315
fueling a case for conservation
where protection is required. As
reliable information on age ranges
and breeding rates is often not
available for small, threatened
populations, ecologists sometimes
use data from other populations of
the same species—or a different but
similar species—to conduct a PVA.
However, the results are variable,
even in populations of the same
species in the same area. In a 2015
study of three colonies of California
sea lion in the Gulf of California,“surrogate” data from one colony
was used to make forecasts on the
other two; they proved valid for one
colony, but not the other.Making a difference
Methods are still being refined, but
PVA has now become a cornerstone
of conservation biology. PVAs have
been applied to populations as
varied as island foxes in California,
sea otters in Alaska, Fender’s blue
butterflies and Northern Spotted
Owls in Oregon, and bottlenoseENVIRONMENTALISM AND CONSERVATION
A population
viability analysis is
conducted to assess
the situation.A management
solution is found to
combat the threat
to the population.A population is identified as being at risk.The population has
a chance to recover.dolphins off the coasts of Argentina
and Australia. With the development
of increasingly efficient computer
programs incorporating ever more
variables, PVA will undoubtedly
be used even more effectively in
the future. It is impossible to predict
every extinction, but PVA provides
tools for identifying endangered
populations and determining the
management actions likely to
be most effective in improving
population viability, and preserving
a species at risk. ■The island foxes of the Channel
Islands, off California, numbered fewer
than 200 in the late 1990s. By 2015,
there were more than 5,000, but on one
island, a subpopulation is still at risk.A Japanese study
The Japanese Rock Ptarmigan
lives in the Japanese Alps at
an altitude of around 8,200 ft
(2,500 m). Its population of some
2,000 birds is divided into
several small communities
on mountain peaks. When a
combination of climate warming
and predators moving further
up the mountains prompted
fears for its survival, ecologist
Ayaka Suzuki and his team set
out to find the minimum viable
population size for the birds on
Mount Norikura. The teamcollected population growth
data, including the number of
female offspring that survived
to the next breeding season and
the annual survival rate of all
birds. Their calculations included
variables for a range of offspring
from each pair.
Their findings indicated that
there was a relatively low risk of
extinction in the next 30 years,
even if the starting population
was only 15. One potential
conclusion is that the Mount
Norikura population is strong
enough to supplement declining
populations on other mountains.Population viability
analysis can indicate how
urgently recovery efforts need
to be initiated in specific
populations.
William F. Morris
American biologistUS_312-315_Population_viability_analysis.indd 315 12/11/2018 17:34