Population dynamics of Virginia's hunted black bear (Ursus americanus) population

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Virginia Tech

The Cooperative Alleghany Bear Study (CABS) was initiated in 1994 by the Virginia Department of Game and Inland Fisheries (VDGIF) and the Virginia Polytechnic Institute and State University (VPI&SU) to investigate population dynamics on Virginia's hunted bear population. CABS personnel handled 746 different bears (1.5M:1F) 1,368 times on its northern study area during June 1994 to September 2000. The sex ratio for summer captures was 1.5M:1F, which differed from 1:1 (n = 1,008, Z = 6.17, P < 0.0001). Sex ratios for the summer captures ranged considerably among years (÷2 = 23.92, df = 6, P = 0.0005) and among age classes (÷2 = 119.22, df = 4, P < 0.0001), with the largest discrepancy among yearlings (5.7M:1F). The sex ratio among captured adults favored females (0.6M:1.0F ). Average age for all captured bears was 3.87 ± S.E. 0.12 years; females (n = 309) averaged 5.20 ± S.E. 0.16 and males (n = 402) averaged 2.84 ± S.E. 0.14 years (t = 10.92, df = 709, P < 0.001). Litter size averaged 2.35 cubs / litter over the 6-year period and sex ratio did not differ from 1:1 (n = 183, Z = 0.74, P = 0.461), but varied among years (÷2 = 16.61, df = 5; P = 0.005).

Three-hundred-and-seventy-six (164M, 212F) of 746 captured individuals were equipped with radio-transmitters. The ratio of radio-collared bears fluctuated from 2.6 F:1M (1998) to 8.6F:1M. We tested a radio-collar effect on survival as a covariate and found a significantly higher survival for radio-collared adult and 3-year-old females in the first 3 years of the study (c2 = 6.64, 1 df, P = 0.01). Estimates using the Kaplan-Meier staggered entry showed survival rates for females (adults = 0.993, subadults = 0.997) higher than for males (adults = 0.972, subadults = 0.917). Estimates using the mark-dead recoveries data showed survival rates of 0.840 for adult females (0.714 for 2-year-olds) and 0.769 for adult males (0.335 for 2-year-olds).

We observed 34 mortalities of radio-collared bears for which hunting mortality accounted for 85%. Four natural mortalities included a 5-year-old female and a 2-year-old male that were killed by other bears, and a 14-year-old and 2-year-old female that died of unknown causes. Among the ear-tagged sample, 2-year-old males experienced the highest mean harvest rate of 45%, with a high of 65% mortality in 1996. Among females, 2-year-olds were most vulnerable with a harvest mortality rate of 22% a year.

Population modeling indicated that population growth rate of black bears in Virginia is most sensitive to changes in adult female reproduction and survival. With current survival and reproductive estimates, simulation indicated that adult female harvest has to increase 44% from current levels to stabilize population growth.

Population size estimates using Bowden's estimate for mark-resight data for a 100 km2 sub-area on the northern study area ranged from 83-131 animals during 1998-2000. When adjusted for the proportion of time radio-collared bears spent on the study area population estimates fell to 63-96 bears. Using the Lincoln-Petersen estimate with Chapman's modification, black bear population estimates for the northern study area ranged from 582-1,026 animals during 1994-1999 on the 860 km2 area.

Visitation rates to bait station sites correlated well with changes in population size estimates (n = 5, r = 0.97, P = 0.007). Black bear harvest in general was weakly correlated to change in population size (n = 6, r = 0.49, P = 0.328), however, archery harvest was highly correlated (n = 6, r = 0.95, P = 0.002). The monitoring indices showed all showed the same trends. We recommend a combination of them rather than relying on only a single index for monitoring Virginia's black bear population.

During winters 1995–2001, located 215 dens of radio-collared bears; 68% were in trees. Ground dens used by bears included nests in laurel thickets, excavations, brush piles, and rock cavities. The proportion of bears using tree dens did not differ between our 2 study areas (n = 203, ÷ 2 = 1.63, 1 df, P = 0.202), the proportion of females using tree dens (65%, n = 127) was greater than that of males (33%, n= 15; ÷ 2 = 10.69, 1 df, P < 0.001) on the northern study area. Sex and age were significant factors in determining the type of den a bear selected. Twenty-six of 66 individual bears handled for 2–6 consecutive years consistently used tree dens, 8 were faithful to rock cavities, and only 4 regularly used ground dens for denning. Twenty-eight bears (42.4%) switched den types over the 6–year period, primarily from tree dens to rock cavities.

survival, denning, mortality, black bear, management, Modeling, density, Reproduction