Download Extinctions: Past and Present

What Role Has Extinction Played in the History
of Life?
Extinctions:
Evolutionary History Has Been Marked by Periodic Mass
Extinctions
Climate Change Contributed to Mass Extinctions
Past and Present
Catastrophic Events May Have Caused the Worst Mass
Extinctions
The extinction of the Dinosaurs was caused by an asteroid
impact with the Earth about 65 million years ago
Continental Drift from Plate Tectonics
Evolution of the Earth with Time: Continental Drift
200 Million Years Ago
50 Million Years Ago
The continents are passengers on plates moving on Earth's surface as a result
of plate tectonics.
(a) About 340 million years ago, much of what is now North America was
positioned at the equator.
(b) All the plates eventually fused together into one gigantic landmass, which
geologists call Pangaea.
150 Million Years Ago
(c) Gradually Pangaea broke up into Laurasia and Gondwanaland, which itself
eventually broke up into West and East Gondwana.
(d) Further plate motion eventually resulted in the modern positions of the
continents.
100 Million Years Ago
Continental Drift - 150 mya
Present
Continental Drift - 100 mya
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Continental Drift - 75 mya
Continental Drift - 50 mya
Continental Drift - 0 mya
Extinction events
Paleontological archives reveal the existence of 5 large
extinction events in the earth history =
Time periods where extinction rates are greater than mean expected
values
Asteroid collision (65 106 yr):
- global cooling
- marine transgression
- volcanic activity
- nuclear winter
Extinction of 50% of marine species
Extinction of whole plant and animal families
(including
organisms like dinosaurs)
Formation of Pangea
(250 dominant
106yr):
- habitat destruction
- climatic modifications
Extinction of 90% of living species
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Greatest Hits: Mass Extinctions
Roughly 2 BYA: Most of life on earth wiped out due
to pollution (O2)
Permian Mass extinction 250 MYA 90 - 95% of marine
species became extinct
K-T (Cretaceous) Event 65 MYA 85% of all species
became extinct
End-Ice-Age Mass Extinction (10 TYA)
Current on-going mass extinction.
Causes of these mass extinctions
Massive environmental perturbation
Extra-terrestrial impacts
Volcanoes
Climate change
Biological agents
Comet Shoemaker- Levy 9 July 1994
Impact size if S-L 9 had hit
Earth (12+ such impacts!)
Permian, K-T extinctions
Show strong signals of
impacts (iridium layer,
shock quartz, other signals)
Climate change following impact event
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Toba extinction event. 75,000 YA
Toba, Sumatra, Indonesia
Toba Caldera energy released about one gigaton of TNT
3000 times greater than Mount St. Helens
Led to a decrease in average global temperatures by
3 to 3.5 degrees Celsius for several years
Believed to have created population bottlenecks
in the various homo species that existed at
the time
Eventually leading to the extinction of all the other
homo species except for the branch that became
modern humans
15000 ybp to 1500 AD
"North American Pleistocene Landscape," by Karen Carr
Human colonization
Humans invaded at end of the last ice age (15,00010,000 ybp)
Spread across the continent and through South
America in < 1000 years!
Clovis hunters migrated south through an ice-free
corridor east of the Canadian Rockies and moved
south of the glaciers around 11,500 years ago
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Pleistocene extinctions
Shortly after glaciers retreated at the end of the
Pleistocene, over thirty-five different genera of
mammals became extinct
Twenty-seven vanished from the world completely,
and eight of them were only in North America.
These mammals are known as “megafauna” for
their massive size and their large populations.
Mastodon (Mammut americanum) source: Illinois State
Museum
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The “Overkill” hypothesis
Entered "the happiest hunting ground ever known."
An abundance of animals wholly unadapted to
human predation.
Humans had easy hunting and exposure to few new
illnesses
The animals simply couldn't keep up with humans.
Effects on megafauna in North
America
Large, slowly reproducing creatures targeted
Precipitated an extinction cascade
When the large herbivores disappeared, so did their natural
predators
Loss of large grazers also caused a major shift in the plant
communities (e.g., from prairie to forest).
Many smaller fauna that fed off the carcasses of the megafauna or
would live off their "leftover" plant food also went extinct.
NORTH AMERICAN
VEGETATION AT 13,000 YBP,
11,000 YBP, AND AT PRESENT
Criticisms
Where are the kill sites? Very few in North
America as they are in New Zealand
Compare with Africa and Asia? Humans and large
animals have co-evolved for long periods?
Also 10 extinct families/genera of birds? All
scavengers?
Synergisms
Changes at the end of the Pleistocene were by far
the most rapid and amazing in the earth's history.
Many large mammals confined to extremely limited
ranges as the result of climatic change
Presumably exceptionally vulnerable to human
predation as a result.
