Chapter 3Extinction

© 2002 by Prentice Hall, Inc.

Upper Saddle River, NJ 07458

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Outline• Rate of extinction• Causes of extinction• Risks confronted by endangered

species• Characteristics of species and

their relationship to extinction

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The Extinction Crisis• Extinction

– All individuals die without producing progeny

• Pseudoextinction– Species disappear over evolutionary

time

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The Extinction Crisis• Pseudoextinction

– Lineage transformed into separate lineages

• Fossil Record– Extinct species to living species –

1,000:1

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Extinction Crisis (cont.)• Fossil Record (cont.).

– Average life span of a species – 4 million years

– Average extinction rate – 2.5 species per year

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Extinction Crisis• Fossil Record

– Total number of species over time – 10 million

• Biased fossil record– Favors successful, geographically

wide-ranging species– Persist longer than the average

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Extinction Crisis• Biased fossil record (cont.)

– Biased toward vertebrates and mollusks

– Background extinction rates are probably higher than indicated in fossil record.• Example Extinction rates 10 times higher

than predicted by fossil record

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Extinction Crisis– Background extinction rates are

probably higher than indicated in fossil record (cont.).•1 every 50 years for birds living today

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Extintion Crisis

• Present extinction rate much high than in the past or predicted.– Effects due to humans – Distant Past

•Correlation between human population growth and the number of extinctions (Figure 3.1)

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Num

ber

of

hum

ans

(bill

ions)

0

1

2

3

4

5

6

1650 1700 1750 1800 1850 1900 1950 2000 1600-1700 1700-1800 1800-1900 1900-2000

Birds

Mammals

Num

ber

of

exti

nct

speci

es

0

10

20

30

40

50

Year Year

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Extinction Crisis

• Effects due to humans (cont.).– Large scale extinctions in North and South

America coinciding with the arrival of humans (11 thousand years ago)• North America lost 73% of its genera of

large mammals

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Extinction Crisis

• Effects due to humans (cont.).– Large scale extinctions in North and South

America (cont.).• South America lost 80% of its genera of

large mammals

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Extinction Crisis (cont.)

• Effects due to humans (cont.).– Large scale extinctions in Australia

coinciding with the arrival of humans (13 thousand years ago)• Lost nearly all of its large mammals, giant

snakes, and reptiles• Nearly half of its large flightless birds

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Extinction Crisis (cont.)– Probable causes of these extinctions

• Hunting• Some climate change

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Extinction Crisis• Effects due to humans – Recent

Past – Devastating effects on islands

•Hawaii – 4th and 5th century Polynesians arrived

– Exterminated 50 out of 100 species of endemic land birds.

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Extinction Crisis– Devastating effects on islands (cont.)

•New Zealand – End of the 18th century– Entire avian megafauna consisting of huge

land birds was exterminated– Accomplished through hunting and habitat

destruction

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Extinction Crisis– Devastating effects on islands (cont.)

•Madagascar – last 1,500 years– Exterminated– Giant elephant bird, largest bird ever recorded– 20 species of lemur, most larger than any

surviving species– 2 giant land tortoises

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Patterns of Extinction• Islands vs. continental areas

(Table 3.1)

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Patterns of Extinction– Reasons for differences in extinction

rate• Island species may consist of a single

population– Single climatic event can lead to extinction

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Patterns of Extinction– Reasons for differences in extinction

rate (cont.)• Island species may have evolved in the

absence of terrestrial predators – Characteristics contributing to extinction

» Flightlessness» Tameness» Reduced reproductive rates

– Ex. Hawaii (Figure 3.2)

                                     

Perc

ent

endangere

d

0

25

50

75

100Habitat loss

Exotic species

Pollution

Hunting

Disease

Continental U.S. birds

HawaiianBirds

ContinentalU.S. plants

Hawaiianplants

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Patterns of Extinction• Causes of extinction (Figure 3.3)

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No cause assigned

Introduced animals

Habitat destruction

Hunting

Other causes

56%

17%

16%

10%

1%

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Patterns of Extinction– Introduced species effects

•Competition– Not been shown to eliminate an entire species

•Predation– Rats, cats, and mongooses have accounted

for at least 112 of 258 extinctions of birds on islands (43%).

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Patterns of Extinction• Introduced species effects (cont.)

– Disease and parasitism•Avain malaria in Hawaii accounted for

the loss of 50% of the local Hawaiian bird species

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Patterns of Extinction– Causes of Extinction (Cont.)

•Habitat destruction– A prime cause of extinction– Ex. Deforestation– Subtle alterations (e.g. pollution) have not yet

been shown to cause extinction

•Direct exploitation – Hunting– Caused numerous extinctions– Ex. Figure 3.4

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Endangered Species• Definition – a species that is

thought to be at risk of extinction in the foreseeable future.

• Factors threatening species with Extinction– Habitat loss or modification

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Endangered Species• Factors threatening species with

Extinction (cont.).– Hunting– Accidental or deliberate introduction

of exotic species– Deliberate eradication

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Endangered Species• Factors threatening species with

Extinction (cont.).– Incidental – Disease, both exotic and endemic

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Endangered Species• Characteristics of Factors

– Human in origin– Species are threatened with several

factors simultaneously– Relative importance as measured by

frequency of occurrence

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Endangered Species• Characteristics of Factors (cont.)

