“We accomplished what we wanted to do through the settlement negotiations: to really reduce the litigation so we could have more control over our priorities and focus our limited resources on the species that need attention the most,” Gary Frazer, who heads the agency’s endangered-species unit, said in an interview. “We definitely see it as a good thing.”

The hundreds of backlogged candidates have waited “an unacceptably long time” for final consideration, he said.

Skeptics might ask whether any species that can wait decades for listing was endangered to begin with. The answer, experts say, is that some aren’t; the wildlife service is likely to remove some from consideration after re-evaluation. But most of the rest are probably in declines lasting decades that would not be arrested without outside help.

“Extinction is not an event. It’s a process,” said Patrick A. Parenteau, an expert on the law at the Vermont Law School in South Royalton, Vt. Habitats slowly shrink, populations wither, inbreeding increases and a species weakens until some outside force — a storm, a fire, a dry spell — administers the coup de grâce. The Interior Department said in 1990 that 34 species had gone extinct while awaiting decisions on listing.

The reasons for the backlog vary. Past lawsuits by conservationists forced the wildlife service to spend time and money on tasks like designating protected habitats for already listed wildlife instead of considering new candidates, Mr. Frazer said. Outsiders say chronic budget shortages, past mismanagement and, most recently, politics have also added to the delays.

Under President George W. Bush, “a lot of the management of the program was taken out of the Fish and Wildlife Service and put in hands of political appointees,” Jamie Rappaport Clark, president of Defenders of Wildlife and a wildlife service director under President Bill Clinton, said in an interview. One Interior Department official resigned in mid-2007 amid charges of politicizing the listing process.

Mr. Frazer, who also headed the endangered species program during Mr. Bush’s first years in office, was demoted to liaison to the United States Geological Survey from 2004 to 2008.