But many conservationists and environmentalists say federal agencies are increasingly using DPSs to add fewer animals to the endangered list while taking more off.
"Originally [the DPS rule was] being used to protect smaller, isolated populations of biological importance," said Patrick Parenteau, senior counsel of the Environmental and Natural Resources Law Clinic at the Vermont Law School.
"But now it's been turned into something that is frankly unrecognizable from what its origins were."
Chris Nolin, chief of conservation at the U.S. Fish and Wildlife Service's Endangered Species Program, strongly disagrees.
Federal officials use the provision to protect a greater portion of species at risk, she said.
"It affords us the opportunity to list something and, more importantly, try to recover something that maybe we wouldn't have been able to," Nolin said.
There were a total of 26 DPSs prior to 1997, according to the USFWS database. Since then, 54 more have been established.
Nolin argues that clearer policy criteria and the increased availability of genetic data have helped fuel the provision's use.
In fact, a September 2007 study published in the journal Conservation Biology showed genetic information played a key role in nearly 40 federal decisions on whether to protect wildlife in the past decade.
Sylvia Fallon, the study's author and a conservation genetics fellow at the Natural Resources Defense Council, noted the quality and quantity of genetic information reviewed by decision-makers varied widely.
Studies that used less data and found fewer genetic distinctions between populations were less likely to result in wildlife protection than those that used more information, she said.
"Now that genetics is becoming more commonly used, it might be worth revisiting how it's being used in these decisions in terms of how you evaluate the data," Fallon said.
Critics maintain that federal agencies are using the DPS classification to deliberately undermine the Endangered Species Act in several ways.
In some cases, agencies designate a DPS to isolate and delist a relatively healthy animal population, such as the Douglas County deer, even if the remainder of the species is threatened.
"What they do is take a species on the list that is doing poorly overall, find a population that is doing relatively well, and then say, Okay, carve that off and say it's recovered," said Kieran Suckling, founder and policy director of the nonprofit Center for Biological Diversity in Tucson, Arizona.
For example, the gray wolf, one of the first species protected under the Endangered Species Act, was divided into three DPSs.
In 2007 the western Great Lakes DPS was delisted, and the northern Rocky Mountain DPS was delisted last month. The Southwestern population remains on the endangered list.
Federal agencies say that the two delisted populations have met their recovery goals and are no longer in need of stringent protection.
But environmentalists are challenging the moves, noting that removing federal protections makes a recovered population more vulnerable to pressure from industry groups and hunters.
Hunting season for the northern Rocky Mountain gray wolf could start this fall under state management plans being developed in Idaho, Montana, and Wyoming, according to news reports.
(Read "Wolves to Be Hunted if Removed From U.S. Endangered List" [February 5, 2007].)
"This is what you get when you have an administration that is universally hostile to wildlife protection," said Andrew Wetzler, director of the Natural Resources Defense Council's Endangered Species Project.
Critics also say the provision has been misused to avoid having to add an entire new species to the list.
Conservationists petitioned in 2001 to list the North American green sturgeon, for example. The National Marine Fisheries Service declined the petition, but a subsequent legal challenge led the agency to establish two populations.
The agency listed the group south of the Eel River in California as threatened in 2006 but did not list the northern population. Officials concluded the northern DPS, while potentially at risk, did not warrant protection because of continued spawning.
But critics remain skeptical.
"If you look at the listings, you can see how the agency is choosing to list at the DPS level in order to reduce the extent of conservation efforts," Suckling said.
Federal agencies also use the DPS provision to lump populations together so that smaller at-risk groups get less protection, critics charge.
One example, Suckling said, is a small group of cactus ferruginous pygmy owls in Arizona.
The state's population of 20 or fewer birds, protected since 1997 as a DPS separate from groups in Mexico and Texas, lived on land in Arizona sought by developers.
The National Association of Home Builders filed a lawsuit in 2001 challenging the DPS designation and the protection afforded for the owls' habitat.
Federal officials ultimately determined that the Arizona population was not significant to the species as a whole, and it was removed from the endangered species list in 2006.
Suckling and others argue that the decision ignored the bird's long-term survival needs. But federal officials said at the time that the agency based its decision on science, policy, and legal considerations.
"All biological information received during our comment period was examined, while considering our distinct population segment policy and direction from the courts," said Benjamin Tuggle, USFWS's acting regional director for the Southwest.
"Our final decision in no way diminishes our interest in protecting wildlife," he said.
And the USFWS's Nolin argues that while "distinct population segment" may not be a commonly used scientific term, it is a consistently applied tool that allows federal officials to conserve species more effectively.
"There's definitely discretion in this," she said. "What Congress meant is to protect things that are important to the ecology of the U.S. and species conservation.
"Some of that may be a policy call or judgment call: How different is different enough?"
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