– Ex. Threats facing terrestrial mammals in Australia and the Americas

• 119 species considered endangered• 75% threatened by more than one factor• 27 species face four or more threats• Major threat – 76% of the species are

experiencing habitat loss or modification• Figure 3.5

                                     

Threat and classes of threats Percent of species affected

0 10 20 30 40 50 60Habitat loss & modification: 76%

Cultivation & settlement

Pastoral development

Logging & plantations

OtherExploitation: 50%

Meat

Fur and hides

Live trade

Predators

Competitors

Limited distribution

Persecution

Disturbance

Incidental take

Disease

Introductions: 18%

Others

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Endangered Species• Significance of hunting – Valuable

fur and wood (Figure 3.6)• Overexploitation

– Overharvesting for commercial interests

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Endangered Species• Overexploitation (cont.). 1998,

– Rare plants are threatened by collectors

• David Wilcove categorized threats to plants and animals in the US– Five categories

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Endangered Species– Five categories (cont.)

•Habitat destruction•Alien species•Overharvesting•Disease (both native and alien)•Pollution

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Endangered Species– Sample size: 1880 species– Results (Figure 3.7)

                                     

All species

Percent of species threatened0 10 20 30 40 50 7060 80 90 100

Habitat loss Exotic species Pollution Over exploitation Disease

Invertebrates

Vertebrates

Plants

Mammals

Birds

Reptiles

Amphibians

Fish

Freshwater mussels

Butterflies

Other invertebrates

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Endangered Species• Categorization of threats by class

of species (Figure 3.8)

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Percentage endangered

0 0.5 1 1.5 2 2.5 3 3.5

Mammals

Fish

Birds

Reptiles

Amphibians

All invertebrates

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Endangered Species• Categorization of threats by

geographic areas (Table 3.2)

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Endangered Species– The majority of threatened mammals

occur in tropical countries•Tropical countries have more species

(therefore should have more endangered species

•Tropical countries have a higher percentage of endangered species as well.

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Endangered Species (cont.)– Bigger countries have more

endangered species than smaller countries (Figure 3.9)

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20

25

30

35

40

45

50

55

60

Num

ber

of

thre

ate

ned m

am

mals

10,000 20,000 50,000 100,000 200,000 500,000 1,000,000 2,000,000

MadagascarIndonesia

IndiaBrazil

ChinaAustralia

Zaire

United States

Argentina

MexicoSouth AfricaNigeria

Thailand

Laos

Vietnam Cameroon

Tanzania

Peru

Colombia

Country area (1000 ha)

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Endangered Species (cont.)– US and endangered reptiles,

amphibians, and fishes•Better monitoring and documenting

activities

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Endangered Species• Correlations between human

factors and extinction– 1995, Kerr and Currie Compared 90

countries– Six indices of human activities (Table

3.3)

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Endangered Species (cont.)

• Correlation between human factors and extinction (cont.)

•Human population explained the most variation in the proportion of endangered species of birds.

•Per capita GNP explained the most variation in mammals

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Species Characteristics and Extinction

• Rarity (Fig. 3.10)– Determined by

•Geographic range,•Breadth of habitat•Local population size

• Ability to disperse (Fig. 3.10)

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Species Characteristics and Extinction (cont.)

– Rescuing a population through immigration

• Degree of specialization (Fig. 3.10)

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Species Characteristics and Extinction (cont.)

• Degree of specialization (cont.)– Organisms that are specialized are

more likely to become extinct•Limited food•Limited habitat

                                     

Rare

Common

Poor dispersal Good dispersal

Habitat destroyed Habitat destroyed

More prone to extinction Less prone to extinction

N N

t t

Can reach newfragment

Not able to reach new fragment

High specialization Low specialization

Habitat fragments Habitat fragments

3) Degree of specialization

2) Dispersal ability

1) Rarity

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Species Characteristics and Extinction (cont.)

• Population variability (Fig. 3.10)– Stable populations are less likely to

go extinct

• Trophic status (Fig. 3.10)– Applies to animals only– Higher trophic levels more at risk

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Sudden population decline can lead to extinction

Population size relatively constant: extinction unlikely

Pyramid of numbers

High trophic status

N N

t t

Low trophic status

More prone to extinction Less prone to extinction

Low variabilityHigh variability

4) Population variability

5) Trophic status

Plants - thousands

Herbivores - hundreds

Carnivores - tens

Top carnivores are very few, so prone to extinction as in 1

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Species Characteristics and extinction (cont.)

• Life span (Fig. 3.10)• Reproductive ability (Fig. 3.10)

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More prone to extinction Less prone to extinction

6) Life span Short life span Long life span

7) Reproductive ability

High reproductive ability Low reproductive ability

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Summary• Important causes of extinction

– Introduced species (39%)– Habitat destruction (36%)– Direct exploitation (23%)

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Summary (cont.)

• Factors threatening species– Habitat destruction

• Deforestation and ecosystem conversion

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Summary (cont.)

• Species characteristics affecting the sensitivity to extinction– Rarity– Ability to Disperse– Degree of Specialization– Population variability

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Summary

• Species characteristics affecting the sensitivity to extinction (cont.)– Trophic Status– Longevity– Reproductive ability

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Discussion Question #1• Which type of organisms do you

think deserve priority in conservation efforts and why?

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Discussion Question #2• What ecological information would

you need in order to list a species as endangered?

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Discussion Question #3• If we are concerned with

protecting rare species, should we also be concerned with protecting subspecies (or races) or even individual populations? Discuss the pros and cons of this issue.