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Appendix C—Population Dynamics and Harvest Management we address both the scientific and conservation basis for maintaining such harvest opportunity. There are many ways that changes in the environment could affect polar bear population dynamics and harvest opportunity; we consider two scenarios here to illustrate the considerations that this Plan recommends for management of removals. In the first scenario (“habitat quantity”), reduction in the extent of the sea-ice platform may reduce access to prey and create a greater competition for resources, reducing the carrying capacity. But if some bears are able to access prey, and thus retain high survival and reproductive rates, the intrinsic population growth rate might remain the same, even if the overall population number declines. In this situation, harvest can be maintained if the total rate of human-caused removals remains at or below h (the removal rate that maintains a population above its mnpl). Annual quotas for human-caused removal would need to be reduced in proportion to the decrease in the population size, but the rate of removal could remain the same. For example, if the removal rate was 3.0% and the subpopulation size was 2,000, up to 60 bears could be taken; if the subpopulation size was only 1000, no more than 30 bears could be removed while meeting MMPA Demographic Criterion 2. This would maintain the population size at roughly the same ratio relative to changing carrying capacity, even as the carrying capacity decreased (Fig. C-5, top panel). In the second scenario (“habitat quality”), an increase in the ice-free period could, for example, increase the fasting period for all bears, reducing reproductive rates (and possibly also survival rates) across the board. In this case, the intrinsic population growth rate and population resilience would decrease. If this happened, the rate of harvest would need to decrease to meet MMPA Demographic Criterion 2. For example, if the intrinsic population growth rate is 7%, then an appropriate removal rate might be 4.5%; but if density-independent effects of climate change caused the intrinsic population growth rate to fall to only 2%, then the removal rate necessary to maintain the population size above mnpl would have to be below 1.5% (Fig. C-5, bottom panel). If the intrinsic growth rate is negative, then a population is incapable of maintaining its current size or growing. In this case, there is no removal rate that can meet MMPA Demographic Criterion 2 and ESA Demographic Criterion 4 (thus, h is 0); indeed, the population would be expected to become extirpated even in the absence of harvest. In reality, some combination of these two effects is also possible. In addition, the precise mechanisms by which climate change effects will affect polar bears are not well understood at this time. Research and monitoring will clarify these issues. But the Plan’s MMPA and ESA criteria relative to human-caused removals take into account both potential mechanisms for the effects of climate change on polar bear populations. The framework for management of human-caused removals will need to be responsive to changes in both the growth rate and carrying capacity. But, provided that the growth rate remains positive, a sustained opportunity for removal remains possible, even with a decline in carrying capacity. Provided that climate change—the threat that is driving the changes in growth rate and carrying capacity—is addressed to the extent described in this Plan, the framework established in this Plan would allow for recovery under the ESA and conservation under the MMPA. The concepts underlying this framework for management of human-caused removals are founded in harvest theory (Wade 1998, Runge et al. 2009) and can be illustrated with yield curves. Yield curves show the annual total removals and the corresponding equilibrium population size for a range of sustainable harvest rates (Fig. C-5). The peak of the yield curve is the maximum net productivity, and removals that keep the population above mnpl will fall somewhere on the right shoulder of the yield curve. An impact on habitat quantity will shrink the yield curve by reducing the carrying capacity (Fig. C-5, top panel), while otherwise allowing the same rate of removal (although the allowable quota decreases). An impact on habitat quality will flatten the yield curve by reducing the intrinsic growth rate (Fig. C-5, bottom panel), thereby reducing the allowable rate of removal as well as the allowable quota. In principle, the strategy for managing humancaused removals described above could work even if the carrying capacity decreased to low levels, but at some point, additional considerations would arise, including the increasing risk of chance