On July 28, 1975, under the authority of the Endangered Species Act (ESA), the U.S. Fish and Wildlife Service listed the grizzly bear (Ursus arctos horribilis) in the lower 48 states as a threatened species (U.S. Fish and Wildlife Service 1993). A primary goal of the ESA is to recover populations of species listed as threatened or endangered to self-sustaining, viable populations that no longer need protection under the act. As part of this goal, recovery parameters for the Yellowstone ecosystem grizzly bear population were established in the 1993 Grizzly Bear Recovery Plan (U.S. Fish and Wildlife Service 1993). Under this plan, three population recovery goals must be achieved before the grizzly bear population is considered recovered. Once these three recovery parameters are met, and a conservation strategy that ensures continued habitat and population protection is implemented, the grizzly bear may be delisted from threatened status.
The following population recovery parameters were designated under the 1993 Grizzly Bear Recovery Plan (U.S. Fish and Wildlife Service 1993):
Recovery Parameters
1) Females With Cubs: Adult female grizzly bears with cubs-of-the-year (COY) are the most reliable segment of the population to count (Knight et al. 1995). The number of cubs per litter as well as pelage-color combinations of different family groups aid in identifying individual adult females. Adult female grizzly bears in the Yellowstone ecosystem generally have a three-year breeding interval. Therefore, the number of different females with COY counted over a three-year period, minus the number of known adult female mortalities, gives an estimate of the number of adult females in the population (Knight and Eberhardt 1985).
Recovery Goal: To have an average of 15 adult females with COY per year on a 6-year running average both inside the recovery zone and within a 10 mile area immediately surrounding the recovery zone.
Rationale: The purpose of this goal is to estimate an average minimum population size and to demonstrate that a known minimum number of adult females are alive so that reproduction is sufficient to sustain existing levels of human-caused bear mortality in the ecosystem. The target number of 15 unduplicated females with COY must be attained as a running 6-year average. A running six-year average accounts for two breeding cycles and will allow at least two years when each adult female alive can be reported with cubs. The 6-year average number of unduplicated females with cubs is not intended to determine precise population size or trend but to derive a minimum population estimate.
Current Status: The annual average number of unduplicated females with COY (1997 - 2002, 6-year average, Table 1) = 38 (225 ¸ 6)
(M. A. Haroldson, Interagency Grizzly Bear Study Team, pers. commun.). This recovery goal is currently being achieved.
2) Distribution of Females With Cubs: To monitor grizzly bear population trends and to analyze the consequences of human activities and development on bears, grizzly bear habitat within the recovery zone has been divided into 18 habitat units. These units were delineated based on the principle that each unit should contain complete spring, summer, and fall habitat for grizzly bears. For most of the units, there is substantial evidence that the habitat contains adequate food sources to support grizzly bears in these three seasons.
Recovery Goal: To have 16 of 18 recovery zone Bear Management Units (BMU's) occupied by females with young from a running 6-year sum of observations, and, no two adjacent BMU's shall be unoccupied. Occupancy requires verified evidence (sightings or tracks) of at least one female with young (COY, yearling, or two-year-old) at least once in each of 16 BMU's over a 6-year period.
Rationale: The purpose of this parameter is to demonstrate an adequate distribution of reproductive females within the recovery zone. This parameter assumes that successful reproduction is an indicator of sufficient habitat being available to bears and provides evidence that available habitat is being managed adequately. The distribution of reproducing females throughout the recovery zone also indicates potential future occupancy of these areas because grizzly bear offspring, especially female offspring, tend to occupy habitat within or near their mothers home range after weaning.
Current Status: From 1997 through 2002, 18 of 18 BMUs were occupied with family groups (1997 - 2001, 6-year sum of observations, Table 1)
(M. A. Haroldson, Interagency Grizzly Bear Study Team, pers. commun.). This recovery goal is currently being achieved.
3) Mortality: The rate of human-caused grizzly bear mortality, especially of adult females, is a key factor influencing the potential recovery of the grizzly bear population in the Yellowstone ecosystem. Known human-caused mortalities in excess of the level sustainable at a given number of females with cubs could result in population decline, while mortalities below this level would likely result in population increase.
