DNA study reveals non-invasive way to track grizzlies

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A grizzly bear eyes a ground squirrel and a hiker in Northwest Montana. (Chris Peterson photo)

Grizzly bears are an iconic creature, most would agree, and an integral part of what makes Northwest Montana such a special place.

One unique characteristic of bears is that they like to scratch and rub on trees, fenceposts, bridges even utility poles. Is it thought that this is the way bears tell other bears that they’ve been in the area — a bear signature that leaves a lot of bear hair behind.

Finding these trees isn’t all that difficult. A person with a trained eye can spot them fairly easily.

By using these bear rub trees, U.S. Geological Survey scientist Kate Kendall was able to track the expansion and movement of the grizzly bear population over the course of eight years in the 8-million acre Northern Continental Divide Ecosystem all through DNA analysis of hair left behind from very furry bears.

Her results were recently detailed in a paper “Using bear rub data and spatial capture-recapture models to estimate trend in a brown bear population,” in the journal Nature.

The DNA data was gathered from a huge effort in 2004, and then again from 2009 to 2012 and involved the work of dozens of technicians and volunteers gathering hair from more than 5,000 bear rubs.

The dataset was enormous, Kendall, who is now retired, noted in an interview last week.

“We were detecting one-third of the bears in the ecosystem each year,” she said. “Just from natural rub trees.”

The DNA isn’t in the hair itself, it’s in the follicle, where it attaches to the skin.

It’s a completely non-invasive way to track a population over time.

When the project started in 2004, bears in the southern end of the ecosystem were sparse, Kendall noted. Some places didn’t have any bears at all.

But through multiple samples in subsequent years, scientists were able to capture more hair and in turn, gather more data and track movements of bears as they rubbed on different trees over time. That data was put into statistical modeling that revealed not just the population size, but the movement of bears over time and the density of the population on the landscape.

DNA comes with a host of other data as well. U.S. Geological Survey Scientist Tabitha Graves, who was a co-author of the study and did a lot of the modeling work, has created family trees of bears from the data. She said another interesting facet from this study was that the female population in Glacier grew from 2004 to 2012 — something that wasn’t expected.

Today, Graves and Kendall agree that Glacier is probably at or near it’s carrying capacity for grizzlies.

Similar work is being done in Glacier National Park with mountain goats, Graves noted.

In that case, DNA is gathered from goat scat, not hair, but the modeling methods are similar.

Goats can be tracked and populations modeled from scat locations over time.

Grizzly bear tracking could also be done using citizen science as well, Kendall noted, if funding was available.

Kendall sees the overarching message as a success story.

“Lots of those areas had no bears or few bears in 2004,” Kendall noted. “It’s a conservation success story. It’s proof the Endangered Species Act can work. We’ve done a good job of bringing the NCDE population back from what it was. Everyone should feel good about what the population is doing.”

The numbers point that out.

Ecosystem wide, female density of grizzlies in 2004 was 7.77 bears per 1,000 square kilometers. By 2012, it was 11.45 bears per 1,000 square kilometers.

That conservation effort included a host of federal, state, and tribal agencies working together, including the Forest Service, Park Service, the Blackfeet and Salish and Kootenai Tribes, the state of Montana and private landowners.

Those agencies also cooperated and helped fund the DNA work.

A link to the full study is available at: https://www.nature.com/articles/s41598-019-52783-5. It is free and open to the public.

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