Chap. 3 Extinction

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Chap. 3 Extinction

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Chap. 3 Extinction

1. Rate of extinction
2. Causes of extinction
3. Risks confronted by endangered species
4. Characteristics of species and their relationship
   to extinction

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3.1 The Extinction Crisis

• Extinction
• All individuals die without producing progeny
• Pseudoextinction
• Species disappear over evolutionary time
• Lineage transformed into separate lineages
• Fossil Record
• Extinct species to living species 1,0001

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• Fossil Record
• Average life span of a species 4 million years
• Average extinction rate 2.5 species per year
• Total number of species over time 10 million
• Favors successful, geographically wide-ranging
  species
• 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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Effects due to humans Distant Past

• Correlation between human population growth and
  the number of extinctions (Figure 3.1)
• 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
• South America lost 80 of its genera of large
  mammals

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6
5
50
4
Birds
40
Mammals
Number of humans (billions)
3
Number of extinct species
30
2
20
1
10
0
0
1650 1700 1750 1800 1850 1900 1950 2000
1600-1700 1700-1800 1800-1900 1900-2000
Year
Year
Fig. 3.1 Population growth and animal
extinctions. (left) Geometric increase in the
human population.(right) increasing numbers of
extinctions in birds and mammals.
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Effects due to humans Distant Past

• 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
• Probable causes of these extinctions
• Hunting
• Some climate change

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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.
• 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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Devastating effects on islands

• 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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3.2 Patterns of Extinction

• Islands vs. continental areas (Table 3.1)
• Reasons for differences in extinction rate
• Island species may consist of a single population
• Single climatic event can lead to extinction
• 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)
• Causes of extinction (Figure 3.3)

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100
Habitat loss
Exotic species
75
Pollution
Percent endangered
Hunting
50
Disease
25
0
Hawaiian Birds
Continental U.S. plants
Hawaiian plants
Continental U.S. birds
Fig. 3.2
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No cause assigned
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Introduced animals
Habitat destruction
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Hunting
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Fig. 3.3 The causes of extinctions
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Other causes
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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).
• Disease and parasitism
• Avain malaria in Hawaii accounted for the loss of
  50 of the local Hawaiian bird species

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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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Fig 3.4
(a) Stellers sea cow
(b) the dodo

• (d) the Carolina parakeet

(c) the passenger pigeon
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3.3 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
• Hunting
• Accidental or deliberate introduction of exotic
  species
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• Disease, both exotic and endemic

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Characteristics of Factors

• Human in origin
• Species are threatened with several factors
  simultaneously
• 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)

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Threat and classes of threats
Percent of species affected
0 10 20 30 40 50 60
Habitat loss modification 76
Cultivation settlement
Pastoral development
Logging plantations
Other
Exploitation 50
Meat
Fur and hides
Live trade
Introductions 18
Predators
Competitors
Others
Limited distribution
Persecution
Fig. 3.5 The factors that threaten mammals in
Australia and the Americas.
Disturbance
Incidental take
Disease
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Overexploitation

• Significance of hunting Valuable fur and wood
  (Figure 3.6)
• Overexploitation
• Overharvesting for commercial interests
• Rare plants are threatened by collectors

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Five categories

1. Habitat destruction
2. Alien species
3. Over-harvesting
4. Disease (both native and alien)
5. Pollution

David Wilcove (1998) categorized threats to
plants and animals in the US.
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Endangered Species

• Sample size 1880 species (Figure 3.7)
• Habitat degradation was by far the most important
  threat (threatening 85 of species).
• Overall, pollution threatens 46 of vertebrates
  and 45 of invertebrates, and of minor importance
  only for plants (7).
• Overexploitation of mammals, birds, and reptiles
  is considerable.

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Percent of species threatened
0
10
20
30
40
50
70
60
80
90
100
All species
Vertebrates
Invertebrates
Plants
Mammals
Birds
Reptiles
Amphibians
Fish
Freshwater mussels
Butterflies
Other invertebrates
Fig. 3-7
Habitat loss
Exotic species
Pollution
Over exploitation
Disease
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Endangered Species

• Categorization of threats by class of species
  (Figure 3.8)
• Mammals are clearly the most endangered taxonomic
  group.
• Categorization of threats by geographic areas
  (Table 3.2)
• 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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Percentage endangered
1
1.5
2
2.5
3
3.5
0
0.5
Mammals
Fish
Birds
Reptiles
Amphibians
All invertebrates
Fig. 3.8 Percentage of Known species classed as
endangered.
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Endangered Species

• Bigger countries have more endangered species
  than smaller countries (Figure 3.9)
• US and endangered reptiles, amphibians, and
  fishes
• Better monitoring and documenting activities

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60
55
Madagascar
Indonesia
50
45
Brazil
India
40
China
Number of threatened mammals
Australia
35
Tanzania
Zaire
30
Peru
United States
Vietnam
Cameroon
Colombia
25
Mexico
Thailand
South Africa
Nigeria
Laos
Argentina
20
10,000
20,000
50,000
100,000
500,000
1,000,000
2,000,000
200,000
Country area (1000 ha)
Fig. 3.9 Relationship between number of
threatened species and area of a country.
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Correlations between human factors and extinction

• 1995, Kerr and Currie Compared 90 countries
• Six indices of human activities (Table 3.3)
• 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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3.4 Species Characteristics and Extinction

• Rarity (Fig. 3.10)
• Determined by Geographic range, Breadth of
  habitat, Local population size
• Ability to disperse (Fig. 3.10)
• Rescuing a population through immigration
• Degree of specialization (Fig. 3.10)
• Organisms that are specialized are more likely to
  become extinct
• Limited food
• Limited habitat

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Less prone to extinction
More prone to extinction
Common
1) Rarity
Rare
N
N
t
t
2) Dispersal ability
Poor dispersal
Good dispersal
Habitat destroyed
Habitat destroyed
Not able to reach new fragment
Can reach new fragment
Habitat fragments
Habitat fragments
3) Degree of specialization
High specialization
Low specialization
Fig. 3.10a
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• 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
• Life span (Fig. 3.10)
• Reproductive ability (Fig. 3.10)

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Less prone to extinction
More prone to extinction
4) Population variability
Low variability
High variability
Population size relatively constant extinction
unlikely
Sudden population decline can lead to extinction
N
N
5) Trophic status
t
t
High trophic status
Low trophic status
Pyramid of numbers
Top carnivores
Carnivores - tens
Herbivores - hundreds
Plants - thousands
Fig. 3.10b Characteristic that make species
particularly vulnerable to extinction.
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More prone to extinction
Less prone to extinction
Long life span
Short life span
6) Life span
7) Reproductive ability
High reproductive ability
Low reproductive ability
Fig. 3.10c Characteristic that make species
particularly vulnerable to extinction.
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