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4.2.3
Species Estimates
&
Rates of Extinctions
Species estimates
• There is no accurate figure for # of species on earth
today
• Organisms that are found are described and then
recorded/stored in institutions (Natural History
Museums etc.)
• There are many species that have not been
discovered. Estimates of the # of undiscovered
species vary widely.
Species estimates
Accepted view:
• 1.5 million species have been described
• May be more than 10 million species actually alive
• Some scientists think the total # species on earth
could be substantially higher than 10 million.
Estimates of total numbers of species
Kingdom
Number of species
Bacteria
4,000
Fungi
72,000
Protoctists (algae and protozoa)
80,000
Plants
270,000
Animals (vertebrates)
52,000
Animals (invertebrates)
1,272,000
Total number described
1,750,000
Possible # unknown species
14,000,000
Adapted from: UNEP-WCMC (2000). Global biodiversity: Earth’s living resources in the 21st
century. Cambridge, World Conservation Press.
Classifications
• When an organism is
classified, it is categorized in to
the following divisions 
• Each category is smaller than
the previous one, and
eventually the organism is
given its name comprising its
genus and species
• There is a larger division than
Kingdom, called Domain
Classifications
• The panther (Panthera pardus)
is classified as follows:
Kingdom – Animalia
Phylum – chordata
Class – Mammalia
Order – Carnivora
Family – Felidae
Genus – Panthera
Species - pardus
• The
domestic cat
(Felis catus)
is in the
same family
as the
panther!
Changes in # of species
• Over geological time, # of species on earth has
changed many time.
• Species become extinct, and new species evolve
• Extinction – the point when a species ceases to exist
or the last known individual of the species dies
• Mass extinctions – times when Earth loses more than
three-quarters of its species in a geologically short
interval.
Changes in # of species
• Mass extinctions have happened 5 times in the past
540 million years.
• All mass extinctions have been caused by major,
natural abiotic events.
• All major extinction events are known from studying
the fossil record.
• For mass extinctions, Paleontologists refer to the % of
families that have become extinct.
Background extinction rate
• Background extinction rate (B.E.R.)– the natural extinction rate
of all species.
• Scientists estimate it should be 1 species per million species
per year, or up to 100 species per year.
• For mammals, there are 5000 species alive today, so the B.E.R.
should be 1 species per 200 years!
• 90 mammal species have become extinct in the last 400 years
and 170 are listed as critically endangered!!
• This suggests the current extinction rate is too high, and it is
humans who are causing the extinctions 
Mass Extinctions
• Average time
between
extinctions =
approx. 100
million years
• 5 mass
extinctions
throughout
Earth’s history.
Mass Extinction #1 – end of Ordovician period
• About 430 million years ago (mya)
• About 25% of all families (categories of species) of both
terrestrial and marine life were exterminated
• Caused by – drop in
sea level as glaciers
formed, followed by
rising sea levels as
they melted.
Mass Extinction #1 – end of Ordovician period
As large ice sheets formed in
the Late Ordovician, sea levels
fell dramatically, draining inland
seas.
Mass Extinction #2 – Late Devonian Period
• About 360 mya
• 19% of all families became extinct
The 10m-long predator Dunkleosteus was one of the biggest
ever placoderms, a group of armoured fish. The entire group
• Cause is not known
met its end in the Late Devonian mass extinction.
Corals suffered badly in the Late Devonian
mass extinction. Extensive reef ecosystems
collapsed.
Mass Extinction #3 – end of Permian era
• About 250 mya
• Loss of 54% of all families (95% of all marine species!) from the
warm shallow seas.
• Cause is uncertain, but could have been caused by asteroid or
flood of volcanic material from the Siberian Traps in eastern
Russia.
• Plants and plankton were destroyed by huge releases of CO2
• O2 levels in the sea were reduced.
• Only a few molluscs survived in the black, deoxygenated mud
that was left.
• It took 20-30 million years for coral reefs and forests to regrow
Mass Extinction #3 – end of Permian Era
The last remaining trilobites
died out in the mass
extinction event 252 million
years ago.
