A. Status of the Spectacled Eider - U.S. Fish and Wildlife Service

scheme (Mace and Stuart 1994) allow use of either growth rates or extinction probabilities but
they are not necessarily consistent. We know that for the same mean rate of decline,
probabilities of extinction will increase as variance in population growth rate increases.
Because the criteria use only one estimate for population growth rate, it will only match to the
probability of extinction for a single variance. According to the rate of decline criteria (the
IUCN criteria are also at r =-0.05 for the Endangered category), Spectacled Eiders on the
YKD would qualify as Endangered under any of the aforementioned criteria. In contrast, the
YKD population does not qualify as Endangered since it does not reach a 20% chance of
extinction in 38 years or 5.2 generations, given the mean generation time (Fig. 1-3). Under
the current IUCN criteria, Spectacled Eiders would qualify for the less risky. Vulnerable
category using the extinction probability criteria. Note that an earlier draft of the IUCN
criteria used a 20% chance of extinction in 10 generations and that the YKD population would
qualify under these less stringent criteria. Endangered classification would be warranted under
the recovery objective for endangered in this plan and we further discuss why in Appendix II,
which covers decision analysis.
The Unknown Population
This section strives to give guidance on setting appropriate criteria for classifying populations
into the different risk categories (i.e., endangered, threatened, delisted). The category of
endangered is examined first. As shown in Figure I-i, probability of extinction is a function
of both population growth rate and the variance in that rate. For population growth rates
ranging from -0.01 to -0.16 and population sizes ranging from 1,000 to 5,000 pairs,
simulations were conducted as follows: (1) choose variation in growth rate from a uniform
distribution ranging from standard deviation (s) = 0.07 to 0.21; (2) project population for
1,000 years. [Notethat s is used rather than CV because as the mean growth rate (r) goes to
zero, the CV goes to infinity.] The median probability of reaching critical population size was
recorded for each of 10,000 simulations (Figure 1-5).
At high rates of decline, increasing population size has little effect on the time to reach
critical. For example, assuming r = -0.15 (the estimated rate from the ground plot surveys) it
would take