The Ecology Book

313

See also: Ecological resilience 150–151 ■ Climax community 172–173

■ Metapopulations 186–187 ■ Mass extinctions 218–223 ■ Deforestation 254–259 Vulnerability of^

small populations

A minimum viable population

has to be of a sufficient size

not only to maintain itself

under average conditions but

also to endure extreme events.

Mark Shaffer likened this to

a reservoir built to withstand

the type of flood that occurs

only once in 50 years, but not

a devastating once-in-a-

century flood.

Small populations are

especially vulnerable to

multiple threats occurring

successively. The Heath Hen

in New England, US, had been

widespread in colonial times,

but relentless hunting for food

and sport caused a dramatic

decline in Heath Hen numbers

by 1908. In that year, the last

surviving population on the

island of Martha’s Vineyard

was given protected status.

However, a catastrophic

wildfire during the 1916

breeding season, severe

winters, inbreeding, disease,

and heavy predation by birds

of prey all combined to push

the Heath Hen population

below a viable level. By 1927,

only two females remained,

and the species was extinct

by 1932.

habitat. This minimum number

also dictates the amount of suitable

habitat that the species needs. PVA

is a useful tool for conservationists

when lobbying governments and

developers to give protected status

to an area. Armed with a PVA, they

can explain precisely why reducing

a stretch of forest, heathland, or

reedbed will threaten certain flora

or fauna. Protecting an area that

is extensive enough to support

a large species also benefits many

smaller organisms sharing the

same environment.

A number of creatures can only

survive in environments where

human disturbance is minimal.

This is especially true for those

that live in specialist habitats,

such as certain owls in old-growth

forest, reptiles on acid heathland,

or amphibians in fast-flowing,

unpolluted streams. However, as

the human population grows, there

is a constant demand for land for

building, agriculture, leisure, roads,

or forestry. This pressure is a

particular threat to species that

cannot easily adapt and move

elsewhere. Where they are already

confined to “islands” of suitable

habitat, it takes no more than a low

level of environmental damage or

human disturbance to nudge them

toward extinction.

Counting grizzlies

In 1975, grizzly bear numbers were

shrinking in Yellowstone National

Park. Only an estimated 136 of the

bears were left, and this isolated

population was considered to be

endangered. As part of his doctoral

research, Mark L. Shaffer began to

study the long-term sustainability

of this geographically isolated

grizzly bear population.

Shaffer, a pioneer of population

viability analysis, applied four

factors that he considered would

decide their fate. The first was

demographic stochasticity:

irregular, unpredictable fluctuations

in numbers, age, gender, and birth

and death rates. For example, if the

overwhelming majority of animals

in a population are males, breeding

success will be poorer than in a ❯❯

ENVIRONMENTALISM AND CONSERVATION

Fender’s blue butterfly was not seen

after the 1930s and was deemed extinct

until it was rediscovered in 1989. It is

endangered, but small populations live

in northwest Oregon.

Uncertainty is just about

the only certainty in PVA.

Steven Beissinger

American conservation biologist

US_312-315_Population_viability_analysis.indd 313 12/11/2018 17:34