Environmental groups say that shows the Obama administration has done little to improve on what they consider a dismal record on endangered species under President George W. Bush.

Nearly two years after taking office, Obama has provided Endangered Species Act protection to 51 plants and animals, an average of 25 a year. By comparison, the Clinton administration protected an average of 65 species per year, and the Bush administration listed about eight species a year.

"Like the Bush administration, the Obama administration is failing to provide prompt protection to wildlife desperately in need of protection," including the plains bison, sage grouse and hundreds of other species, said Noah Greenwald, endangered species program director at the Center for Biological Diversity, an Arizona-based group that has filed lawsuits seeking greater protection for those and other species.

Greenwald said Interior Secretary Ken Salazar has failed to correct a longtime "culture of delay and foot-dragging" at the Fish and Wildlife Service, which oversees the endangered species program. The agency has been without a permanent director since February, when former director Sam Hamilton died. All but one of the service's eight regional directors are holdovers from the Bush administration.

Tom Strickland, assistant Interior secretary for fish, wildlife and parks, acknowledged the growing backlog, but compared it to a list of schools that need repair.

"We know what we need to do. We don't have the resources to do it all at once," he said.

Many of the species listed as candidates for protection have been waiting for such a designation for decades, including the Oregon spotted frog, found in three West Coast states, and the eastern massasauga rattlesnake, found in nine Midwest and Great Lakes states. The frog has been a candidate for the endangered species list since 1991, the snake since 1982.

Bob Irvin, senior vice president for conservation at Defenders of Wildlife, another environmental group, compared species on the waiting list to patients in a crowded emergency room.

"Species on the candidate list continue to deteriorate while waiting for care," Irvin said. "The 251 species now under consideration for federal protection are glaring reminders that we can and should do more to safeguard our valuable natural resources."

Candidates for the endangered species list get no formal protection. Officials say the designation raises awareness among private landowners and federal land managers that the species need help.

Delays can have consequences. At least 24 species have gone extinct after being designated as a candidate for protection, including the Louisiana prairie vole, Tacoma pocket gopher, San Gabriel Mountains blue butterfly, Sangre de Cristo peaclam from New Mexico and numerous Hawaiian invertebrates.

The federal government spent nearly $1.4 billion last year on programs and land acquisition related to endangered species, up from just under $1 billion the previous year. A total of 793 plants and 578 animals are listed as threatened or endangered in the United States, including 83 mammals and 139 species of fish.

Strickland, who oversees the endangered species program, said it was unfair to evaluate the program based on how many species are listed each year. Some species are in greater danger than others, he said.

"We make judgments based on limited resources, but also the peril with which the species is faced," Strickland said, noting that several species have jumped onto the protected list when they faced an imminent threat.

Strickland, who also serves as Salazar's chief of staff, said the Obama administration has taken steps to restore credibility to the endangered species program, which he said had been damaged under the previous administration.

First, Salazar directed that listing decisions be based on science rather than politics, in response to a scandal involving Julie MacDonald, a former Bush official who was found to have exerted improper political interference on range of endangered species decisions.

Second, the department reinstated a rule, dropped by the Bush administration, requiring government agencies to consult with the Fish and Wildlife Service on actions that could affect endangered species.

Damien Schiff, a lawyer for the Pacific Legal Foundation, a California-based property rights group that calls the Endangered Species Act a regulatory nightmare, said criticism of Obama by conservation groups is overblown.

"Goes to show that one can never satisfy the environmentalists," he said.