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Disease? (Ross McPhee)
Other similar extinction events
Humans could have brought with them microbes
Infested with, but adapted to the pathogens perhaps carried by the lice in their hair, the fleas on
their dogs, or within their own bodies
Island extinctions
Humans next conquered the islands.
Madagascar, New Zealand, Cyprus, the Caribbean,
and the Pacific.
Large, tame, and flightless birds vanished.
New Zealand and Madagascar, lost
their large flightless birds almost
instantaneously with the arrival of
people 1,000 years ago.
The moas
of New
Zealand
Polynesia expansion
Reached last inhabitable areas – Pacific Islands – within last
1,000-4,000 years
Bird bones persist to, but not through, first indication of
human presence
Every one of ~800 Pacific Islands likely had its own species
of rail
A new species
of flightless bird
has been
discovered on
Calayan, a
remote island in
the Philippines
18/08/2004
With Stone Age technology, exterminated >2,000 bird
species, or ~15% of world’s total
Estimate may be 50% too low
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Continental Extinctions
European Expansions
John James Audobon wrote of the
passenger pigeon in 1844 in The Birds
of America:
"In the autumn of 1813, I left my house at
Henderson (Kentucky), on the banks of the
Ohio, on my way to Louisville. In passing over
the barrens a few miles beyond Hardensburgh, I
observed the pigeons flying from northeast to
southwest, in greater numbers than I though I
had ever seen them before…”
The passenger pigeon became extinct in the wild. This is “Martha", the
last passenger pigeon, died at 1:00 p.m. on September 1, 1914, at age 29,
in the Cincinnati Zoological Garden.
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Tom Nixon once
shot 120 bison in
40 minutes.
Also killed over
3200 bison in 35
days!
Lions and wolves were virtually eliminated from
eastern North America, and grizzly bears all but
the remotest parts of the Rockies.
Some 31 million
bison were killed
for hides between
1868 and 1881.
200,000 Native American Plains
Indians were thought to harvest 2
million buffalo a year, a mere third of
the bison's annual birth rate.
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Current extinctions
Comparison islands-continents
Islands
Molluscs
Birds
Mammals
Others
Total
151
104
90%
34
59%
74
62%
363
75%
11
10%
24
41%
46
38%
121
25%
79%
Continents
40
21%
85 (2.1%) mammals, 113 (1.3%) birds have
gone extinct since 1600; most in the last 150
years.
Number of species extinct since 1600
Current extinctions
50% of fungi species in Europe may have
become extinct in past 60 yrs.
The Big Picture
Background rate that is thought to have occurred
during most of the Earth’s history.
Based on the fossil record for marine animals, it has
been estimated that the annual extinction rate was
about 1 species in every million to ten million
species.
If there are 1 - 10 million species on Earth, then 1-10
species should go extinct each year.
Current extinctions
20% of the world’s freshwater fishes extinct
or in serious decline.
Comparisons
Based on recent extinctions among birds and
mammals about 1% of species have gone extinct
every 100 years.
Extrapolated means that about 1 in every 10,000
species are currently going extinct each year.
These estimates, suggest that the current extinction
rate is about 100-1000 times greater than the
background rate (other estimates 40 - 10,000 x
background rate).
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Extrapolations
Extrapolate that ~ 34 species go extinct daily based on
so many species occur in rain forest that we can ignore
other habitats,
z = 0.15,
rainforest is lost at about 1% per year.
Tremendous uncertainty but all conclude that 10,000s of
species will go extinct in the next few decades.
Oceanic islands - Z = 0.18 -0.37 .
Habitat islands - Z = 0.17 - 0.72 .
Ecological Theory of Extinction
MacArthur-Wilson Theory of Island Biogeography
Metapopulation dynamics
Demographic Stochasticity
MacArthur-Wilson Island Biogeography (1967)
Interested in predicting species numbers
on islands
Numbers represent a balance between
extinction and immigration
Prediction: lower extinction rate
on larger islands
Prediction: higher immigration rates
on islands closer to mainland
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Size does matter: Species-area curves
One of MacArthur & Wilson’s key observation
was the species-area curve, predicting the
number of species S simply from area A.
S = CAz
Log(S) = C + z*log(A)
Key is species-area exponent z
How can we apply this to conservation??
Complications
Species-area slope (z): between islands vs.
patches within an island
Even if species-area curve exact, only tells
us how many species will be lost,
NOT which particular ones
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Non-equilibrium Island Biogeography
Application to conservation biology
Isolated patches of habitats are
essentially islands
When isolation is increased, the island
is no longer in equilibrium and the number of
species is expected to decline
Hence, loss of species on ever-isolated
islands is expected
Need to maximize patch size
Need to maximize exchange between patches
When should you NOT maximize exchange?
Risk of disease/pathogens spreading
Patches are sufficiently genetically different
End
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