Extinction Risks
In
1997, the western stock of Steller sea lions in Alaska was listed as
endangered under the United States Endangered Species Act. This
listing was due in part to a modeling study by researchers at the National
Marine Fisheries Service (NMFS), which suggested that the stock would
reach a critically low size in the near future if the observed declining
trend in the number of Steller sea lions in the western stock continued.
The Steller Sea Lion Recovery Team is now revisiting
the status of the species in light of information that has been collected
during
the past 5-10 years. Consortium Researcher Arliss Winship is working
in collaboration with the Recovery Team to develop a new model that
will be used to assess the risk of extinction of the Steller sea lion.
This model will also be used to help establish criteria to determine
when the status of the species has changed.
Observed counts (dots) and number of
sea lions predicted by the model (lines) at a hypothetical
Steller sea lion breeding
area (rookery) from the late 1970s through the present and
into the future. In this example, the status of the population
in the future, as predicted by the PVA, ranges from moderate
recovery to extinction within 30 years. |
|
Winship’s
computer model is an age-structured simulation of Steller sea
lion populations, which quantifies the probability of them becoming
extinct over a specified time period. This type of model is referred
to as a population viability analysis
(PVA).
As
shown
by the
graph, the model first tries to replicate the observed count
data (dots) to determine what the carrying capacity and vital
rates (birth and survival) were during the past 25 years. It
then simulates
the population into the future (dashed lines) under different
assumptions about what vital rates and carrying capacity may
be. |
The model's predictions of the risk of
extinction are heavily dependent on assumptions about future
birth rates, death rates, and carrying
capacity. Consistent with the original NMFS PVA, the model predicts
that many rookeries in the western stock will reach critically low
numbers over the next 100 years if current trends continue. The model
also suggests that the number of sea lions may be lower now simply
because the carrying capacity has been reduced (i.e., the current environment
cannot support as many sea lions as it did in the past). Whether or
not the assumption that carrying capacity might have changed is correct
does not alter the fact that Steller sea lions in western Alaska
are
at risk and are susceptible to random fluctuations in the environment
because their numbers are so low. Current and future research will
help to determine which of these scenarios is most likely.
Probabilities of extinction
of Steller sea lion populations at different breeding sites (rookeries)
in western Alaska predicted
by one set of simulations. If current trends continue, many rookeries
are at risk of becoming extinct in the near future, although populations
at some rookeries have a reduced risk due to their larger size and
lower rates of decline.
