Grizzly Bears

Grizzly Bears
		by *David J. Mattson* National Biological Service *R. Gerald Wright* National Biological Service *Katherine C. Kendall* National Biological Service *Clifford J. Martinka* National Biological Service

Fig. 1. Approximate distribution of grizzly bears in 1850 compared to 1920 (a; Merriam 1922) and 1970-90 (b). Local extinction dates, by state, appear in (a). Populations identified in (b) are NCE -- North Cascades ecosystem, SE -- Selkirk ecosystem, CYE -- Cabinet-Yaak ecosystem, BE -- Bitterroot ecosystem, NCDE -- Northern Continental Divide ecosystem, GYE -- Greater Yellowstone ecosystem. As indicated in (b), a grizzly was killed in the San Juan Mountains of southern Colorado in 1979.

Grizzly bears (Ursus arctos) once roamed over most of the western United States from the high plains to the Pacific coast (Fig. 1). In the Great Plains, they seem to have favored areas near rivers and streams, where conflict with humans was also likely. These grassland grizzlies also probably spent considerable time searching out and consuming bison that died from drowning, birthing, or winter starvation, and so were undoubtedly affected by the elimination of bison from most of the Great Plains in the late 1800's. They are potential competitors for most foods valued by humans, including domesticated livestock and agricultural crops, and under certain limited conditions are also a potential threat to human safety. For these and other reasons, grizzly bears in the United States were vigorously sought out and killed by European settlers in the 1800's and early 1900's.

Between 1850 and 1920 grizzlies were eliminated from 95% of their original range, with extirpation occurring earliest on the Great Plains and later in remote mountainous areas (Fig. 1a). Unregulated killing of grizzlies continued in most places through the 1950's and resulted in a further 52% decline in their range between 1920 and 1970 (Fig. 1b). Grizzlies survived this last period of slaughter only in remote wilderness areas larger than 26,000 km² (10,000 mi²). Altogether, grizzly bears were eliminated from 98% of their original range in the contiguous United States during a 100-year period.

Because of this dramatic decline and the uncertain status of grizzlies in areas where they had survived, their populations in the contiguous United States were listed as threatened under the Endangered Species Act in 1975. High levels of grizzly bear mortality in the Yellowstone area during the early 1970's were also a major impetus for this listing. Grizzly bears persist as identifiable populations in five areas (Fig. 1b): the Northern Continental Divide, Greater Yellowstone, Cabinet-Yaak, Selkirk, and North Cascade ecosystems. All these populations except Yellowstone's have some connection with grizzlies in southern Canada, although the current status and future prospects of Canadian bears are subject to debate. The U.S. portions of these five populations exist in designated recovery areas, where they receive full protection of the Endangered Species Act.
Grizzlies potentially occur in two other areas: the San Juan Mountains of southern Colorado and the Bitterroot ecosystem of Idaho and Montana. There are no plans for augmenting or recovering grizzlies in the San Juan Mountains, and serious consideration is being given to reintroducing grizzlies into the Bitterroots as an "experimental nonessential" population.
Status and Trends
Recent research in the Northern Continental Divide, Yellowstone, and Selkirk ecosystems has produced growth and size estimates for these grizzly bear populations. Study results, however, have been compromised by either small sample sizes, incomplete coverage, or possibly unrepresentative samples. These types of studies are also relatively expensive and require the capture and radio tagging of bears, although without the aid of radio tagging, it is even more difficult to directly count or otherwise monitor grizzly bear populations in their extensive, typically forested, ranges.

Because of these difficulties, we have only rough estimates of size for U.S. grizzly bear populations. Many grizzlies exist only in the Northern Continental Divide and Yellowstone ecosystems. We can be confident that there are at least 175 bears in the Northern Continental Divide ecosystem and 142 in the Yellowstone ecosystem, and a minimum of about 360 in the entire contiguous United States (Table). On the other hand, it is unlikely that more than 75 animals inhabit each of the Cabinet-Yaak, Selkirk, and North Cascade populations.

Table. Recent population and trend estimates for areas in the contiguous United States occupied or potentially occupied by grizzly bears (NCDE -- Northern Continental Divide ecosystem, GYE -- Greater Yellowstone ecosystem, CYE -- Cabinet-Yaak ecosystem, C -- Cabinet portion only (95% confidence interval), SE -- Selkirk ecosystem, NCE -- North Cascades ecosystem, BE -- Bitterroot ecosystem, SJE -- San Juan ecosystem).

