Tabitha Graves was an ecologist with a conundrum. She had a big question and no way to find 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 Ecosystem. But traditional methods of scientific investigation — such as mark-recapture, tracking technology, and observational assessments — don’t typically produce sufficient or comprehensive results for solitary and wide-ranging large carnivores like grizzlies.
Many sleepless nights later, Graves found her answer. There existed an “incredible, rare” data set of grizzly bear genetic information collected by Katherine Kendall, Graves’ USGS predecessor. 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 individual bears across the 7.9 million-acre ecosystem, a designated grizzly bear recovery zone that stretches from Canada 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 relations 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 variation in a group’s genes that lends adaptability to challenging conditions such as the introduction of a new disease or climate changes.
Pedigrees have long been used in conservation, mostly with the goal of increasing diversity in captive populations, 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 pedigrees in four regions of the ecosystem and offers one explanation for varying, but improving, diversity in different regions.
“We were very excited that the results show this population moving such a positive direction,” Graves said. “Our results inform conservation and management of grizzly bears, and this family tree is the first step in understanding the ecology of recovering species, particularly bears in this system.”
Graves and Mikle first used Kendall’s data set to evaluate the baseline genetic diversity of bears in 2004, the year when Kendall expanded her sampling systematically across the entire ecosystem. Using six measures of diversity, Graves found that the ecosystem’s highest genetic diversity existed in the north, with less diversity toward the south.
Geography offers some explanation for this pattern. Glacier National Park has long been a “stronghold” for the grizzly population, which was listed as threatened in 1975, because it offers 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 supported lower densities of bears that were “probably semi-isolated” from the others, according to the paper.
Such isolation often spurs interbreeding and can be a “recipe for decline in genetic diversity,” Graves said.
When the researchers began comparing 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 diversity in the southern regions had increased “really fast,” as Graves said, by statistically meaningful amounts.
It was time to turn to the bear genealogy for answers. Statistical analysis of a family tree could elucidate the underlying genetic demographic processes responsible for the uptick in genetic diversity. Graves used a computer program that places individuals into clusters of family groups by genotype to draw up a tree configuration based on the scientific principle that the simplest explanation is often the best one.
With this family tree in hand, the scientists found what seemed to be a very clear explanation for lower genetic diversity. The small, isolated populations of the south were home to dominant males with high reproductive success rates, meaning that a few males were responsible for producing a large percentage of the next generation.
Graves and Mikle also found a straightforward 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 rates dropped. Individuals are now contributing smaller, but more equal amounts, of offspring to the next generation.
“Connectivity brought new genes into areas that had historically been dominated by a single or couple different bear families,” Mikle said.
“The cause of low diversity was the incredible success of those males,” Graves added. “And the cause of increasing diversity was large amounts of animals moving in.”
In the southeast region, which overlaps with the Scapegoat Wilderness, Graves and Mikle discovered one individual 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 reproductive rates, and mated with each other, further establishing the dominance of their genetic material within the group. Graves found a similar male in the southwest 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 isolated groups because only a small number of bears were passing down their genes to the next generation, dramatically whittling down the range of genetic material available to future cubs. The case study demonstrated the textbook effects of isolation on a small population — but it also offers a reason to be optimistic about the recovery of genetic diversity.
“The effects of over 10 generations of semi-isolation and reproductive bottlenecks … were erased by a influx of immigrants,” the study reads. And what’s most astounding is the “rapid” speed of genetic diversity recovery — that corrective erasure happened in less than the timespan of just one generation, which, for bears, is approximately 10 years.
To aid conservation and recovery efforts, Graves and Mikle hope to further develop their family tree, which now stretches out over 20 feet of paper, and to continue tracing the reproductive, spatial, and temporal movements of grizzlies in the Northern Rockies.