Grizzly Bear Population Ecology and Monitoring, Denali National Park and Preserve, Alaska

Keay, J. A. 2001. Grizzly bear population ecology and monitoring, Denali National Park and Preserve, Alaska. U.S. Geological Survey, Alaska Biological Science Center. Final project report, 20pp.

ABSTRACT: The dynamics and ecology of a naturally regulated grizzly bear population were studied in Denali National Park and Preserve, Alaska, from 1991 through 1998. Bears were captured by helicopter darting and fitted with radio-transmitters containing mortality sensors. Bears were located at least monthly during the non-denning period. Density was determined by capture-mark-resight procedures. Serum antibody prevalence for infectious canine hepatitis, canine distemper, and leptospirosis were low indicating disease was not an important factor affecting population dynamics. Grizzly bear density, during fall 1995, was 27.1 independent bears/1000 km2 (95% C.I. = 25.1 – 30.2) with 61% of the population marked. Adjusting survey area to reflect forage-producing habitat for more direct comparison with adjacent studies indicated 34.7 bears/1000 km2 (95% C.I. = 32.2 to 38.7). Sight-ability of both females and males was high during the fall survey (60.7 % and 86.2%, respectively). The male age structure was unimodal, was 10% subadults, and included adults up to 16 years of age. The female age structure was strongly bimodal and 20% were subadults. The female segment experienced a shift in age distribution between 1991 and 1997; each mode advanced 6 years but remained intact. There were 14% subadults in the 1991 female population and 3% in the 1997 population. Cubs-of-the-year experienced the lowest annual survival rates of all age classes at 0.341. Yearling survival was 0.455 and that of dependent 2-year-olds was 0.785. Subadult survival was 1.000 for females and 0.943 for males. Adult survival rates were 0.970 for females and 0.983 for males.
Average age of first reproduction was 7.6 years and varied from 6 to 10. Litter size of cubs-of-the-year at the time of den emergence averaged 2.1. Dependent bears separated from their mothers at an average age of 2.9 years. Reproductive rates were 0.091 for 6- year-olds, 0.307 for bears 7 to 9 years old, 0.384 for bears 10 to 26, and 0.071 for bears 27 to 30 years of age. Bears averaged 23% to 27% body fat prior to den entrance in September. Females and subadult males had less than 10% body fat following den emergence in May. Adult males were 15% fat during May. Females lost 30% to 35% of total body mass and 18% to 20% of lean body mass during winter hibernation. High survival rates of independent bears and lack of human interference during the past 80 years suggests the Denali population is likely at carrying capacity. High dependent bear mortality coupled with periodic recruitment may allow the population to oscillate mildly but, in the long term, is likely stable. Implications for long-term monitoring are discussed.