JACKSON, Wyo.-- Bear biologists are refraining from assigning a single reason for a two-fold increase in the rate of natural grizzly bear deaths in the Yellowstone National Park region.
Ten of the 16 grizzlies that have met their ends this summer have died of natural causes, according to data from the Interagency Grizzly Bear Study Team.
Historically, humans are to blame for more than 75 percent of bear deaths in the first half of the summer.
While the rate is just 37 percent this year, it is derived from a small sample size, and biologists aren’t jumping to any conclusions.
"We should be careful not to make too much of this," said Frank van Manen, team leader for the group. "We’re seeing the typical range of conditions that we’d see with grizzly mortality.
"The fact that there were two females with cubs that were killed inflates the numbers a little bit," van Manen said. "We’re seeing an aging of the population as well. It wouldn’t surprise me if we start to see more of these bears dying from old age."
A major increase in natural mortality could have bearing on policy.
The study team is tasked with conducting research that will determine whether greater Yellowstone area grizzlies retain "threatened" status under the Endangered Species Act. In November, the 9th Circuit Court of Appeals prohibited the federal government from pulling ESA protections. The court upheld Judge Donald Malloy’s ruling that the U.S. Fish and Wildlife Service failed to account for the potential harm to grizzlies from the decline of whitebark pine trees.
Whitebark pine seeds are critical late-season food for grizzlies. That makes it "probably too early to say" if the high-elevation conifer is connected to this year’s natural mortality rate, van Manen said.
Data gathered early this season showed "pretty reasonable" production of seeds in whitebark pine stands that haven’t been that productive in the past few years, he said.
Because more bears are likely to die before going into hibernation, it is also too early to say if the grizzly population, thought to be about 600 ecosystem-wide, can sustainably absorb the overall mortality rate, van Manen said.
The female grizzly population can withstand about 9 percent mortality and the male population about 15 percent mortality. Positing two grizzly deaths for every one that is detected, the study team’s best estimate for overall mortality is 8 percent.
But, those are mortality rates for the year to date. Typically, the rates spike in the fall, van Manen said.
This is in part because of conflicts with hunters and also because bears tend to move around more at that time of year, he said. The proportion of human-caused deaths also increases.
Inside Grand Teton and Yellowstone national parks, mortality dynamics are drastically different, said Kerry Gunther, Yellowstone’s bear management biologist.
In a typical year, "humans probably account for 85 percent of the mortality outside the park, Gunther said.
Inside the park, it’s much different, he said, adding that Yellowstone usually records more natural than human-caused grizzly deaths.
This year, every one of Yellowstone’s nine deaths has been natural.
In Grand Teton National Park, the proportion is one human-caused death to zero natural.
The only recorded grizzly death in Grand Teton this year was of a bear that is presumed to be "Brownie," the yearling male cub of the 16-year-old female grizzly known as bear 399.
The slow creep in distribution of grizzlies outward from their core population near Yellowstone is another factor that influences mortality, van Manen said.
"You see more deaths on the periphery (of the range)," he said.
With grizzly population increasing in density along the range’s fringes, deaths there might increase as well.
"We’re at a point now where we’re getting pretty close to having all suitable habitat occupied," van Manen said. "What you’d expect is that with less room for the bears to space out there’d be a greater possibility for conflict."
Once the year-end data trickle in, Yellowstone area grizzly managers will convene a meeting to assess the significance of the mortality trends, van Manen said.