Estimating Black Bear Population Size, Growth Rate, and Minimum Viable Population Using Bait Station Surveys and Mark-Recapture Methods
O'Neill, Deborah M.
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We initiated bait station surveys for black bears in southwestern Virginia in 1999. Bait station surveys are intended to be used as an index to follow bear population trend over time. We compared the bait station visitation (black bear visitation) to black bear harvest and mast surveys 1999 = 2002. The mean bait station visitation rate during 1999 - 2002 was 15.3% (SE = 2.89, n = 4). The number of bears harvested in the 3 counties that also had bait station surveys was 48 (31 males, 17 females), 59 (44 males, 15 females), 45 (32 males, 13 females), and 43 (26 males, 17 females) in 1999, 2000, 2001, and 2002, respectively. Harvest of males and females differed (n = 2, F = 19.44, df = 1, P = 0.0045). Bait station visitation and female harvest had a strong functional relationship with a negative slope (n = 4, r = -0.78, P = 0.22). The strongest relationship was between male harvest and total harvest (n = 4, r = 0.97, P = 0.03). Mean index to mast production for 1999 - 2002 was 2.3 (range 1.5 - 3.1), 2.7 (range 1.8 - 3.4), 2.3 (range 1.6 - 3.6), and 1.6 (range 1.2 - 2.4), respectively. The overall summary for mast production for the same years was described as fair, good, fair, and poor to fair. Mast production was significantly different between years (n = 4, F = 3.44, df = 3, P = 0.0326), and soft and hard mast production appeared to be above average in 2000. This corresponded with the lowest visitation (10.2%) of the 4 years. There was no correlation between bait station visitation and mast production (n = 4, r = 0.11, P = 0.87). Since 1998, the annual bear harvest in Virginia has exceeded 900 individuals (with the exception of 824 in 2001), and peaked in 2000 when 1,000 bears were harvested. Though harvest rates were high, a reliable population estimate did not exist for black bears in Virginia. We estimated population size, growth rate, and minimum viable population size using data collected between 1995-2000. We used Jolly-Seber, direct recovery, and minimum population size methods to estimate population size. The Jolly-Seber method estimate of adult female density was 0.23-0.64 bears/km2, and 0.01 bears/km2 for adult males. We estimated a density of 0.09-0.23 bears/km2 for all sex and age classes using direct recovery data. Using minimum population size, we found adult female density was higher than any other sex or age class (n = 6, t = 2.02, df = 40, P < 0.0001) with an average density of 0.055 adult females/km2. We used mark-recapture data collected from 148 individual bears (96 males:52 females) captured 270 times in program MARK to estimate survival using recapture, dead recovery, and Burnham's combined models. Adult females had the highest survival rate of 0.84-0.86, while yearling males had the lowest with 0.35. Using direct recovery data, adult females again had the highest survival rate with 0.93 (0.83-1.0) and 3-year old males had the lowest with 0.59 (0.35-0.83). We estimated growth rate using population estimates from Jolly-Seber, direct recoveries, and minimum population size methods. The lowest growth rate estimated was for all females (ages lumped) using minimum population size data (Î»=0.82). Direct recovery data for all bears (sex and age lumped) during 1995 - 2000 showed the highest positive annual growth rate (Î» = 1.24). We developed a population model using Mathcad 8 Professional to determine population growth rate, MVP, and harvest effects for an exploited black bear population in southwestern Virginia. We used data collected during the CABS study (1995 - 2000) in the model including population estimates derived from direct recovery data, age and sex specific survival rates, and cub sex ratios. When we used actual population values in the model, the bear population in southwestern Virginia did not go extinct in 100 years (l = 1.03, r = 0.03). When we reduced adult female survival from 0.94 to 0.89, the probability of extinction in 100 years was 3.0% and l = 0.99 (r = -0.01; Table 3.2). When the survival was reduced by an additional 0.01 to 0.88, the probability of extinction increased to 13.0% (l = 0.99, r = -0.01). Growth rate and extinction probabilities were very sensitive to adult female survival rates. Two-year old and 3-year old females did not impact extinction probabilities and growth rates as much as adult females. Their survival could be decreased by 44.0%, and still be less than the 5.0% extinction probability.
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