169
11
Variability and Measurement Error in
Extinction Risk Analysis: The Northern
Spotted Owl on the Olympic Peninsula
Lloyd Goldwasser, Scott Ferson, and Lev Ginzburg
Introduction
The degree of endangerment of a biological species can be described on a variety
of different scales. For instance, U.S. federal law recognizes and protects a species
as endangered if it is in danger of extinction and threatened if it is likely to become
endangered within the foreseeable future (Fig. 11.1). The precise quantitative
meanings of danger, likely, foreseeable, and even extinction were left undefined
by Congress, except for the requirement that listing be based solely on the best
available scientific and commercial data.
The classification suggested by the International Union for the Conservation of
Nature and Natural Resources (IUCN 1994) recognizes a threatened species as
vulnerable, endangered, critical, or extinct (Fig. 11.2). The criterion defining
extinct status is that the species has not been sighted in the wild for an extended
period of time. Mace and Lande (1991) proposed quantitative criteria for assigning species to the vulnerable, endangered, and critical categories. Each category is
defined by a probability of extinction over some time scale: species having a 50%
or greater probability of extinction over 5 years or two generations (whichever is
longer) are designated as critical; a 20% probability of extinction over the longer
of 20 years or 10 generations defines a species as endangered; and 10% probability of extinction over 100 years defines vulnerable status. Although they recognize that risk of extinction and its time scale are both relevant to the degree of
endangerment, the extended time periods in their proposed criteria can impose
high levels of uncertainty in the probability estimates when measurement errors
are accounted for.
The analysis of extinction risk involves both the projection of population sizes
into some point in the future and the assessment of the uncertainty associated with
that projection. The choice of the time scale of the projection may strongly affect
the strength of the conclusions because, although uncertainty inevitably increases
with time, different processes contribute to uncertainty at different rates. The main
sources of uncertainty that have been incorporated in most risk analyses to date
have been the variability of the environment, which may raise or lower the vital
rates of a population from year to year in an unpredictable fashion, and demo-
11
Variability and Measurement Error in
Extinction Risk Analysis: The Northern
Spotted Owl on the Olympic Peninsula
Lloyd Goldwasser, Scott Ferson, and Lev Ginzburg
Introduction
The degree of endangerment of a biological species can be described on a variety
of different scales. For instance, U.S. federal law recognizes and protects a species
as endangered if it is in danger of extinction and threatened if it is likely to become
endangered within the foreseeable future (Fig. 11.1). The precise quantitative
meanings of danger, likely, foreseeable, and even extinction were left undefined
by Congress, except for the requirement that listing be based solely on the best
available scientific and commercial data.
The classification suggested by the International Union for the Conservation of
Nature and Natural Resources (IUCN 1994) recognizes a threatened species as
vulnerable, endangered, critical, or extinct (Fig. 11.2). The criterion defining
extinct status is that the species has not been sighted in the wild for an extended
period of time. Mace and Lande (1991) proposed quantitative criteria for assigning species to the vulnerable, endangered, and critical categories. Each category is
defined by a probability of extinction over some time scale: species having a 50%
or greater probability of extinction over 5 years or two generations (whichever is
longer) are designated as critical; a 20% probability of extinction over the longer
of 20 years or 10 generations defines a species as endangered; and 10% probability of extinction over 100 years defines vulnerable status. Although they recognize that risk of extinction and its time scale are both relevant to the degree of
endangerment, the extended time periods in their proposed criteria can impose
high levels of uncertainty in the probability estimates when measurement errors
are accounted for.
The analysis of extinction risk involves both the projection of population sizes
into some point in the future and the assessment of the uncertainty associated with
that projection. The choice of the time scale of the projection may strongly affect
the strength of the conclusions because, although uncertainty inevitably increases
with time, different processes contribute to uncertainty at different rates. The main
sources of uncertainty that have been incorporated in most risk analyses to date
have been the variability of the environment, which may raise or lower the vital
rates of a population from year to year in an unpredictable fashion, and demo-