March 18, 1998 – The Peninsular bighorn sheep was listed as endangered.
December 21, 1998 – The Center sued the U.S. Fish and Wildlife Service to force habitat protection for the species.
1999 – The Service agreed to reconsider its decision to withhold habitat protection.
September 2000 – After the Center intervened, the city of Rancho Mirage refused to approve a massive development project in Peninsular bighorn sheep habitat. The development would have destroyed as much as 406 acres of sheep habitat.
October 25, 2000 – The Service published a recovery plan for the sheep.
February 1, 2001 – The Fish and Wildlife Service designated 844,897 acres of critical habitat for the sheep.
February 28, 2001 – In a landmark settlement with the U.S. Forest Service, the Center and allies won protections for more than 50 endangered species living in Southern California's four national forests, including the Peninsular bighorn sheep. The settlement instituted such sweeping changes as critical habitat protections, removing cattle from 226,026 acres of habitat, and closing two roads within the sheep's lambing grounds.
2006 – When the Agua Caliente tribe and industry groups filed suit to vacate the Fish and Wildlife Service's 2001 critical habitat designation, the Center and Sierra Club intervened. The final settlement, though, vacated 5 percent of Peninsular bighorn critical habitat (mostly on tribal lands) and remanded critical habitat designation to the Service. The Service was required to publish new proposed critical habitat by late September 2007.
April 20, 2007 – The Center filed suit against the Army Corps of Engineers and the Fish and Wildlife Service for approving grading in Peninsular bighorn critical habitat and a plan that would have cut off an essential movement corridor for the bighorn to access the northern portion of their range.
May 16, 2007 – In response to a Center and Sierra Club lawsuit, a Superior Court judge issued a preliminary injunction to prevent the Shadowrock development from performing grading in Peninsular bighorn sheep critical habitat in the heart of Chino Canyon.
October 10, 2007 – The Service published a new critical habitat proposal for the bighorn, which would reduce the originally protected area by nearly 55 percent.
August 26, 2008 – The Fish and Wildlife Service released a revised critical habitat proposal for the bighorn that would add just 36, 240 acres to the October 2007 plan, leaving the bighorn with less than half the acreage protected in 2001.
December 18, 2008 – The Center, Sierra Club, and California Wilderness Coalition filed suit challenging a flawed Bureau of Land Management plan amendment that would open Dunn Road, an illegal road built through bighorn critical habitat, to commercial vehicles.
March 20, 2009 – The Center and the Sierra Club filed suit against the city of Palm Desert and the California Department of Fish and Game, challenging the city's approval of the "Cornishe of Bighorn" housing project. The project would intrude on the recommended buffer around the Bighorn Institute, the U.S. Fish and Wildlife Service-recognized recovery center for the Peninsular bighorn.
April 13, 2009 – The U.S. Fish and Wildlife Service released a proposal that would designate just 376,938 acres of critical habitat for the Peninsular bighorn — a 55-percent reduction from the 2001 designation. The proposal was apparently made to accommodate urban sprawl.
June 11, 2009 – The Center, the Sierra Club, Desert Protective Council, Desert Survivors, and the San Bernardino Audubon Society filed a notice of intent to sue the Service for slashing the bighorn's critical habitat.
January 5, 2010 – After appeals by the Center and allies, the U.S. Forest Service withdrew one grazing decision and reversed a second on 51,000 acres of public land in the San Jacinto Mountains, effectively halting cattle grazing in important Peninsular bighorn habitat.