A federal judge for the District of Idaho says the U.S. Fish and Wildlife Service must reconsider its decision not to list slickspot peppergrass as an endangered species.

The plant with tiny white flowers grows almost exclusively in the Snake River Plain and on the Owyhee Plateau.

In his ruling handed down Wednesday, U.S. District Magistrate Judge Mikel Williams says the Fish and Wildlife Service wrongly downplayed scientific evidence and emphasized uncertainties when it decided not to list the plant. The ruling was in favor of Western Watersheds Project, which sued Interior Secretary Dirk Kempthorne and the Fish and Wildlife Service over the agency's failure to protect the plant. Idaho Gov. C.L. "Butch" Otter joined the federal agency as a defendant-intervenor.

The plant had been on Fish and Wildlife's radar since 2000, when the agency began preparing a draft rule to list it as endangered. The agency's director approved a rule listing it as threatened in 2003, but reversed course after the U.S. Air Force challenged the decision. Western Watersheds challenged the reversal in 2005 and the agency began more research in 2006.

The issue, Williams wrote, was that Fish and Wildlife abruptly reversed itself again during the second research phase. In the span of five months, one research panel's recommendation that the species be listed as threatened was replaced by a second panel's assurance that the plant was fine, despite concerns from agency employees.

"The Court finds that the FWS's decision to change course based on the new information, without submitting it for review and evaluation by any group of experts resulted in a decision that was not based on the best available science," Williams wrote in his decision.

Katie Fite, biodiversity director for Western Watersheds, hailed the decision as a victory for science and said grazing in the Jarbidge area and near Glenns Ferry is harming the plants. She criticized Otter for leaping to the defense of the small number of ranchers with cattle near the plants, and said reasonable management practices would make things better for everyone involved.

"It's as if they're pursuing this purposeful policy of extinction," Fite said. Later, she added, "Ultimately, no matter how many times it takes, this plant is going to be listed."

Jeff Foss, field supervisor of Fish and Wildlife's Snake River Office, said the agency is disappointed in the ruling and is still examining the decision. No timeline is set for revisiting the decision, he said.

Foss himself was mentioned in the decision as one of those concerned about inconsistencies between agency decisions and scientific data.

"We applied what we considered to be the best available information in reaching our decision," Foss said.

Nate Poppino may be reached at 208-735-3237 or npoppino@magicvalley.com.

The Associated Press contributed to this report.