events (stochasticity) on small populations and the possibility of Allee effects. Because of these considerations, the Plan recommends a three-level approach to management of human-caused removals, such that the rate of removal would decrease as the risk to the population of removal increased (red and yellow zones of Fig. 8). The framework for managing human-caused removals also needs to be sensitive to the quality of data that supports it, and to our ability to distinguish the different mechanisms that might change the dynamics. The management strategy described above is predicated on being able to monitor the population size and the number of removals on a regular basis, ideally annually. Regehr et al. (2015) and Regehr et al. (in press) show that as the sampling error and interval between monitoring events increase, removal is still possible, but the removal rate needs to be set at a more cautionary level to guard against dropping below mnpl. Further, changes driven by 102 Polar Bear Conservation Management Plan
Appendix C—Population Dynamics and Harvest Management loss of habitat quantity as opposed to loss of habitat quality will be difficult to distinguish, yet they have different effects on the sustainable level of removal (Fig. C-5). The Plan recommends that these considerations be included in the deliberations of co-management groups as they establish guidelines for removal of polar bears. Historical condition K = 70% of historical K = 50% of historical removal rate 4.5% APPENDIX C — LITERATURE CITED Hilborn, R., and C.J. Walters. 1992. Quantitative Fisheries Stock Assessment: Choice, Dynamics, and Uncertainty. Chapman and Hall, New York, New York, USA. Regehr, E.V., R.R. Wilson, K.D. Rode, M.C. Runge. 2015. Resilience and risk: a demographic model to inform conservation planning for polar bears. U.S. Geological Survey Open-File Report 2015-1029:1-56. Annual Harvest 0 Annual Harvest 3.0% 1.5% 0 Historical K Population Size removal rate 4.5% r = 0.07 r = 0.049 r = 0.035 r = 0.020 3.0% 1.5% Regehr, E.V., R.R. Wilson, K.D. Rode, M.C. Runge, H. Stern. In press. Harvesting wildlife affected by climate change: a modeling and management approach for polar bears. Journal of Applied Ecology. Runge, M.C., J.R. Sauer, M.L. Avery, B.F. Blackwell, and M.D. Koneff. 2009. Assessing allowable take of migratory birds. Journal of Wildlife Management 73:556-565. Taylor, M.K., D.P. DeMaster, F.L. Bunnell, and R.E. Schweinsburg. 1987. Modeling the sustainable harvest of female polar bears. Journal of Wildlife Management 51:811-820. Taylor, M.K., P.D. McLoughlin, and F. Messier. 2008. Sex-selective harvesting of polar bears Ursus maritimus. Wildlife Biology 14:52-60. Wade, P.R. 1998. Calculating limits to the allowable human-caused mortality of cetaceans and pinnipeds. Marine Mammal Science 14:1-37. Walters, C.J., A.M. Parma. 1996. Fixed exploitation rate strategies for coping with effects of climate change. Canadian Journal of Fisheries and Aquatic Sciences 53:148-158. 0 0 Historical K Population Size Figure C-5. The effects of reduced carrying capacity and reduced growth rate on harvest yield curves. Each graph shows the sustainable annual harvest against the corresponding equilibrium population size; three reference lines show removal rates of 4.5%, 3.0%, and 1.5%. The top panel shows three scenarios where the carrying capacity changes, but the intrinsic rate of growth remains the same (r = 0.07). The bottom panel shows four scenarios where the intrinsic rate of growth changes in the same proportion as the carrying capacity. The graphs were derived using a theta-logistic population model with θ = 5.045, which roughly corresponds to dynamics for polar bears (Regehr et al. 2015, Regehr et al. in press). Polar Bear Conservation Management Plan 103
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U.S. Fish and Wildlife Service Pola
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Polar Bear Conservation Management
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Contents Executive Summary.........
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Executive Summary Today, polar bear
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and connectivity in each recovery u
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I. Background I. BACKGROUND Polar b
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I. Background the extent and charac
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II. Conservation Strategy II. CONSE
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III. Management Goals and Criteria
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B. Conservation Criteria under the
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IV. Conservation Management Strateg
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