When it comes to population trends, one male grizzly can have a big impact.
At the Waterton-Glacier Science and History Day, United States Geological Survey research ecologist Tabitha Graves discussed the results of her grizzly bear family tree project.
Graves found that in the southeast region of the Northern Continental Divide Ecosystem, which has the lowest genetic diversity, one griz male was related to 68 percent of the 101 descendant living bears. The male had three very successful offspring, Graves noted, including the two most productive females in the area.
Graves’ dataset is the product of almost two decades of bear DNA collection. Kate Kendall started the project in 1998 and Graves has been continuing it since 2004.
Researchers collect DNA from bear hair, gathered either from hair snares – bears are lured to a site and their hair snags on barbed wire – or rub trees strung with barbed wire, which Graves noted is even more desirable to itchy bears.
Glacier National Park bears have been the subject of eight years of surveys between 1998 and 2012. Between 2004 and 2012, Graves said, the population has been increasing.
“This is very good news and it’s what we would expect,” she explained.
The density distribution of bears has changed as well. Bears used to be concentrated in the northern portion of the Park, but they’re growing in density in the southern areas of the ecosystem too. This could be the result of population growth in isolated subpopulatons, Graves speculated.
The effective population size, or number of breeding individuals, has also increased since 2004.
Over 1,000 unique individuals were identified and the team made 1,287 parentage assignments, including 435 triads, in which both mother and father are known.
“We don’t detect every single one of the bears,” she noted. But if they can detect a mom and her offspring, they can track the migration of the offspring.
And as for the “Genghis Khan of bears,” the male related to 68 percent of the bears in the southeast NCDE, this is indicative that a few individuals have a large genetic impact in areas with low genetic biodiversity. Immigration into areas of low diversity has increased the gene flow dramatically since 2004.
“This data is not only serving to explain what happened in a population of bears, but confirming what we already thought about diversity,” Graves said. “I don’t think you can really overstate the role that Glacier has played in the recovery of bears.”
Another fascinating result of the family tree work is the effect of nurture on griz-human conflicts. If nature, or genetics, affected whether offspring would be problem bears, the prevalence of problems with offspring would match the prevalence of problem fathers.
There’s no correlation between problem offspring and problem fathers, Graves said, but there’s a strong correlation between problem mothers and problem offspring.
This is the first dataset to show that problem sows have problem cubs, and that therefore the cubs are learning bad behavior from mom rather than having a genetic predisposition to griz-human conflict.
Currently, Graves is reconstructing the tree to include Canadian bears. In the future, she hopes to evaluate whether landscape factors affect long-distance dispersal of bears.