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Surprisingly enough, we know very little
about natural extinctions
In the past, known only from fossil records
Physical evidence of cause rarely
preserved
Cause and Effect hard to establish
Post hoc ergo propter hoc danger
Even if cause established, what’s the
mechanism?
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Habitat Disruption
› Volcanic Eruptions
› Asteroid Impact
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Habitat Modification
› Climate Change
› Mountain-Building
› Sea Level Change
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“Exotic” Species
› Continental Drift
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Epidemics
› Rapid co-evolution of disease and host
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Evolution of New Competitors in Place
› Existing organisms already well-adapted
Excessive Predation (Food, fur,
collecting, pest eradication, etc.)
 Habitat Destruction
 Destruction of keystone species
 Introduction of Exotic Species
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› Competitors
› Predators
› Diseases
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Pollution and Contamination
Humans Show Up and Megafaunas Go
Extinct
 Australia 40,000 years ago
 Americas 15,000 years ago
 Madagascar 1000 years ago
 New Zealand 1000 years ago
Contentious: Threatens Image of Early
Humans As Stewards of Environment
 Immigrants From Arctic Wouldn’t Have
Fine-tuned Cultural Sense of How to
Manage Temperate Environment
 American Fauna Not Accustomed to
Humans
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Climate Change?
› Rode out 20+ Previous Glacial Cycles
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Change in Ecology?
› C3 and C4 Grasses
Refers to chemical reactions during
photosynthesis (3- versus 4-carbon
molecules)
 C3 Grasses are cool climate, C4
grasses are warm climate
 C4 grasses are richer in silica particles
and wear teeth faster
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Why Didn’t All Megafauna go Extinct?
› Bison, Pronghorn, Deer, Grizzly Bears
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Did Humans Really Hunt Megafauna?
› Central Asian Mammoth-bone Huts, but
Rabbits Are Main Bones in Food Dumps
› What killed off Saber-Tooth Cats?
Did Humans Kill Off Some Keystone
Species?
 Timing is Sure Suspicious
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Steller’s Sea Cow
› Cold-Water Relative of Manatee
› Extinct 1768
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Great Auk
› Flightless, Penguin-like North Atlantic Bird
› The Original “Penguin”
› Nice Example of Convergent Evolution
› Extinct 1844
May once have been the most
numerous bird on the planet
 Estimated 5 billion
 Made up 30-40% of all North American
birds
 Flocks 1 mile wide, 300 miles long
 Evolved to travel and breed en masse
 Protection against most predators
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Unlike other predators, humans
exploited the mass flocks of the
passenger pigeon
 Netting, mass shooting
 Railroads shipped pigeons to market,
created demand
 Declines noted by 1860
 Species could probably have survived
even this predation, except….
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 Pigeons
were hunted in nesting
sites
 Hunters used telegraph to learn
of colonies
 Conservation laws too little, too
late
 Last wild pigeons shot
Wisconsin, 1899 and Ohio, 1900
 Scattered
birds could not
breed
 Captive breeding attempts
failed
 Last bird died in Cincinnati Zoo,
September 14, 1914, 1 PM
 The only extinction we can time
to the minute
Eastern race of the prairie chicken
 Once ranged from Maine to Virginia
 Hunting caused visible decline by
1800, steep by 1830
 By 1870, restricted to Martha’s
Vineyard, Massachusetts
 By 1906, only 50 left
 1907, Sanctuary established
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 1907:
Sanctuary established
for last 50 birds
 By 1915, number had grown
to 2000
 Species had been rescued?
1907-1915: Heath hen had grown from
50 to 2000 birds
 1916: Fire destroyed most of refuge
 Harsh winter and influx of hawks further
damaged species
 Flock attacked by disease from
domestic turkeys
 By 1927, only 13 left, mostly male
 Last bird died, 1932
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Only Parrot Native to U.S.
