,” individual reproductive success rate dropped. Indi- viduals are now contributing smaller, but more equal amounts, of o spring to the next generation.
“Connectivity brought new genes into areas that had historically been domi- nated by a single or couple di erent bear families,” Mikle said.
“The cause of low diversity was the incredible success of those males,” Graves added. “And the cause of increas- ing diversity was large amounts of ani- mals moving in.”
In the southeast region, which over- laps with the Scapegoat Wilderness, Graves and Mikle discovered one indi- vidual bear that had 101 descendants — which, in 2004, included every single resident in the group. The male fathered three bears (one male, two female) that also had unusually high successful repro- ductive rates, and mated with each other, further establishing the dominance of their genetic material within the group. Graves found a similar male in the south- west region with 61 descendants.
“It’s literally a soap opera,” she joked.
The family tree revealed that the genetic diversity was so low in these iso- lated groups because only a small number of bears were passing down their genes to the next generation, dramatically whit- tling down the range of genetic material available to future cubs. The case study demonstrated the textbook e ects of iso- lation on a small population — but it also o ers a reason to be optimistic about the recovery of genetic diversity.
“The e ects of over 10 generations of semi-isolation and reproductive bottle- necks ... were erased by a in ux of immi- grants,” the study reads. And what’s most astounding is the “rapid” speed of genetic diversity recovery — that corrective era- sure happened in less than the timespan of just one generation, which, for bears, is approximately 10 years.
To aid conservation and recovery e orts, Graves and Mikle hope to fur- ther develop their family tree, which now stretches out over 20 feet of paper, and to continue tracing the reproductive, spatial, and temporal movements of griz- zlies in the Northern Rockies.
clare@ atheadbeacon.com
DECEMBER 7, 2016 // FLATHEADBEACON.COM
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