WASHINGTON – The Interior Department has struck an agreement with a liberal conservation group to decide within six years whether 251 imperiled species deserve to be placed on the endangered species list, Interior officials announced Tuesday.

The settlement between the U.S. Fish and Wildlife Service and WildEarth Guardians delivers at least a partial truce in the long-standing battle between left-leaning environmental groups and Fish and Wildlife Service officials, who have consistently said that they lack the resources to add all the species that deserve protection to the federal endangered species list.

Two groups in particular — WildEarth Guardians and the Center for Biological Diversity — have filed the vast majority of petitions since 2007, asking that more than 1,230 plants and animals be added. That’s roughly five times as many requests as in the 12 years that preceded it.

“We and the government agree that the day has come to address the future of the endangered species candidates. This will be an important step toward protecting the rich biodiversity in the U.S. and stemming the extinction crisis,” said Nicole Rosmarino, wildlife program director of WildEarth Guardians.

Currently, the service has identified 251 species, including Gunnison’s prairie dog and the Pacific walrus, that face the risk of extinction but cannot receive federal protection because the government lacks the money to implement conservation measures such as identifying critical habitat for them. The agency estimates that it will be able to make final listing decisions within a year — which is mandated by law — on just 4 percent of the flora and fauna that deserve to make it on the endangered list.

“The focus should be getting species protected and avoiding unnecessary resources being devoted to the courtroom,” said John Kostyak, vice president of wildlife conservation for the National Wildlife Federation.

Last month, the Fish and Wildlife Service submitted a 2012 budget request to Congress in which it asked lawmakers to set a limit on how many plants and animals it has to consider for listing in a given year. It wrote, “The many requests for species petitions has inundated the listing program’s domestic species listing capabilities.”