Forest Guardians and a coalition of conservation groups (collectively Forest Guardians) sought to enjoin the Animal and Plant Health Inspection Service (APHIS) and the United States Forest Service (USFS) from killing mountain lions (Felis concolor) to protect livestock in the Santa Teresa Wilderness Area in Arizona, arguing that it violated the Wilderness Act of 1964 (Wilderness Act),[1] the Arizona Wilderness Act,[2] and the National Environmental Policy Act (NEPA).[3] The district court granted summary judgment in favor of APHIS and USFS. The Ninth Circuit affirmed, ruling that 1) lethal predatory control to protect livestock in the Santa Teresa Wilderness was consistent with the Wilderness Act and the Arizona Wilderness Act, 2) the fact that USFS's manual might bar predator control was unpersuasive because agency manuals do not bind the agency and are not entitled to deference, and 3) USFS conducted adequate environmental assessments of the effects of predator control because the agency has discretion to determine the geographic scope of its NEPA analysis.

The court initially determined that USFS's authorization of lethal predator control did not violate either the Wilderness Act or the Arizona Wilderness Act because neither act expressly prohibited predator control in wilderness areas.[4] The court noted, however, that both acts authorize preexisting grazing activities to continue in wilderness areas and that livestock grazing necessarily entailed activities necessary to support grazing, including lethal predator control methods. The court consequently deferred to the agency's conclusion that the Wilderness Act authorizes lethal predator control.

The court also rejected Forest Guardians' argument that congressional guidelines incorporated into the Arizona Wilderness Act[5] disallowed lethal predator control. The court reasoned that even though the congressional guidelines permitted preexisting grazing to continue once an area was designated as wilderness, the guidelines did not necessarily prohibit predator control if the activity had not predated the wilderness designation. In addition, the court ruled that even if the USFS manual prohibited the use of lethal predator control in this circumstance, a manual, which does not have the force of law and does not bind the agency, is not entitled to judicial deference.

Finally, the court deferred to USFS's decision to review the impact of controlling mountain lions in the wilderness area as reported in a statewide study. The court reasoned that the statewide study addressed the effects of lethal predator control in the Santa Teresea Wilderness Area and that, under Kleppe v. Sierra Club,[6] USFS has discretion to define the scope of its environmental assessment under NEPA.