Essentials of Ecology

188 CHAPTER 9 Sustaining Biodiversity: The Species Approach

SCIENCE FOCUS

Estimating Extinction Rates Is Not Easy

searchers know that their estimates of extinc-

tion rates are based on insufficient data and

sampling and incomplete models. They are

continually striving to get more data and to

improve the models used to estimate extinc-

tion rates.

At the same time, they point to clear evi-

dence that human activities have accelerated

the rate of species extinction and that this

rate is still increasing. According to these bi-

ologists, arguing over the numbers and wait-

ing to get better data and models should not

be used as excuses for inaction. They agree

with the advice of Aldo Leopold (Individuals

Matter, p. 22) in his thoughts about prevent-

ing premature extinction: “To keep every cog

and wheel is the first precaution of intelligent

tinkering.”

Critical Thinking

How would you improve the estimation of

extinction rates?

onservation biologists who are try-

ing to catalog extinctions, estimate

past extinction rates, and project future rates

have three problems. First, the extinction

of a species typically takes such a long time

that it is not easy to document. Second, we

have identified only about 1.8 million of the

world’s estimated 4 million to 100 million

species. Third, scientists know little about the

nature and ecological roles of most of the

species that have been identified.

One approach to estimating future extinc-

tion rates is to study records documenting

the rates at which mammals and birds (which

have been the easiest to observe) have be-

come extinct since humans arrived and to

compare this with fossil records of extinctions

prior to the arrival of humans. Determining

the rates at which minor DNA copying mis-

takes occur can help scientists to track how

long various species typically last before be-

coming extinct. Such evidence indicates that

C

under normal circumstances, species survive

for 1 million to 10 million years before going

extinct.

Another approach is to observe how the

number of species present increases with the

size of an area. This species–area relationship

suggests that, on average, a 90% loss of hab-

itat causes the extinction of 50% of the spe-

cies living in that habitat. This is based on the

theory of island biogeography (Science Focus,

p. 90). Scientists use this model to estimate

the number of current and future extinctions

in patches or “islands” of shrinking habitat

surrounded by degraded habitats or by rap-

idly growing human developments.

Scientists also use mathematical models

to estimate the risk of a particular species

becoming endangered or extinct within a

certain period of time. These models include

factors such as trends in population size,

changes in habitat availability, interactions

with other species, and genetic factors. Re-

Blue whale, giant panda,

rhinoceros

Blue whale, giant panda,

Everglades kite

Elephant seal, desert

pupfish

Bengal tiger, bald eagle,

grizzly bear

Blue whale, whooping

crane, sea turtle

African violet, some

orchids

Snow leopard, tiger,

elephant, rhinoceros,

rare plants and birds

California condor, grizzly

bear, Florida panther

Low reproductive

rate (K-strategist)

Specialized

niche

Narrow

distribution

Feeds at high

trophic level

Fixed migratory

patterns

Rare

Commercially

valuable

Large territories

Characteristic Examples

Figure 9-5 Characteristics of species that are prone to ecological and

biological extinction. Question: Which of these characteristics helped

lead to the premature extinction of the passenger pigeon within a

single human lifetime?

officially listed as endangered and protected under the

U.S. Endangered Species Act.

Some species have characteristics that make them

especially vulnerable to ecological and biological ex-

tinction (Figure 9-5). As biodiversity expert Edward O.

Wilson puts it, “The first animal species to go are the

big, the slow, the tasty, and those with valuable parts

such as tusks and skins.”

Some species also have behavioral characteristics that

make them prone to extinction. The passenger pigeon

(Core Case Study, Figure 9-1) and the Carolina

parakeet nested in large flocks that made

them easy to kill. Key deer, which live only in the U.S.

Florida Keys, are “nicotine addicts” that get killed by

cars because they forage for cigarette butts along high-

ways. Some types of species are more threatened with

premature extinction from human activities than oth-

ers are (Figure 9-6).

RESEARCH FRONTIER

Identifying and cataloguing the millions of unknown species

and improving models for estimating extinction rates. See

academic.cengage.com/biology/miller.