Species vs. Ecosystem Recovery

Although endangered populations are typically delisted once they reach a certain population threshold, this doesn’t tell the whole story. Yellowstone Ecosystem wolves and grizzly bears are close to being delisted — but new research suggests that their wide-ranging ecological effects have not yet been restored.

“What will we have achieved by putting wolves and grizzly bears in the Yellowstone Ecosystem if their ecological roles are not complete?” ask Sanjay Pyare, who did this work at the Wildlife Conservation Society, and who is now at the Denver Zoological Foundation in Missoula, Montana, and Joel Berger of the Wildlife Conservation Society in Moose, Wyoming, in the September 2003 issue of Biological Conservation.

Delisting thresholds may be adequate for the survival of wolves (Canis lupus) and grizzly bears (Ursus arctos) in the Yellowstone Ecosystem. But there is more at stake because these carnivores affect their ecosystems at many levels. For instance, wolves and bears decrease the densities of some hooved mammals, which decrease browsing on willow communities, which in turn increase the diversity and nesting densities of migratory songbirds.

Pyare and Berger investigated a potential measure of “ecosystem recovery” for carnivores: whether their prey respond to them appropriately. The researchers used snowballs soaked in wolf urine or grizzly bear feces to compare the anti-predator responses of moose (Alces alces) in two areas: 1) those where there are so many wolves and grizzly bears that they commonly prey on young moose (Alaska’s Talkeetna Mountains and Denali National Park and Preserve) and 2) those where wolves and grizzly bears are recolonizing (Wyoming’s Grand Teton National Park, which is part of the Yellowstone Ecosystem).

Moose responses to predators range from vigilance (such as putting their ears forward) to aggression (such as erecting their nape fur, which makes them look bigger and more menacing) to leaving the area.

The researchers found that the Alaskan moose were three times more likely to respond aggressively to the carnivore odors than were the Wyoming moose (about 15 percent vs. 5 percent). In addition, Alaskan moose exposed to grizzly bear odor were five times more likely to leave the area than were Wyoming moose (about 35 percent vs. 7 percent). Whereas about 15 percent of the Alaskan moose exposed to wolf odor left the area, hardly any Wyoming moose did.

These findings suggest that for large carnivores, populations that are big enough for delisting may not yet be big enough for ecosystem recovery. “Efforts should be directed to determining not just at what level a population is viable, but what additional population increases may restore ecological functionality,” say Pyare and Berger.

—Robin Meadows

Note: This research made Rush Limbaugh’s May 2003 newsletter. The piece was called “Yellow Snow” and featured a cartoon of men in lab coats pelting a moose with snowballs. Berger comments, “I just can’t figure out where some people come from when they ridicule applied science steeped in conservation. It seems that nothing could be more wholesome than wanting a good healthy environment.”

Pyare, S. and J. Berger. 2003. Beyond demography and delisting: Ecological recovery for Yellowstone’s grizzly bears and wolves. Biological Conservation 113:63-73.

Gray Wolf (Canis lupus) Photo by Gary Kramer/USFWS

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