	Minimum population estimate		Population estimate assuming 60% sightability^a
Area	Average (mean)	Range (95% CI)	Average (mean)	Range (95% CI)	Trend estimate^b	Long-term viability
NCDE^c	242^d	175-308	404	293-514	Stable to slightly +^e	?
NCDE^f	302^d	219-384	502	365-640
GYE^g	197^d	142-252	329	237-420	ca. +0.01^h	?
CYEⁱ	<15		29	9-55 ^j	?	Not viable
SE^k	26-36 ^l		?		0 to +0.02^k, recently-^m	Not viable
NCEⁿ	10-20 ^l		<50^l		?	Not viable
BE	0		Possible presence		?	Not viable
SJE	0		Possible presence		?	Not viable


^a	Based on results from Aune and Kasworm (1989) suggesting that 60% of adult females were observed in their study area. Accordingly, minimum population estimates are divided by 0.6.
^b	Expressed as an increasing (+) or decreasing (-) population, where available in terms of per capita rate of increase or decrease per year. A "?" indicates populations for which there are no substantive or reliable estimates of trend.
^c	Data from USFWS (1993) and MFWP (1993).
^d	Mean and 95% confidence intervals for 3-year sums of "unduplicated" adult females observed in an area (n = 4 years, except for CYE n = 3 years) minus known adult female mortality for the corresponding 3-year period, divided by 0.284 (the assumed proportion of adult females in the population) for NCDE and GYE.
^e	From Keating (1986), Aune and Kasworm (1989), McLellan (1989), and MFWP (1993).
^f	Using 22.8% adult females in the population and assuming a 1:1 adult sex ratio, based on the upper 95% confidence interval for estimates of percentage of adults in grizzly bear populations from the NCDE (MFWP 1993).
^g	Data from Knight et al. (1993).
^h	From Knight et al. (1988).
ⁱ	Data for 1986-90 from MFWP (1993). Minimum population estimate is for the Cabinet portion only. Data from USFWS (1993).
^j	The lower confidence interval = 0, but 9 bears were radio-marked and known to be alive.
^k	From Wielgus (1993).
^l	Including bears in adjacent Canada.
^m	From R.B. Wielgus, (University of British Columbia, Vancouver, personal communication, 1994).
ⁿ	From Almack et al. (1991).

