check updated study questions and help file on calculating background rates of extinction. exam...

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