Chapter 6: Mass Extinctions and
Global Change
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Extinction
• Termination of an evolutionary linkage by
demographic failure or loss of genetic
distinctiveness
Evolutionary Time
Species A
Anagenesis
Species B
Speciation
Species C
Species D
Extinction by
genetic swamping
Species D'
Hybridization
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Lifespan of a species
• Based on the fossil record, 99.9% of all
species that have existed on earth have
gone extinct
• Average species lifespan = 1 million years
• Outcomes after 1 million years
– evolution into a similar, but distinct, new species
– reach an evolutionary dead end
– become a living fossil (horseshoe crabs, sharks)
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Extinction as a normal process
• Over geological time, long periods of uniform rates
of extinction with ~9 catastrophic mass extinctions
Figure 6.1. The rise
and occasional fall of
biodiversity as indicated
by the fossil record of
families of marine
organisms. Marine
organisms are used as
an index of past
biodiversity because
they have left the most
complete fossil record.
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Mass extinction timeline
Permian
(245 mya)
OrdovicianSilurian
(438 mya)
TriassicJurassic
(208 mya)
Devonian
(360 mya)
500
400
Cretaceous
(65 mya)
300
200
Millions of years ago
Recent
100
Present
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Notable mass extinctions
• 430-500 million years ago (late
Ordovician)
– Growth and decay of Gondwanan ice sheet
– Followed sustained period of environmental
stability associated with high sea level
• 360 million years ago (late Devonian)
– Global cooling associated with widespread
anoxia in the oceans
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Notable mass extinctions
• 250 million years ago (end of Permian)
– Sustained period of very cold temperatures and
reduction in area of warm, shallow seas
– Extinctions
• 54% of all marine families
• 84% of all marine genera
• 96% of all marine species
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Notable mass extinctions
• 250 million years ago (end of Permian)
Sarcopterygian
fish
Trilobites
Crinoids
Monuran
insects
Therapsid
reptiles
Bryozoans
Nautiloid
molluscs
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Notable mass extinctions
• 65 million years ago (end of Cretaceous)
– Meteorite impact producing catastrophic
environmental disturbance (reduction in sunlight
due to dust clouds, decrease in temperature)
– Extinctions
• dinosaurs, pterosaurs, many plants and invertebrate
species
• 47% of all genera
• 76% of all species
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Notable mass extinctions
• 11,000 years ago (Pleistocene / Recent)
– Often associated with human migrations though
North and South America
– North American extinctions
• 2 genera, 21 species of small mammals
• most all species of large birds
• 27 genera, 56 species of large mammals
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15,000 to
11,000 ybp
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Model of colonization of the New World by Paleo-Indian
hunters, coupled with extinction of megafauna by overkill.
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Extinction and early humans
• Pleistocene extinctions in North America
– American mastodon
– Giant capybara
– Giant short-faced bear
– Long-legged llama
– North American cheetah
– Stag-moose
– Yesterday’s camel
– North American horse
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Extinction and early humans
• Pleistocene extinctions in North America
– Sabertooth cats
– Giant ground sloth
– Steppe bison
– Woolly mammoth
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Table 6.1. Numbers of plant and animal species by major
taxon, listed by the World Conservation Monitoring Centre
(www.redlist.org) as having become extinct since 1600.