Sea scorpions, the eurypterids, were probably the largest
arthropods ever to have lived. They disappeared in the world’s
most devastating mass extinction to date.
Map of the world as it was 260 million years ago. The
supercontinent Pangaea stretched from pole to pole, creating
environmental conditions that put many species under stress.
Huge volcanic eruptions in
Siberia are likely to have been
a major trigger of the endPermian extinction event.
Mass Extinction #4 – end of Triassic Period
• About 200 mya
• 23% of all families (50% of marine invertebrates and 80% of
land quadrupeds) became extinct
• Quadruped – 4 legged animal
• Plant species were not badly affected
• Probable cause – flood-like lava escaping form a volcano in the
Atlantic Ocean
Mass Extinction #3 – end of Triassic Period
Most mammal-like reptiles and large amphibians disappeared,
Mammal-like reptiles such as Thrinaxodon, which lived about
251 million years ago, thrived in the Early Triassic. But the next as well as many dinosaur groups.
mass extinction at the end of the Triassic resulted in the
In the sea, the largest entire group to die out was the strange
group's demise
eel-like conodonts. Reef ecosystems were decimated again.
Ammonites, brachiopods and bivalves were also badly affected,
with the latter losing over 90% of its species.
Mass Extinction #5 – Cretaceous-Tertiary Extinction
• 65 million years ago
• Famously associated with extinction of dinosaurs
• Virtually no large land animals survived, plants were greatly
affected and nearly 50% of marine life was wiped out.
• Total loss of families was 17%
• Global temp. was 6-14oC warmer than present
• Sea levels over 300 meters higher than today. Oceans flooded
up to 40% of continents
• Cause – enormous asteroid/comet colliding in the sea near the
Yucatan peninsula in Mexico (currently accepted cause)
Mass Extinction #5 – Cretaceous-Tertiary Extinction
90 mya
65 mya
Dinosaurs were around for about 165 million years, evolving
into hundreds of different species. But they were already
struggling before the famous asteroid strike.
How the world looked 90 (top) and 65
(bottom) million years ago. The substantial
drop in sea level drained many inland seas.
Summary of 5 mass extinctions
Extinction Time
(mya)
1
439
Geological era
Ordovician
Loss of species
(estimates)
25% families
2
364
Devonian
19% families
3
251
Permian-Triassic
54% families
4
199-214 End Triassic
23% families
5
65
17% families
(including all dinosaurs)
Cretaceous-Tertiary
Adapted from: UNEP-WCMC (2000). Global biodiversity: Earth’s living resourcs in the 21st
century. Cambridge, World Conservation Press.
The Sixth Mass Extinction
• Wait, I thought there were only 5 mass extinctions!
• Well, the 6th mass extinction is happening RIGHT NOW
• Rapid decline in populations of many species
• Unlike the 5 previous mass extinctions, the 6th is due to human
activity
• It is happening at a much faster rate
• Species have been unable to adapt b/c of the speed of changes
on Earth, largely caused by humans.
The Sixth Mass Extinction
• Human pop. is growing exponentially…more people = more
land being taken and more species being exploited
• Species do not have the chance to move to new areas
• Pollution and climate change have added to the destruction of
ecosystems
The Sixth Mass Extinction
• Key factors responsible for current mass extinction
1. Loss of habitat due to agriculture, cities, roads, industry
2. Overexploitation of resources such as timber and fish, and in
hunting and agriculture
3. Pollution
4. Introduction of alien species as humans move species from
one continent to another
The Sixth Mass Extinction
• International Union for Conservation of Nature (IUCN) –
organization dedicated to nature conservation
• IUCN assessed species which are most at risk
• 30% of amphibians and 28% of reptiles are seriously under
threat
• Man smaller invertebrates may become extinct before they are
even discovered!
• Some scientists calculate that 25% of all species will become
extinct in the next decade.
http://www.smithsonianmag.com/smart-news/extinction-rates-are-biased-and-much-worse-than-you-thought-24290026/?no-ist