The Ecology Book

314

more evenly balanced population,

and will influence its chances of

survival. The second consideration

was environmental stochasticity:

unpredictable fluctuations in

environmental conditions, such

as habitat and climate changes,

which may affect the availability

of food and shelter. The third

was natural catastrophes, such

as forest fires or floods. The fourth

of Shaffer’s factors was genetic

changes, including problems

created by inbreeding. For each

of these, statistical modeling can

determine a range of possibilities.

Since Shaffer’s initial research

in the 1970s and ’80s, and

subsequent new management and

conservation strategies, grizzlies

have extended their habitat by

more than 50 percent within the

extensive Greater Yellowstone

Ecosystem—an area of 34,375 sq

miles (89,031 sq km) that has the

national park at its core. In 2014,

the US Geological Survey estimated

that around 757 bears lived in the

ecosystem, based on 119 sightings

of grizzly sows and cubs. However,

the population had dropped to

around 718 in 2018, and population

POPULATION VIABILITY ANALYSIS

A female grizzly and her cubs forage

in Yellowstone. A female’s home range

is 300–550 sq miles (775–1,400 sq km),

while a male’s is as much as 2,000 sq

miles (5,000 sq km).

modllers have suggested that

Yellowstone may have reached its

maximum carrying capacity—the

largest number of animals an area

of suitable habitat can support. In

2017, grizzlies were briefly removed

from the threatened species list,

but their protections were restored

by a federal judge in 2018.

How studies are devised

PVA studies are now conducted in

several ways. The simplest type is

the time-series PVA, which looks at

the entirety of a population over a

period of time in order to calculate

a rough average growth trend and

any variations. In such studies, all

individuals are treated as identical.

Demographic PVAs tend to be

more precise and detailed. They are

based on estimated reproductive

and survival rates for different age

bands within the population. Such

analyses require much more data,

but can provide extra information

on the needs and vulnerability of

different sections of the population,

Technology is increasingly

allowing scientists and

policymakers to more closely

monitor the planet’s

biodiversity and threats to it.

Stuart L. Pimm

American–British biologist

US_312-315_Population_viability_analysis.indd 314 12/11/2018 17:34