The grizzly bear (Ursus horriblis) formerly ranged over much of North America from the mid-plains westward to California and from central Mexico north to Canada and Alaska . During the early 1880s there were approximately 50,000 grizzlies in the lower 48 states . Between 1850 and 1920 the species was extirpated from 95% of its continental U.S. range . In 1922, 37 populations remained , declining to six by 1975 when grizzly bear was listed as “threatened” under the federal Endangered Species Act in the conterminous U.S. . At the time of listing, fewer than 1,000 grizzlies remained in about two percent of the species' historic range .
Early declines in grizzly populations were largely a result of persecution by European settlers; grizzly bears were shot, poisoned, and trapped wherever they were found . Despite protection under the Endangered Species Act, human-caused mortality, largely that resulting from human bear conflicts, has remained the biggest threat to grizzlies . Approximately 88% of U.S. grizzly bear deaths documented during studies over the past 20 years were killed by humans, both legally and illegally . Habitat degradation associated with rural or recreational development, road building, and energy and mineral exploration is also a major threat to grizzly bear survival . Habitat destruction in valley bottoms and riparian areas is particularly harmful .
The 2005 population was between 1,200 and 1,400 grizzly bears in Wyoming, Montana, Idaho, and Washington states [1, 3]. Grizzly bears have also been reported in the San Juan Mountains of Colorado in recent years, but have not been confirmed since a grizzly was killed in 1979 . Only two populations (Yellowstone and Northern Continental Divide) contain >350 individuals . There are six grizzly bear recovery areas in the conterminous U.S. including Yellowstone National Park and surrounding areas.
The Yellowstone recovery areas consists of 9,200 square miles of northwest Wyoming, eastern Idaho, and southwest Montana . When the grizzly bear was listed in 1975, the population was variously estimated at 136, 229, 234, and 312 bears . Prior to the mid 1980s, low adult female survival was thought to be driving the decline of this population . In the early 1980s, with the development of the first Grizzly Bear Recovery Plan, agencies began work to control mortality and to increase adult female survivorship . In 1999, the population was estimated to have increased to between 400 and 600 . Some research suggests that population trends seen through the mid-1990s were largely tied to the success/failure of the whitebark pine (an important food source) crop . Starting in 1993, counts of female bears with cubs-of-the-year were used to calculate minimum grizzly bear population estimates . These estimates suggest that the grizzly population increased at a rate of 4 to 7% per year starting in the early 1990s . The 2005 population was estimated at "over 580" bears  then re-estimated at "over 500" bears . The Yellowstone grizzly's expanded by 48% since 1970  and 68% of suitable habitat outside of the recovery zone is now thought to be occupied by grizzly bears .
The 1993 grizzly bear recovery plan outlined three demographic recovery criteria to be met in order to consider delisting of the Yellowstone grizzly: 1) A minimum of 15 females with cubs-of-the-year be maintained over a running 6-year average both inside the Recovery Zone and within a 10-square mi area immediately surrounding the Recovery Zone 2) Sixteen of 18 bear management units within the Recovery Zone must be occupied by females with young, with no 2 adjacent bear management units unoccupied, during a six-year sum of observations 3) The running six-year average for total known, human-caused mortality should not exceed 4% of the minimum population estimate in any two consecutive years; and human-caused female grizzly bear mortality should not exceed 30% of the above total in any two consecutive years . The U.S. Fish and Wildlife Service believes these criteria have been met .
On March 29, 2007 the U.S. Fish and Wildlife Service declared the Yellowstone grizzly bear is a distinct population segment which is fully recovered and removed it from the endangered species list . The decision is opposed by many conservation groups and scientists who assert that the population is too small to be viable in the long-term, habitat and hunting/persecution threats will not be adequately address without Endangered Species Act protection, and global warming is devastating white-bark pine habitat [10, 11].A notice of intent to sue to strike down the delisting was filed on April 2, 2007 .
 U.S. Fish and Wildlife Service. 2005. Grizzly Bear Fact Sheet. Accessed at <http://mountain-prairie.fws.gov/species/mammals/grizzly/factsheetGrizzlybear111405.pdf>.
 Mattson, D.J., R.G. Wright, K.C. Kendall, and C.J. Martinka. 1995. Grizzly Bear In LaRoe, E.T., G.S. Farris, C.E. Puckett, P.D. Doran, and M.J. Mac, (eds). Our living resources: a report to the nation on the distribution, abundance, and health of U.S. plants, animals, and ecosystems. U.S. Department of the Interior, National Biological Service, Washington, DC. 530 pp. Accessed at <http://biology.usgs.gov/s+t/noframe/c032.htm>.
 U.S. Fish and Wildlife Service. 2005. Grizzly Bear Recovery. Mountain-Prairie Region, Endangered Species Program. Website <http://mountain-prairie.fws.gov/species/mammals/grizzly/> accessed 3/2006.
. Honnold, D.L. and L. Lucas. 2007. Notice of Violations of the Endangered Species Act in Designating and Delisting the Yellowstone Grizzly Bear Distinct Population Segment. 60-day notice of intent to sue filed on behalf of the Sierra Club, Natural Resources Defense Council, Alliance for the Wild Rockies, Humane Society of the United States, Center for Biological Diversity, Western Watersheds Project, Great Bear Foundation, and Jackson Hole Conservation Alliance, April 2,2007.
 U.S. Fish and Wildlife Service. 2005. Grizzly Bear (Ursus Horriblis). Website <http://ecos.fws.gov/docs/life_histories/A001.html> Updated 9/2005, accessed 3/2006
 Schwartz, C.C., M.A. Haroldson, K.A. Gunther, and D Moody. 2002. Distribution of Grizzly Bears in the Greater Yellowstone Ecosystem, 1990–2000. Ursus 13:203-212.
 U.S. Fish and Wildlife Service. 2000. News Release: Public to Review Strategy for Managing a Recovered Population of Grizzly Bears in the Yellowstone Ecosystem. Accessed at <http://mountain-prairie.fws.gov/PRESSREl/00-05.htm>.
 Pease, C. and D.J. Mattson. 1999. Demography of the Yellowstone grizzly bears. Ecology 80(3):957-975.
 U.S. Fish and Wildlife Service. 2005. Grizzly Bear Recovery: Yellowstone. Mountain-Prairie Region, Endangered Species Program. Website <http://mountain-prairie.fws.gov/species/mammals/grizzly/yellowstone.htm> accessed 3/2005.
 NRDC. 2005. Grizzly Bears in Peril: Plan to remove protections for Yellowstone grizzlies threatens their long-term survival. Website <http://www.nrdc.org/wildlife/animals/bears.asp> updated 8/24/2005, accessed 3/2006.
 U.S. Fish and Wildlife Service. 2007. Final Rule Designating the Greater Yellowstone Area Population of Grizzly Bears as a Distinct Population Segment; Removing the Yellowstone Distinct Population Segment of Grizzly Bears From the Federal List of Endangered and Threatened Wildlife; 90-Day Finding on a Petition To List as Endangered the Yellowstone Distinct Population Segment of Grizzly Bears. March 29, 2007 (72 FR 14866)