Recovery Goal: The 6-yr average of known human-caused mortality shall not exceed four percent of the current population estimate based on the most recent three year sum of females with cubs, minus known, adult female deaths. In addition, no more than thirty percent of the known human-caused mortality shall be females. These mortality limits cannot be exceeded during any two consecutive years for recovery to be achieved.
Rationale: The level of sustainable mortality is directly related to the number of females with cubs in the population. Harris (1985), suggests that grizzly bear populations can sustain six percent human-caused mortality without population decline. To facilitate recovery and to account for unknown, unreported, human-caused mortality, the human-caused mortality level was set at no more than four percent of the minimum population estimate, and no more than thirty percent of this mortality can be females. The most recent 3-year sum of unduplicated females with cubs can be used to calculate a minimum population estimate. This method applies the proportion of adult females in a population to the minimum number of adult females known to be alive. This method allows annual recalculation of the mortality limits based on population monitoring.
Current Status: The allowable, known, human-caused mortality limit for 2002 was 16.6 bears (4% of the population estimate of 416 bears). The population estimate is calculated by dividing the three year total of adult females (35+42+50=127) minus the three year total of known adult female mortality (3+6+4=13) by the estimated percent (0.274) of the grizzly bear population which is adult females [(127-13)/0.274 = 416]. The annual average number of known, human-caused grizzly bear deaths for 1997-2002 (Table 2) was 10.5 bears/yr (63 ¸ 6) or 2.5% of the present minimum population estimate of 416 bears (M. A. Haroldson, Interagency Grizzly Bear Study Team, pers. commun.). This is lower than the maximum limit of four percent. The 2002 allowable human-caused mortality of females was 5 bears (30% of the total allowable of 16.6 bears). The current annual average of known, human-caused adult female mortalities for 1997-2002 is 2.8 females/yr (17 ¸ 6) or 17% (2.8 ¸ 16.6). This is below the limit of thirty percent. This recovery goal is currently being achieved.
The Yellowstone ecosystem grizzly bear population met all 3 Population Recovery Parameters in 2002 (Table 3) and has met them in each of the last 5 years (1998 - 2002).
Conservation Strategy
A draft conservation strategy is completed and is currently being reviewed by the Interagency Grizzly Bear Committee. The conservation strategy will detail the habitat and population management and monitoring methods that will be used in the Primary Conservation Area if and when the population is removed from the threatened species list. State management plans for Wyoming, Montana, and Idaho have been completed and will guide grizzly bear management in areas outside of the primary conservation area if and when the population is delisted. These plans are designed to ensure that proper habitat and population management and monitoring will remain in place to ensure the continued survival of the grizzly bear without the protection of the ESA. The conservation strategy must be signed by all agencies prior to any consideration of delisting the species.
Literature Cited
Harris, B. B. 1985. Editor. Results of the workshop on grizzly bear population genetics. Sponsored by the Office of the Grizzly Bear Recovery coordinator. U.S. Fish and Wildl. Service. Missoula, Mont. 8pp.
Knight, R. R., B. M. Blanchard, and L. L. Eberhardt. 1995. Appraising status of the Yellowstone grizzly bear population by counting females with cubs-of-the-year. Wildl. Soc. Bull. 23(2):245-248.
Knight, R. R., B. M. Blanchard, and M. A. Haroldson. 1997. Yellowstone grizzly bear investigations, report of the Interagency Grizzly Bear Study Team, 1996. U.S. Dep. Inter., Natl. Park Serv. 47pp.
Knight, R. R. and L. L. Eberhardt. 1985. Population dynamics of Yellowstone grizzly bears. Ecol. 66(2):323-334.
U.S. Fish and Wildlife Service. 1993. Grizzly Bear Recovery Plan. U.S. Dep. Inter., U.S. Fish and Wildlife Service, Missoula, Mont. 181pp.
Yell 706
Information Paper No. BMO-6
Kerry A. Gunther
Bear Management Office Wildlife Biologist
Yellowstone National Park May 2003