We have few reliable estimates of population trends for the same reasons that we have few reliable estimates of population size. In most cases we do not have any information on trends or the populations are so small (as in the Selkirks) that the death of only a few individuals can turn a growing population into a declining one (Table). Current best estimates for the Northern Continental Divide and Yellowstone areas suggest that these largest populations have been stable or slightly increasing in recent years. Even for these relatively well-studied populations, however, obtaining a reliable estimate of trends is difficult because of large and diverse study areas, small samples, and potentially biased observations.
Long-term viability of a population or species is achieved when there are enough animals and sufficient secure and productive habitat to ensure that the population or species will survive for the indefinite future. Certainly, direct mortality that accompanied the arrival of European settlers had catastropic consequences for grizzly bears. Other catastrophes related to disease, climate change, and changes in human values could yet be visited upon grizzlies.
Viability analysis is not an exact science, yet there are some rules of thumb that can be used to identify populations at substantially greater risk of extinction than others. For example, among existing isolated populations of brown bears (also U. arctos) and grizzly bears worldwide, only populations that were reduced to no fewer than about 450 bears responded with rapid growth when given protection. Conversely, even with protection, populations smaller than 200 continued to decline (Mattson and Reid 1991). All of these smaller populations also occupied areas less than 10,000 km² (3,900 mi²) at the time they were given legal protection. This relationship between range size and vulnerability is consistent with the fact that only North American grizzly populations occupying areas larger than 26,000 km² (10,500 mi²) in 1920 survived to the present. The Selkirk and Cabinet-Yaak ecosystems are about 5,200 km² (about 2,000 mi²) and the remaining ecosystems are about 24,600-29,500 km² (about 9,500-11,400 mi²). We expect populations with current ranges less than 29,500 km² (11,400 mi²) to be at substantially greater risk of extinction.
Exchange of genes among individuals and populations is also important to survival of populations. Allendorf et al. (1991) estimated that populations of about 500 interbreeding grizzlies may be required to maintain normal levels of genetic diversity. This genetically effective population size equates to total population sizes of around 2,000 because not all bears breed. Given that the maximum documented movement of grizzly bears away from their mother's range is 45-105 km (28-65 mi; Blanchard and Knight 1991), it is unlikely that populations separated by a greater distance exchange breeding animals. Furthermore, bear movement across these gaps is entirely dependent upon their surviving often hostile conditions.
No grizzly bear population in the contiguous United States could be considered robust by our rules of thumb for population viability. Clearly, the small populations of the North Cascade, Selkirk, and Bitterroot ecosystems, the San Juan Mountains, and the U.S. portion of the Cabinet-Yaak ecosystem are not viable. Although the North Cascade ecosystem is close to 26,000 km² (10,000 mi²), its prospects are compromised by its isolation, even from populations in Canada. Similarly, although the Cabinet-Yaak and Selkirk populations can potentially receive bears that have dispersed from other populations, their 5,200-km² (2,000-mi²) ranges are within the size boundaries of many U.S. populations that went extinct between 1920 and 1970 and are similar to those of European populations that appear to be declining toward extinction.
Prospects for the larger Northern Continental Divide and Greater Yellowstone populations are better but still uncertain. The Yellowstone population is probably no larger than 420 animals (Table) and is very isolated, making its long-term status tenuous. The Northern Continental Divide population probably has the best prospects because it is the largest population, in the largest area, and within the range of movement of other grizzly bear populations. Nonetheless, even this population is near the thresholds of 450 animals and the 26,000-km² (10,000-mi²) range size historically associated with persistence of grizzlies in the United States and Europe.
The prognosis for the Selkirk, Cabinet-Yaak, and Northern Continental Divide populations might be improved if their connections with Canadian grizzly populations were considered. These southern Canadian grizzlies, however, do not have protection comparable to the U.S. Endangered Species Act and, outside of national parks, they are all hunted. There is also serious debate over the status of Canadian grizzly populations, especially in southwest Alberta and the northern Selkirks. Thus, there is no evidence that Canadian grizzlies will guarantee the long-term survival of neighboring U.S. populations.
Implications
Since listing of the species under the Endangered Species Act in 1975, populations have probably stabilized in the Yellowstone and Northern Continental Divide ecosystems. Little if any of the former range has been reoccupied, however, and five of seven potential or existing populations do not have optimistic prospects, and even the two largest populations remain at risk.
About 88% of all grizzly bears that have been studied and died within the United States during the last 20 years were killed by humans, both legally and illegally. Humans remain the almost exclusive source of grizzly mortality, despite protection under the Endangered Species Act. Improved protection of these populations is accordingly dependent upon reducing the frequency of contact between grizzly bears and humans, primarily by managing levels of human activity in areas where we want grizzly bears to survive.
The Selkirk and Cabinet-Yaak grizzly bear populations may also need to be augmented by management if they are to survive beyond the next 100 years, whereas the North Cascade, Bitterroot, and San Juan populations will require the import of bears from elsewhere if they are to grow or persist even in the short term. The Yellowstone and Northern Continental Divide populations will need at least existing levels of protection, along with reliable monitoring and timely management.
		For further information: David J. Mattson National Biological Service Cooperative Park Studies Unit Department of Fish and Wildlife Resources University of Idaho Moscow, ID 83843

