Sierra Club: Grizzly mortality limits flawed
by Mike Koshmrl/Jackson Hole News & Guide | NOVEMBER 11, 2016
Conservation groups worry that high death rates of boar grizzlies in a post-delisting world will not be captured by population models and that bear numbers will unknowingly fall into decline as a result.
Modeling conducted by former federal grizzly researcher David Mattson and distributed by the Sierra Club predicts that within eight to nine years population estimates of Greater Yellowstone Ecosystem grizzlies will be erroneously inflated by about 200 bears. The reason, Sierra Club staffer Bonnie Rice said, is that male grizzlies will be increasingly vulnerable to hunting outside the national parks. At the same time annual assessments of grizzly numbers depend on females with cubs, not males, which are later factored in using an algorithm that predicts sex ratios.
“This won’t be detected,” Rice said. “And so the states are going to think they have all this discretionary mortality remaining that they don’t really have.”
Plans tied to delisting grizzlies as a threatened species under the Endangered Species Act set mortality limits for male, female and dependent young grizzlies on a sliding scale that varies depending on the population level.
Mattson’s model, Rice said, assumed that the death rate would be at the maximum level permitted before states would have to cease “discretionary mortality.” That could mean hunting grizzlies or killing them in response for preying on livestock.
“We’re really concerned about this,” Rice said, “and of course we’re really concerned about the timing of it.”
One of the three primary documents tied to delisting — a conservation strategy — is expected to be completed at a multiagency subcommittee meeting for Yellowstone grizzly bears that’s set for Nov. 16 and 17 in Cody. The document contains the mortality limits that worry Rice.
“If there’s a problem with these limits,” Rice said, “we need to know and the public needs to know now, and this needs to be seriously addressed by the Fish and Wildlife Service before any conservation strategy is signed and certainly before any delisting rule is issued.”
One man not alarmed about Mattson’s modeling is Frank van Manen, who is the lead scientist for the Interagency Grizzly Bear Study Team.
Mattson’s predictions, he said, lack the detail needed for a meaningful assessment.
“There’s nothing I can do with this information, because there’s no methods described,” van Manen said. “For all I know this was a little kid drawing a bunch of lines on a graph.
“In the realm that I work,” he said, “I can only look at something that has scientific methods behind it.”
Van Manen agreed there might be something to the argument that male grizzlies will be killed at higher rates. In the ecosystem today, he said, sex ratios are about even. But he believes there are other means of detecting diminishing numbers of boar grizzlies in population estimates.
“We wouldn’t necessarily pick up the change in males by monitoring females, but we would still pick up that change in other ways,” van Manen said.
Besides the standard female-based population model, federal grizzly scientists also monitor grizzlies via radio telemetry, which combined with other methods lets them look at sex and age of the hundreds of grizzlies that have been captured and marked over the decades, he said.
The other methods, van Manen contended, all distinguish between males and females and can be used to adjust gender ratios.
“I don’t think we missed something,” van Manen said. “I think we’re well aware that under the proposed population thresholds and mortality thresholds that males would be the larger proportion of that discretionary mortality, and so there is total awareness that we might have to change the sex ratio over time and that we need to be able to have data to account for that, and I’m confident that we do.”