Grizzlies roam, but where? A new study offers clues

According to a new study released this month by University of Montana’s Sarah Sells, you’d primarily favor mountainous areas but would also follow waterways through open valley landscapes. But your destination would depend in large part on where you started, and whether you were on a mission to go somewhere else or simply exploring beyond your home range. 

The conclusions came from a modeling program that predicted pathways through Montana between the bears’ current core habitat areas. The two biggest, each with about a thousand grizzlies, are the Northern Continental Divide Ecosystem around Glacier National Park and the Bob Marshall Wilderness Complex and the Greater Yellowstone Ecosystem surrounding Yellowstone National Park.

The study determined likely pathways bears may take between ecosystems. Such connectivity is key for promoting genetic diversity among grizzly populations, which courts have ruled is a requirement for delisting the species from protection under the Endangered Species Act. The study carried the caveat that its conclusions were not meant to predict where grizzly bears might someday settle, but rather to better understand habitats “with good potential for occupancy” as grizzlies roam farther each year from designated recovery zones. 

Sells, the primary author of the study, holds a doctorate in fish and wildlife biology from UM and is a wildlife researcher there. She works in the Montana Cooperative Research Unit, a collaboration between UM and the U.S. Geological Survey. Paul Lukacs, a UM researcher and professor with a doctorate in fisheries and wildlife biology from Colorado State University, was another author. Other authors on the study were biologists Cecily Costello, Lori Roberts and Milan Vinks of the Montana Department of Fish, Wildlife & Parks. 

Grizzlies were listed as threatened under the Endangered Species Act in 1975, when only a few hundred remained in the Lower 48 states across 2% of their original habitat. Now, the NCDE and GYE have significant numbers of grizzlies, but other recovery areas struggle to hold viable populations.

The Cabinet-Yaak Ecosystem population in far northwest Montana numbered about 60 as of 2017, but may be declining according to more recent U.S. Fish and Wildlife Service data. The Selkirk Ecosystem around the intersection of northern Idaho, northeast Washington and Canada has at least 83 bears, according to the new study, with more than 44 in the U.S. There are no bears known to permanently reside in the Northern Cascades Ecosystem in Washington or the Bitterroot Ecosystem along the Montana-Idaho border. 

By forecasting the routes grizzlies might travel more frequently and extensively across western Montana, the authors wrote, the study could help inform management activities. It could suggest which communities may need human-bear conflict mitigation programs, or where to plan highway crossing structures and conservation easements. 

Bears are likely to move along two primary paths between the NCDE and GYE, the study found. One path ran west of Helena and Bozeman along the Big Belt and Bridger Ranges to the Gallatin Range and GYE. And another path ran from northeast of Missoula, along the Garnet Range to the Avon area, southeast toward Boulder and the Elkhorn Mountains, along Bull Mountain to the Tobacco Root Mountains and finally to the Madison and Gravelly ranges just outside the GYE.

Pathways between the NCDE and Cabinet-Yaak Ecosystem in far northwest Montana “transected much of the Salish and Cabinet Mountains and were generally diffuse networks that interconnected and split regularly. Other pathways connecting the NCDE and CYE involved the Reservation Divide and Ninemile Divide Mountains.” 

Pathways between the Cabinet-Yaak and Bitterroot ecosystems were also a web of diffuse routes.

Between the NCDE and Bitterroot, pathways “were well distributed within the Reservation Divide, Rattlesnake, Garnet, Bitterroot, and Sapphire Mountains, but were relatively sparse in the Missoula and Bitterroot Valleys,” the biologists reported. Mapping produced by the study indicated connectivity of likely grizzly habitat where the northeast end of the Sapphire Mountains nears the northern Bitterroots around Lolo and Florence — a location grizzlies have increasingly explored in recent years. 

For more wandering “exploratory” travels, rather than movements between designated grizzly ecosystems, the study found that simulated bears’ movements stayed relatively closer to their origin ecosystem, but were nonetheless well distributed in outside-ecosystem mountain ranges including the Rattlesnake, Garnet, Nevada, Boulder, northern Big Belt, Centennial, Gravelly southern Tobacco Root, Madison, Gallatin and southern Bridger mountains. 

Separate studies have found that modeling possible connectivity corridors can be accurate: Humans’ relative scarcity during COVID lockdowns in spring 2020 allowed brown bears in Eastern Europe to move through areas that studies had previously identified as likely bear movement corridors. And the authors of this study confirmed their model’s accuracy by plotting known locations of grizzlies outside designated ecosystems over their maps. The data corresponded with where modeling predicted the bears would be found. 

Sells’ study, published this month in the journal Biological Conservation, built on two previous studies by her and others. The first study, published last year, used GPS tracking data to refine researchers’ understanding and modeling of how grizzly bears move around in the NCDE. That offered a better understanding of how the bears use, or don’t use, various habitats. The second study, released earlier this year, showed that those refined models for NCDE bears accurately predicted movements of bears in other populations, meaning the models could simulate bear movements beyond only the NCDE. 

Putting that model to use more broadly, the latest study evaluated western Montana for how likely or not bears would be to use habitat. The modeling produced a variety of shaded maps classifying terrain from scoring 1–3 at the low end (unlikely a bear would use that habitat) to 10 at the high end (grizzlies were most likely to use that habitat). For each male and female bear, the researchers modeled bear movement for travels specifically between two habitats and for “exploratory” travels with no set destination. 

For simulated travels where a bear might have a specific destination but also wander a bit, the study found that bears favored mountainous areas and, secondly, streams and rivers through open valleys. 

“Predicted paths repeatedly converged in these areas, despite variation in habitat use among individual grizzly bears, variable start and end nodes, and different values of (exploration),” the study stated. “Pathways were generally similar for females and males.”