References
Allendorf, F.W., R.B. Harris, and L.H. Metzgar. 1991. Estimation of effective population size of grizzly bears by computer simulation. Pages 650-654 in E.C. Dudley, ed. The unity of evolutionary biology: Proceedings of the Fourth International Congress of Systematic and Evolutionary Biology. Vol. 2. Dioscorides Press, Portland, OR. Almack, J.A., W.L. Gaines, P.H. Morrison, J.R. Eby, R.H. Naney, G.F. Wooten, S.H. Fitkin, and E.R. Garcia. 1991. North Cascades grizzly bear ecosystem evaluation: final report. Interagency Grizzly Bear Committee, Denver, CO. 146 pp. Aune, K., and W. Kasworm. 1989. Final report: East Front grizzly studies. Montana Department of Fish, Wildlife, and Parks, Helena. 332 pp. Blanchard, B.M., and R.R. Knight. 1991. Movements of Yellowstone grizzly bears. Biological Conservation 58:41-67. Keating, K.A. 1986. Historical grizzly bear trends in Glacier National Park, Montana. Wildlife Society Bull. 14:83-87. Knight, R., J. Beecham, B. Blanchard, L. Eberhardt, L. Metzgar, C. Servheen, and J. Talbott. 1988. Report of the Yellowstone grizzly bear population task force: equivalent population size for 45 adult females. Interagency Grizzly Bear Committee, Denver, CO. 8 pp.	Knight, R.R., B.M. Blanchard, and D.J. Mattson. 1993. Yellowstone grizzly bear investigations: annual report of the Interagency Study Team 1992. National Park Service, Bozeman, MT. 26 pp. Mattson, D.J., and M.W. Reid. 1991. Conservation of the Yellowstone grizzly bear. Conservation Biology 5:364-372. McLellan, B.N. 1989. Dynamics of a grizzly bear population during a period of industrial resource extraction. 3. Natality and rate of increase. Canadian Journal of Zoology 67:1865-1868. Merriam, C.H. 1922. Distribution of grizzly bears in U.S. Outdoor Life (December):405-406. MFWP. 1993. Five year update of the programmatic environmental impact statement: the grizzly bear in northwestern Montana, 1986-1990. Montana Department of Fish, Wildlife, and Parks, Helena. 228 pp. USFWS. 1993. Grizzly bear recovery plan. U.S. Fish and Wildlife Service, Missoula, MT. 181 pp. Wielgus, R.B. 1993. Causes and consequences of sexual habitat segregation in grizzly bears. Ph.D. thesis, University of British Columbia, Vancouver. 88 pp.

References

Allendorf, F.W., R.B. Harris, and L.H. Metzgar. 1991. Estimation of effective population size of grizzly bears by computer simulation. Pages 650-654 in E.C. Dudley, ed. The unity of evolutionary biology: Proceedings of the Fourth International Congress of Systematic and Evolutionary Biology. Vol. 2. Dioscorides Press, Portland, OR.

Almack, J.A., W.L. Gaines, P.H. Morrison, J.R. Eby, R.H. Naney, G.F. Wooten, S.H. Fitkin, and E.R. Garcia. 1991. North Cascades grizzly bear ecosystem evaluation: final report. Interagency Grizzly Bear Committee, Denver, CO. 146 pp.

Aune, K., and W. Kasworm. 1989. Final report: East Front grizzly studies. Montana Department of Fish, Wildlife, and Parks, Helena. 332 pp.

Blanchard, B.M., and R.R. Knight. 1991. Movements of Yellowstone grizzly bears. Biological Conservation 58:41-67.

Keating, K.A. 1986. Historical grizzly bear trends in Glacier National Park, Montana. Wildlife Society Bull. 14:83-87.

Knight, R., J. Beecham, B. Blanchard, L. Eberhardt, L. Metzgar, C. Servheen, and J. Talbott. 1988. Report of the Yellowstone grizzly bear population task force: equivalent population size for 45 adult females. Interagency Grizzly Bear Committee, Denver, CO. 8 pp.

Knight, R.R., B.M. Blanchard, and D.J. Mattson. 1993. Yellowstone grizzly bear investigations: annual report of the Interagency Study Team 1992. National Park Service, Bozeman, MT. 26 pp.

Mattson, D.J., and M.W. Reid. 1991. Conservation of the Yellowstone grizzly bear. Conservation Biology 5:364-372.

McLellan, B.N. 1989. Dynamics of a grizzly bear population during a period of industrial resource extraction. 3. Natality and rate of increase. Canadian Journal of Zoology 67:1865-1868.

Merriam, C.H. 1922. Distribution of grizzly bears in U.S. Outdoor Life (December):405-406.

MFWP. 1993. Five year update of the programmatic environmental impact statement: the grizzly bear in northwestern Montana, 1986-1990. Montana Department of Fish, Wildlife, and Parks, Helena. 228 pp.

USFWS. 1993. Grizzly bear recovery plan. U.S. Fish and Wildlife Service, Missoula, MT. 181 pp.

Wielgus, R.B. 1993. Causes and consequences of sexual habitat segregation in grizzly bears. Ph.D. thesis, University of British Columbia, Vancouver. 88 pp.

Grizzly Bears

Status and Trends

Implications