104 



Miiiiinnil\ — Our Living Resources 



Table. Recent population and 

 trend estimates for areas in the 

 contiguous United States occupied 

 or potentially occupied by griz/ly 

 bears (NCDE — Northern 

 Continental Divide ecosystem, 

 GYE — Greater Yellowstone 

 ecosystem, CYE — Cabinet- Yaak 

 ecosystem, C — Cabinet portion 

 only (959c confidence interval), 

 SE — Selkirk ecosystem, NCE — 

 North Cascades ecosystem. BE — 

 Bitterroot ecosystem, SJE — San 

 Juan ecosvstem). 



Status and Trends 



Recent reseaicli in the Nonhem 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 other- 

 wise 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 ani- 

 mals inhabit each of the Cabinet- Yaak. Selkirk, 

 and North Cascade populations. 



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 individu- 

 als can turn a growing population into a declin- 

 ing one (Table). Current best estimates for the 

 Northern Continental Di\ide and Yellowstone 



Minitnuin 

 population estimate 



Population estitriate 

 assuming 60% sightability^ 



Area Average (mean) Range (95% CI) Average (mean) Range (95% CI) Trend estimate'' Long-term viability 



aBased on results from Aune and Kasworm (1989) suggesting that 60% of adult females were observed in tfieir study 

 area. Accordingly, minimum population estimates are divided by 0.6 



''Expressed as an increasing (-i-) or decreasing (-) population, wfiere available in terms of per capita rate of increase or 

 decrease per year. A "?" indicates populations for whicfi ttiere are no substantive or reliable estimates of trend. 

 cData from USFWS (1993) and IVIFWP (1993) 



''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 lor the corresponding 3-year penod. divided by 284 (the 

 assumed proportion of adult females in the population) for f^CDE and GYE. 

 eprom Keating (1986). Aune and Kasworm (1989). McLellan (1969). and MFWP (1993). 



'Using 22 8% adult females in the population and assuming all adult sex ratio, based on the upper 95% confidence inter- 

 val for estimates of percentage of adults in gnzzly bear populations from the NCDE (MFWP 1993). 

 SData from Knight etal (1993) 

 ''From Knight etal. (1988). 



'Data for 1986-90 from f^FWP (1993). Minimum population estimate is for the Cabinet portion only. Data from USFWS 

 (1993). 



iThe lower confidence interval = 0, but 9 bears were radio-marked and known to be alive. 

 l<From Wielgus (1993). 

 'Including bears in adjacent Canada. 



■"From R.B. Wielgus, University of British Columbia. Vancouver, personal communication (1994). 

 "From Almack etal, (1991). 



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 esti- 

 mate of trends is difficult because of large and 

 diverse study areas, small samples, and poten- 

 tially biased observations. 



Long-term viability of a population or 

 species is achieved when there are enough ani- 

 mals and sufficient secure and productive habi- 

 tat 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. arclos) and grizzly bears world- 

 wide, 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 popula- 

 tions also occupied areas less than 10.000 km- 

 (3.900 mi-) 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 occu- 

 pying areas larger than 26.000 km- (10.500 

 mi-) in 1920 survived to the present. The 

 Selkirk and Cabinet- Yaak ecosystems are about 

 5,200 km- (about 2,000 nfi-) and the remaining 

 ecosystems are about 24,600-29,500 km- 

 (about 9,500-11,400 mi-). We expect popula- 

 tions with current ranges less than 29,500 km- 

 ( 1 1,400 mi-) to be at substantially greater risk 

 of extinction. 



Exchange of genes among individuals and 

 populations is also important to survival of pop- 

 ulations, 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 pop- 

 ulation 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 ani- 

 mals. 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 



