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Ecology of Bobcats Relative to Exploitation and a Prey Decline in Southeastern Idaho

Steven T. Knick
Wildlife Monographs
No. 108, Ecology of Bobcats Relative to Exploitation and a Prey Decline in Southeastern Idaho (Apr., 1990), pp. 3-42
Published by: Wiley on behalf of the Wildlife Society
Stable URL: http://www.jstor.org/stable/3830671
Page Count: 40
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Ecology of Bobcats Relative to Exploitation and a Prey Decline in Southeastern Idaho
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Abstract

Bobcat (Felis rufus) population responses to exploitation and a decline in prey populations were studied in southeastern Idaho from January 1982 through December 1985. Reproduction and death rates were compared between a harvested population in the Box Canyon region of the Big Lost River and an unharvested population on the Idaho National Engineering Laboratory (INEL). No radio-marked yearling females in either population (n = 12) raised kittens even though some were physiologically capable, as indicated by placental scars. The harvested population contained a higher proportion of yearlings and a lower proportion of adults than the unharvested population. Recruitment to the harvested population was primarily by immigration because yearlings did not successfully rear young and the population contained few breeding adults. Annual survival rates were lower for the harvested population (0.49) compared to the unharvested population (0.67) because of high mortality during the harvest period (survival = 0.61 vs. 0.87, respectively). Populations of jackrabbits (Lepus californicus) declined during the study at an annual finite rate (λ) of 0.08 to 0.53 after a cyclic peak in 1981; cottontail rabbits (Sylvilagus nuttallii) also declined (λ = 0.09-0.73) between 1983 and 1985. Bobcats ate small mammals during summers of the lagomorph decline (frequency of occurrence = 56%) but continued to eat cottontail rabbits (frequency of occurrence = 56-63%) during winters. As lagomorph populations declined, the average size of home ranges increased and by 1984-85 was 5 times larger than in 1982-83. Bobcats made extraterritorial forays during winters of the decline to areas of jackrabbit aggregations or to lava flows. In an apparent effort to conserve energy, denning females with dependent young hunted closer to the den (x̄ = 1.1 vs. 2.3 km) and traveled shorter total distances (x̄ = 2.7 vs. 5.3 km) when small rodents were the primary prey (frequency of occurrence = 56%) in 1983 compared to 1982 when jackrabbits and cottontail rabbits were 90% (frequency of occurrence) of the diet. When lagomorphs were scarce, bobcat numbers declined at an annual finite rate of 0.52 between 1982 and 1985 in both study areas because fewer females raised litters. Adult survival ($S_{{\rm x}}$) during summers after the lagomorph decline was 0.78-1.00, but spring 1985 survival on the INEL was 0.22 because of the starvation of 2 radio-collared adults after a severe winter. Computer simulations based on social organization and population dynamics of female bobcats were used to determine yield at different harvest intensities. Recommended harvest rates were <20% of the fall population. Increases in mortality of productive females that orphaned kittens (which subsequently died) had a greater impact on yield than did increases in kitten mortality. The predicted size of refugia needed to maintain a harvested population must be large enough to completely enclose 3-5 bobcat territories.

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