 Once ranged from Virginia to Texas
 Adapted readily to agriculture and
became regarded as a pest
 Widely hunted
 Rare by 1880’s
 Last Seen in Florida about 1920
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Cheetahs once ranged worldwide
 Remaining 20,000 are genetically
identical
 Near extinction 10,000 years ago
 Generations of close inbreeding
 Were able to re-occupy large range
because nothing had filled ecological
niche
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American Chestnut was once a major
food crop and lumber source
 Accounted for half the value of
eastern timber
 Devastated by blight 1904-30
 Isolated trees and viable roots still
survive
 Research on blight immunization
 Even if blight cured, other trees have
filled ecological niche
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Scales of organization
 genetic -- diversity of genetic information
found within species and populations
 species -- diversity of species
 community -- diversity of community
composition
 ecosystem -- diversity of assemblages of
communities (Fox River watershed)
 landscape -- diversity of assemblages of
ecosystems (Western Great Lakes)
Single islands (mountain tops) always
have fewer species than areas on the
“mainland” of similar size
 Because islands are isolated, it will be
harder for species to immigrate to them,
lowering the rate of immigration.
 Because of limited resources on islands,
carrying capacity will be lower,
decreasing population sizes and
increasing extinction rates.
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Theory of island biogeography has been
termed the 'First Law of Conservation
Biology.'
Because of human actions, natural habitats
are becoming increasingly isolated and
island-like.
By identifying potential mechanisms
underlying the loss of species diversity, Island
Biogeography Theory may help suggest
ways in which we can design nature
reserves to maximize their ability to maintain
diversity.
Biodiversity often increases when
habitats are fragmented
 Many species need large areas
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› Typically large ranges
› Availability of food
› Protection from predators and invaders
(Example: cowbirds and songbird
decline)
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Corridors as solution?
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Volunteers – natural chance immigrants
(cattle egrets)
Unintentional (rats, English sparrows)
Escaped ornamentals (kudzu, purple
loosestrife)
Escaped pets (feral cats, house finches)
Escaped domestic animals (pigs, goats)
Bio-control gone haywire (mongooses)
Most exotics not street smart
Vigorous exotics have no natural predators
Hawaii: 80% overrun by exotic species
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A large reserve is better than a small
reserve
A single undivided reserve is better than
a number of small reserves
A few large reserves are better than a
number of small reserves
Reserves should be spaced equally from
another, not linearly
Linear reserves should be connected
with corridors
If reserve is small and isolated, it should
be circular and not linear
Sixth Extinction by Richard Leakey and
Roger Lewin 1995
 Are we creating a mass extinction to rival
the other major events in the geologic
past?
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The higher the taxonomic level, the
lower the extinction level
 Easy to wipe out a species, hard to
wipe out a family
 250 m.y. ago: 90% of species lost, 50%
of families, some orders, no phyla
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Groups that vanish during mass
extinctions and then reappear
 Where do they go?
 Why don’t they change?
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About 2100 dinosaur fossils in museums
 285 genera, 336 species
 May have been 1000-1300 genera
total
 Compare to 1300 living mammal
genera
 About 30,000 marine invertebrate
genera (more genera living now)
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Incomplete
 Many organisms will never be fossilized
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› No hard parts
› Rare or very restricted
› Environments where fossilization unlikely
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Often impossible to distinguish species
› Have to rely on skeletons, shells, hard
parts
› No information on coloration
› No information on internal organs
Most sediment is transported by
running water
 Most fossils are in water-laid rocks
 Bias toward aquatic organisms
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› Shells
› Favorable setting
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Terrestrial fossils preserved erratically
Dusky seaside sparrow (color variant
only)
 Ivory-Billed Woodpecker, California
Condor, Steller’s Sea Cow (never
abundant)
 Most rain forest species (too restricted,
not likely to be fossilized)
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Extinction of Pleistocene megafauna
 Extinction of Passenger Pigeon
 Reduction in range of bison, large
carnivores
 Expansion of human domestic animals
 Reduction in rain forest, changes in
land cover
 Humans and artifacts
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The biggest change so far
(Pleistocene extinction) was prehistoric
 Have been very significant shifts in
vegetation and fauna
 Not many extinctions would show up
in the fossil record
 Little change in easily fossilized marine
faunas
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