Animals
Molluscs
Crustaceans
Insects
Other invertebrates
Fishes
Amphibians
Reptiles
Birds
Mammals
Total
Plants
303 Mosses
9 Gymnosperms
3
1
73 Angiosperms
4
92
Dicots
Monocots
83
3
5
22
131
87
726
90
This list includes 40 species that still survive in captivity
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Estimating modern rates of extinction
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Global rates of extinction
• If using 10 million as the estimated number
of species on earth and assuming that about
5% of all species have been described…
• Estimates of annual rates of extinction
– terrestrial vertebrates: 1-10
– aquatic vertebrates: 10-100
– plants: 10-100
– aquatic invertebrates: 100-1,000
– terrestrial invertebrates: 1,000 – 10,000
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Major causes of extinctions of vertebrate species
Percent of all extinctions
Group
Human
exploitation
Introduced
species
Habitat
disruption
Other
cause
Unknown
cause
Mammals
24
20
19
1
36
Birds
11
22
20
2
37
Reptiles
32
42
5
Fish
3
25
29
21
3
40
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Role of global climate change
• Astronomical cycles generating long-term
cycles in climate related to extinction
– Tilt of the earth on its axis
• average 23.5°
• 22-25° over the course of 41,000 years
– Shape of the earth’s orbit around the sun
(eccentricity)
• varies on a cycle of 100,000 years
– Precession of the equinoxes
• where the earth is in its orbit when the solstices and
equinoxes occur
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Figure 6.3. Three long-term cyclical changes in the earth’s
movements collectively generate a 100,000-year cycle of
climate.
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Figure 6.4. Global mean temperature record of the past
150,000 years.
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Response of organisms to global
climate change
• Adaptation by shifting geographic range to
an area with suitable climate (e.g., 18,000
years ago
– response to cold temperatures
• boreal forest and tundra tree species were found in
what is now known as Virginia
• in mountains, species moved downhill
– response to less precipitation
• equatorial species may have shrunk into refugia
• Most species respond individualistically
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Response of organisms to global climate change
Figure 6.5. Changes in the geographic ranges of American beech and
eastern hemlock indicate that these two species are responding to their
environment independently of one another (Ka = 1000 years ago).
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Observations related to presentday climate change
• Greenhouse gases
– carbon dioxide (CO2), methane (CH4), nitrous
oxide (N2O), chlorofluorocarbons(CFCs),
ozone (O3), etc.
– allow solar radiation to penetrate the earth’s
atmosphere and warm the surface of the earth
– inhibit re-radiation of heat energy back into
space
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Greenhouse effect
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Observations related to presentday climate change
• Atmospheric concentrations of many of
the major greenhouse gases have been
increasing since pre-industrial
– atmospheric CO2 concentrations have increased
30%
– atmospheric methane concentrations up 2-fold
• Mean global temperatures appear to be
rising
– 0.3 to 0.6°C since 1860
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Adapting to future climate change
• Many species that survived previous climate
changes may not be able to adapt to another
one – due to human impacts
– populations of many species are currently stressed
by habitat degradation and loss, by
overexploitation and other factors
– due to human alteration of landscapes, chance of
species moving to suitable habitat is reduced
• reduction in total amount of suitable habitat
• fragmentation by roads, easements, agriculture,
urbanization
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Adapting to future climate change
Figure 6.6. Biota particularly
sensitive to global climate
change include some unlikely
bedfellows. For example,
some inhabitants of tropical
mountains, like this
Panamanian golden frog,
occupy narrow thermal niches
that are easily disrupted
(photo: N. E. Karraker),
whereas polar creatures, like
these polar bears (photo: M.
Hunter) rely on predictable and
also easily disrupted
patterns of ice pack formation
to reach seals, their main prey.
Chytridiomycosis “chytrid” fungal
infections
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Adapting to future climate change
Table 6.2. The ability of organisms to shift their geographic ranges
depends on their mobility, both as individuals and between generations.
Mobile between generations
Sedentary between generations
Mobile as individuals
Migratory, early successional birds
Migratory, philopatric birds (i.e., return
to birthplace every year)
Insects of ephemeral ponds
Insects of deep lakes
Pelagic fishes
Anadromous fishes
Sedentary as individuals
Territorial fishes with planktonic larvae
Desert spring fishes
Early-successional plants; selfincompatible annuals
Late-successional plants; selfcompatible perennials
Intertidal molluscs
Terrestrial molluscs
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Adapting to future climate change
Figure 6.7. Average decadal
changes in the timing of
important biological events in
various organisms from
around the world (negative
values indicate a tendency to
shift to earlier dates).
(Redrawn from Root et al.
2003.)
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