By Ruffin Prevost
MAMMOTH HOT SPRINGS, WYO. — In the ongoing deliberations over federal protections for Yellowstone area grizzly bears, debate often focuses on a fixed number of total bears living in the region.
Counting bears—or more appropriately, estimating grizzly bear populations—is essential to helping determine when government recovery goals have been met. But just as important is tracking trends in population changes, and trying to determine the causes of those changes.
It’s the latter part of that task—figuring out why growth in grizzly bear populations is slowing or has flattened—that will prove critical in addressing concerns raised by environmentalists and federal courts the next time wildlife managers seek to remove grizzlies as a threatened species under the Endangered Species Act.
“One of the issues that remains to be resolved is the potential impact of whitebark pine on populations,” said Frank T. van Manen, a wildlife biologist with the U.S. Geological Survey.
Federal courts have ruled that the U.S. Fish and Wildlife Service did not fully consider dwindling whitebark pine numbers when the agency sought to drop grizzlies from the threatened list. Whitebark pine trees are dying at alarming rates cross the region, and bears rely on their seeds as an important late-season food.
“You can’t look at whitebark pine in isolation. There are a lot of other important resources that bears use,” van Manen said in October during the Biennial Scientific Conference on the Greater Yellowstone Ecosystem.
A recent study by van Manen and other members of the Interagency Grizzly Bear Study Team looked at why growth in overall grizzly populations is slowing, or even has stalled in some areas. The slowing growth rate has generally coincided with rising mountain pine beetle infestations and whitebark pine losses. But it also has aligned with rising grizzly population densities inside the core recovery zone.
“Is this resource-driven or are there also density-dependent effects going on?” van Amen said.
Researchers compared changes in bear populations from 1983-2001 against the same metrics from 2002-2011. They focused on where the bears lived, how many cubs were produced and the causes of bear mortality.
In the 1980s and 1990s, most of the growth in grizzly populations came outside of Yellowstone National Park, but inside the recovery zone, an adjacent area where management efforts focus on mitigating bear-human conflicts and protecting critical habitat.
In more recent years, growth rates outside the recovery zone are catching up, “so things have equalized across the system,” van Amen said.
“There simply are a lot more bears spending time outside the recovery zone,” van Amen said. “Bears are now occupying a lot of the suitable habitat in the ecosystem. There’s probably still some room for expansion in the Wind River and Wyoming Range, but it’s getting to the point where the ecosystem is becoming saturated.”
Bears outside of the park and recovery zone are still much more likely to die from human-related causes like vehicle accidents or shootings, but the overall proportion of females producing cubs hasn’t changed drastically between the two study periods, van Amen said.
“We were unable to provide conclusive evidence for either density dependence or resource decline because effects on grizzly bear vital rates would be much the same under both scenarios,” van Amen writes in summarizing his results.
Another study in the works will gather more data on the role of whitebark pine seeds as a food source. But what van Amen calls the “ecological plasticity of grizzly bears”—the trait that makes them so surprisingly adaptable in their approach to diet and habitat—also makes them so challenging to study, especially when looking to link cause and effect.
Contact Ruffin Prevost at 307-213-9818 or [email protected].