and dangerous for both researchers and bears. In conjunction, these issues resuh in Hmited 

 sample sizes for statistical analyses. Thus, population estimates and dynamics calculations are 

 often contested. Generally, researchers do not contest the facts that grizzlies have low 

 reproductive rates and that grizzly populations are very susceptible to human impacts. Also 

 recognized is that bear numbers are very sensitive to changes in female survival rates. For 

 grizzlies in the Yellowstone area, breeding occurs in late spring with cubs bom in the den the 

 following winter. The average litter size is two cubs (range 1 -4) and females produce cubs every 

 third year. Age at first reproduction is generally 5.5 for females (range 4-7 years). Offspring 

 remain with the female 2-4 years before weaning. Brown bears are promiscuous. Females mate 

 with multiple males and may have a litter with offspring sired by different males. Males can sire 

 litters with multiple females in a breeding season. Male bears are sexually mature around 5.5 

 years of age. 



Rates of population change within the PC A are calculated using the Lotka equation. The 

 solution to the equation relies on accurate measurements of parameters such as survival rates for 

 various demographic classes of bears, age at first reproduction, rate of reproduction (a factor of 

 litter size and ft-equency of litters), and life expectancy. The calculation of these parameters 

 requires long-term monitoring of a representative subset of the population. 



For the GYE, these parameters have been measured by the Craighead team for the pre- 1970 

 population and by the IGBST after 1975. Thus, vital measurements are available for the same 

 population before and after a significant decrease following the dump closures. Current 

 information indicates the population in the PCA is increasing at 4+% per year. 



As with all other bear population in the world, it is not possible to determine definitively the 

 actual numbers of bears in the GYE. Therefore, any figure will be a result of some form of 

 estimation. Estimated values have always been a matter of contention and many different 

 estimates are found in the literature. Using garbage dump census data collected by the Craighead 

 team, and a census efficiency determined by ratios of collared to uncollared mortalities inside 

 and outside Yellowstone National Park, the pre-dump closure bear population was estimated at 

 312 animals. This value is now the widely accepted figure for the population for this period. 

 Taken in conjunction with the Craighead demographic data of 43.6% adults and 53.7% females, 

 an adult female population of 73 may be determined for that same time period. Dump census 

 data indicated that this population was growing at an annual average rate of 2.4%. 



The populafion probably decreased by a factor of two, following closure of the dumps beginning 

 in 1969. A minimum of 1 58 grizzly mortalities was recorded during and immediately after dump 

 closures, between 1969 and 1972. The majority were killed in control actions, as bears were 

 forced to exploit new sources of forage. The grizzly was listed as threatened under the ES A in 

 1975. 



An apparent decline in this bear population continued through the 1980s. Researchers modeled 

 continuing declines based on a downward trend in females with cubs-of-year (COY). They 

 calculated a population decline of 1.8%) per year and also concluded that age of first reproduction 

 had increased from 5 to 6 years, and that average litter size had declined since dump closure. 

 These changes were attributed to decreases in available food. 



17 



