Keeping Bears Connected to Bolster Genetic Diversity

Grizzly family in sage
Grizzly bears in Greater Yellowstone form the Lower 48's second largest population of the species, but genetic isolation remains a threat to their long-term survival.

By Kayla Heinze, Communications Specialist

In the fall of 1985, five years before Vital Ground was founded, the U.S. Fish and Wildlife Service hosted a workshop on grizzly bear genetics in Missoula, Mont. At the meeting, scientists raised concerns about dwindling bear numbers in four of the Fish and Wildlife Service’s six designated recovery zones and the status of genetic diversity within those populations. The other two recovery zones represented strongholds for grizzlies in the lower 48.

Still true to this day, the Greater Yellowstone Ecosystem (GYE) and the Northern Continental Divide Ecosystem (NCDE) account for nearly all of the remaining grizzly bears south of the Canadian border, with an estimated 727 and 1,092 living in each respectively. But almost 40 years after that meeting, biologists are still trying to describe the dynamic genetic reality of the Lower 48’s grizzly populations to better inform wildlife conservation plans.

As proposals surface to transport bears between ecosystems to promote genetic exchange and wildlife officials confirm sightings of grizzlies in key passageways between the GYE and NCDE, I wanted to know to what extent the dire situation voiced all those years ago was pertinent to grizzly conservation efforts today. 

When Genetic Diversity Drifts

Overlooked at times in favor of raw population numbers, genetic diversity stands separate as a uniquely crucial measure of a species’ vitality. The reason why is perhaps best understood by looking at what can go wrong when genetic diversity is reduced.

It typically goes like this: low population numbers lead to a pruning of the gene pool. Known as genetic drift, this process is marked by genes completely disappearing from a small population due to random chance and becomes more powerful as the population shrinks. Traits homogenize and inbreeding increases. As the population’s genetic possibilities decrease so does its ability to adapt to unfavorable environmental conditions and other disturbances, like disease.

If this pattern becomes extreme enough, an entire population can enter what is known as an extinction vortex (see Southern California’s cougars) — which, yes, is as disastrous as it sounds.

Map showing Vital Ground's vision of One Landscape for wildlife and people
Vital Ground’s One Landscape Initiative is a vision for linking grizzly recovery zones.

On the flip side, high levels of genetic diversity bolster the resiliency of a population. For grizzly conservation, this means bear populations having a wide array of genetic resources available to help them cope with hazards such as changing food sources. With enough genetic diversity, a population can adapt and prosper for a very long time. 

Genetic drift has been a consistent concern for bear populations in the Selkirk and Cabinet-Yaak ecosystems, with biologists still estimating fewer than 50 grizzlies in the American portions of each. The Selway-Bitterroot Ecosystem and American portion of the North Cascades Ecosystem, meanwhile, do not have documented resident grizzly populations, with no evidence of reproducing females living in either. 

The stories of genetic diversity in the NCDE and GYE are less straightforward. With growing population numbers, it might superficially appear that these regional populations are genetic strongholds. But that appearance is misleading, and scientists clarifying the murky reality are among those calling for increased connectivity to enable genetic exchange.

The Mixed Bag of Grizzly Genes

Understanding genetic diversity is difficult, encumbered by two twists. First, grizzly populations are just hard to study in general, due to the creature’s often solitary and wide-ranging movements through remote terrain. And even when scientists can capture reliable demographic data through tracking technology, it is not always an accurate indicator of genetic diversity. Genetic variation takes longer to rebound than population numbers, because random mutations must accumulate which can take many generations. A population may seem stable based on its total numbers but have low enough genetic diversity that inbreeding and drift still pose a threat. 

A 2022 literature review published by Conservation Genetics Resources found that the GYE not only had a gene pool, “too small for long-term adaptation,” but, according to some studies, also a declining level of genetic diversity in recent decades. Currently isolated from the NCDE and other grizzly populations, this genetic island could eventually be linked to other habitat areas through connectivity projects like Vital Ground’s One Landscape Initiative.     

Their elusiveness is part of what makes grizzlies so fascinating to research, but these practical challenges mean concrete answers about grizzlies can be as few and far between as the species they describe. Scientists like those at the Northern Rocky Mountain Science Station have prevailed, however, capturing a remarkably comprehensive picture of genetic diversity within the NCDE. 

Biologists collaring NCDE grizzly
Researchers in the Northern Continental Divide Ecosystem take measurements and attach a radio tracking collar to a tranquilized grizzly to gain a better understanding of the often solitary creature’s life history. (Photo courtesy of Glacier National Park)

Pulling data from 1,115 individuals over 14 years, the team led by Dr. Tabitha Graves found that there was a stable level of genetic diversity within the subpopulation at the core of the NCDE, centered around Glacier National Park and the Bob Marshall Wilderness. Other research has indicated that the NCDE has, overall, a higher level of diversity than the GYE. Despite this, Graves’ results showed that three periphery NCDE subpopulations located on the southern and eastern boundaries of the NCDE, where grizzlies have been expanding their range, had low levels of diversity. 

These edge populations also showcased “highly ephemeral patterns in genetic diversity,” according to the study’s authors. The reproductive behavior of a single individual can have a large impact on the overall level of genetic diversity, as was the case when a single male sired an estimated 10 of the 16 studied bears in the southwestern subpopulation. 

Edge populations are important for the long-term health of the grizzly. Typically the meeting place of resident bears and immigrants, they present opportunities for new genes to enter a population and increase its resiliency. But grizzlies in these regions also tend to live closer to developed areas, making them more vulnerable to human-induced mortality. In both the NCDE and GYE, a source-sink dynamic has been observed on the private lands surrounding protected parks, with high bear mortality rates outpacing birth rates.

Reaching Long-Term Resiliency

Progress has been made since 1985, yet even within two of the most robust grizzly strongholds genetic diversity has not yet reached high and stable enough levels to ensure the population will be able to adapt to evolving stressors. 

Canine adenovirus and infections from parasites like hookworms are just two diseases that can be serious and fatal to bears. Increasing local temperatures are also shifting the seasonal availability of berries and have decreased the abundance of another bear favorite, the protein-rich seeds of Whitebark pine (Pinus albicaulis). 

Steven Freygang photo of grizzly bear crossing highway
Grizzlies on the edges of ecosystems are threatened by human-related mortality risks, such as roads. Vital Ground’s connectivity projects target these key corridor areas that connect larger habitat cores. (Photo: Steven Freygang)

Equally arduous as unveiling grizzly genetic trends is predicting the specific ways these threats will unfold in the future. As conservationists deeply concerned with the long-term health of grizzlies, Vital Ground considers it a priority to encourage genetic exchange and, therefore, resiliency. While population trends in the NCDE and GYE are promising indicators that the species is on a positive trajectory toward recovery, the unstable levels of genetic diversity in those populations, not to mention those in their much smaller neighboring ecosystems, inspire us to continue being ambitious with our connectivity projects. 

Our work building wildlife corridors, aimed at linking the GYE, NCDE and other ecosystems, often takes place on those crucial edge regions where bears can conduct their genes to isolated groups but are also at a higher risk of being killed. Through habitat conservation projects and conflict mitigation partnerships, we are committed to both protecting open space and making park-adjacent communities safer for bears.

Your support of Vital Ground’s One Landscape Initiative gives grizzlies a chance to overcome the odds and repopulate the lands they once roamed freely. Connectivity establishes bridges for genetic exchange, allowing bears in each recovery zone to share the best of their genetic resources with other populations and adapt to whatever changes lie ahead. 

Learn more about our goals for grizzly recovery here…

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