Check updated study questionsand help file on calculating

background rates of extinction.Exam Resources Section

Cenozoic EraEpoch

Holocene (Recent)

Pleistocene

Paleocene

.01

1.8

5

24

37

58

65

Millionyears ago

Paleogene

Neogene

New periodnames

Cretaceous

Eocene

Oligocene

Miocene

Pliocene

Quaternary

Tertiary

Traditionalperiodnames

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

http://www.sciam.com/2000/0900issue/0900nemecekbox2.html

Early Sites in the Americas

• Clovis People expand into the Americas

• 15,000 years ago• Maybe 20-40,000

years ago

Clovis People’sspear and arrowpoints, scrapers

Human Predation

• Overkill Hypothesis– Archeological evidence of hunting tools– Arrow points etc. embedded in fossilized bones

• Blitzkrieg– Naïve predators or ineffective defenses

New WorldBlitzkrieg?

But . . . Predator-Prey Theory?P

rey

Den

sity

Time

Predator switches to more common prey

Prey is maintainedat low density

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 mega-vertebrates 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

Owen-Smith:KeystoneHerbivoreHypothesis(1987)

Removal of keystone herbivores causes extinction cascade for midsize herbivores and their predators (Domino Effect)

The Chicken and the Egg

Large mammals selected against:

Taxa of South American land mammals

Existingbeforeman’s arrival

Genera: 153 56 54

Extinct10,000years ago

Large-bodiedspecies

96%

Large and small species

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 % Extinct

Africa 7 42 49 19.3

N. A. 33 12 45 73.7

S. A. 45 12 57 78.9

Australia 19 3 22 86.4

What About Africa?

• Human history?

• Climate change?

• Equivocal evidence