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Demographic Response of Black Bears at Cold Lake,
Alberta, to the Removal of Adult Males

Introduction


Natural regulation of bear populations ultimately involves density-dependent processes (Taylor 1994). These processes, however, are poorly understood and a subject of continuing controversy. Some have suggested that adult males regulate bear populations by killing or evicting younger males and, to a lesser extent, females (Bunnell and Tait 1981, McCullough 1981, LeCount 1993). If so, removing adult males by hunting might increase survival rates of subadult and female bears. Others contend that evidence of density dependence is lacking for black bears and warn against harvest strategies that anticipate increases in survival or reproduction when populations are reduced (Miller 1990a,b; Garshelis 1994; Taylor 1994).

Interpretations of an experiment begun by Kemp (1972, 1976) and continued by Young and Ruff (1982) are central to this ongoing debate (Garshelis 1994). To determine whether adult males mediated density-dependent changes in demographic parameters of a black bear population at Cold Lake, Alberta, Kemp (1976) and Young and Ruff (1982) used mark-recapture to estimate population size before (1968-70), during (1971-72), and after (1973-75) an experimental removal of adult males. The population was thought to be unexploited and naturally regulated prior to the removal (Kemp 1976). After 4 years of relative stability before the removal, population estimates more than doubled from 1971 to 1973 and remained relatively high through 1975. Moreover, disproportionately large numbers of subadult males were captured annually during 1972-75. Kemp (1976) and Young and Ruff (1982) concluded that adult males regulated population density by controlling recruitment of subadults. These results are often cited as evidence of density dependence in black bears (Garshelis 1994).

Garshelis (1994) subsequently identified potential sources of bias in population estimates of Kemp (1976) and Young and Ruff (1982): Perceived changes in population might have resulted from (1) age- and sex-related differences in recapture probabilities, combined with the removal of adult males from the population, and (2) increased numbers of transient bears passing through the area when male density was low. Though Kemp (1976) inferred an increase in settling rates of subadults from increased recapture rates following the removal, Garshelis (1994) pointed out that these high rates were observed in only 1 year; thus the data did not support inferences about changes in emigration, immigration, or settling rates. Garshelis (1994) concluded that Kemp (1976) and Young and Ruff (1982) reached unjustified and possibly erroneous conclusions about changes in the Cold Lake black bear population and called for a re-analysis of their data.

We analyzed data first presented by Kemp (1972, 1976) and Young and Ruff (1982), unpublished mark-recapture records for 1976 and 1977, and unpublished survival records for bears radiomarked during 1974-77. Objectives of this paper are to (1) use improved methods of mark-recapture analysis to reassess evidence for an increase in bear densities following the removal of adult males from the CLSA; (2) estimate mortality rates, identify causes of death, and reconsider the conjecture that bear densities were naturally regulated prior to the removal; (3) determine whether numbers of subadult males captured on the CLSA were indicative of trends in numbers using the area; and (4) reinterpret the Cold Lake study with respect to density-dependent population regulation in black bears.


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