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Check updated study questions
and help file on calculating
background rates of extinction.
Exam Resources Section
Cenozoic Era
Traditional
period
names
Million
Epoch
years ago
New period
names
.01
Quaternary
1.8
5
Holocene (Recent)
Pleistocene
Pliocene
Neogene
Miocene
Tertiary
24
Oligocene
37
Eocene
Paleogene
58
65
Paleocene
Cretaceous
American Pleistocene Extinctions
• 135 species of large mammals went extinct
in N. and S. America about 11,000 years
ago.
• Why?
– Climate change
– Human mediated
Early Sites in the Americas
• Clovis People expand
into the Americas
• 15,000 years ago
• Maybe 20-40,000
years ago
http://www.sciam.com/2000/0900issue/0900nemecekbox2.html
Clovis People’s
spear and arrow
points, scrapers
Human Predation
• Overkill Hypothesis
– Archeological evidence of hunting tools
– Arrow points etc. embedded in fossilized bones
• Blitzkrieg
– Naïve predators or ineffective defenses
New World
Blitzkrieg?
But . . . Predator-Prey Theory?
Prey Density
Predator switches to more common prey
Prey is maintained
at low density
Time
Mammoth Hunter . . .
or Fish Catcher?
http://www.sciam.com/2000/0900issue/0900nemecek.html
Human-Mediated Extinction
• Predation
– Overkill
– Blitzkrieg
• Disease
• Fire
Human Rats
http://www.sciam.com/interview/2001/010201macphee/index.html
Gradual Extinctions?
Climate-Change Hypothesis
• ME took place during the last glacial
retreat 10 to 11,000 years ago.
• Less savanna and grasslands, and more
deciduous forests and swamp
environments.
• Hypothesis: Mega-herbivores went
extinct and destabilized communities.
The whole pyramid tumbled.
Evidence Against
• No large-scale extinctions of megavertebrates for previous glacial retreats.
• More extinctions expected in areas most
affected by climate change
– north of N.A. and tropics of S.A
Interaction of Hunting and
Climate Change
• Mega-herbivores as
“ecosystem engineers”
– Keystone Herbivore
Hypothesis
• Domino Extinctions
Keystone
Removal of keystone herbivores
causes extinction cascade for
midsize herbivores and their
predators (Domino Effect)
Owen-Smith:
Keystone
Herbivore
Hypothesis
(1987)
The Chicken and the Egg
Large mammals selected against:
Taxa of South American land mammals
Large and small species
Existing
before
man’s
arrival
Genera:
153
Extinct
10,000
years ago
Large-bodied
species
56
54
96%
Pleistocene Survivors - I
• Holarctic Distribution
– grizzly bear, moose, elk, wolves, musk-oxen
• But . . .
– llama, tapirs, prong-horn antelope, mountain
goat
Pleistocene Survivors - II
• Susceptibility to environmental change
– Grizzly bears vs. sabre-toothed tigers
• Vulnerability to human hunters
– Mountain goat vs. woolly mammoth
% Extinctions of Terrestrial
Genera ( >44 kg adult)
Extinct
Living Total
Africa
7
42
49
%
Extinct
19.3
N. A.
33
12
45
73.7
S. A.
45
12
57
78.9
3
22
86.4
Australia 19
What About Africa?
• Human history?
• Climate change?
• Equivocal evidence