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Pages 111-123

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From page 111...
... In general, it is probably more useful to estimate extinction probabilities as a function of time for different population sizes than to identify some specific MVP. One conceptually simple way of relating risk to the mean extinction time is to assume that if the current ecological conditions remain stable, the probability of extinction per generation also remains stables.
From page 112...
... Efforts to jointly integrate genetic, demographic, and environmental stochasticity into spatially explicit frameworks are badly needled. · Most extinction models primarily address the mean extinction time.
From page 113...
... 1986. The practical difficulties and financial implications of endangered species breeding programs.
From page 114...
... 1987. Effective population size, genetic variation, and their use in population management.
From page 115...
... 1990. Is captive breeding an appropriate strategy for endangered species conservation?
From page 116...
... 1991. Determining minimum viable populations un(ler the Enclangere(1 Species Act, NOAA Tech.
From page 117...
... In this chapter, we build on that information to discuss how important understanding and assessing risk is in ESA decision making and the question of whether different levels of risk should apply to different decisions. Finally, because decisions regarding endangered species must always be made in the face of uncertainty regarding estimates of extinction risk and future events, we suggest ways of improving agency decisions involving risk and uncertainty.
From page 118...
... endangered species for which population size data were available, 15 (28%) had recovery goals set at or below the existing population size at the time the plan was written (Tear et al., 19931.
From page 119...
... by the act, such as listing a species as either threatened or enciangered or clecIaring a species recovered. To ensure that ESA decisions protect endangered species as they are intended to and do so in a scientifically defensible way requires objective methods for assessing risk of extinction (see Chapter 7)
From page 120...
... Another scientific consideration is the time scale for natural cycles of disturbance and regeneration in the species' habitat. Evaluating the success of endangered species management over only a portion of the natural habitat cycle runs the risk of confusing natural fluctuations in population size with adverse reactions to management.
From page 121...
... Such a system could be based on objective criteria, such as some combination of population size and number, believed to represent a specific level of extinction risk. The need to develop a listing system based on objective criteria has been recognizes!
From page 122...
... Assessing the a(lde(l risk from specific human actions is usually an even more clifficult task than estimating the overall extinction risk to a species. Individual human actions, such as developing a few acres of habitat, pose small incremental risks of extinction.
From page 123...
... a threatened species as one that is likely to become an endangered species implies that a species listed as enciangerect is at greater risk of extinction than a threatened one. The determination that a species shouIc!


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