Transcript Biodiversity_Chapter6

Extinction: past, present, future
Gwen Raitt
Available at http://planet.uwc.ac.za/nisl
BCB 705:
Biodiversity
What is Extinction?
 Extinction is the process through which a species or higher
taxonomic category ceases to exist.
 Extinction may also be defined as the disappearance of any
evolutionary lineage (from populations to species to higher
taxonomic categories) because of death or the genetic modification
of every individual.
 Where a lineage has changed such that a new (daughter) species is
recognised, the extinction of the original (parent) species may also
be called pseudoextinction.

The new and original species are
known as chronospecies.
 Extinction may be regarded as the result
of failing to adapt to environmental
changes.
 Extinction is a natural process.
The Fossil Record – Key to the Past
The Occurrence of Fossil-Bearing Rocks
 Fossils are usually found in sedimentary rocks.
 Sedimentary deposits are most likely in low-lying areas.
 Each site may have fossils representing a limited fraction of
geological time because:

Sediment deposition was
not continuous,

Sedimentary rocks erode.
 The further back in time, the
fewer the sedimentary deposits that are available because
of:

Erosion,

Metamorphosis.
The Fossil Record – Key to the Past
An Incomplete Record
 The fossil record is known to be incomplete.

Some time periods are poorly represented
by sedimentary rock formations.

Lazarus taxa

Many large extinct species are poorly represented.

The rate of description of new fossil species is steady.
 Fossil formation depends on the durability of the specimen, burial
and lack of oxygen. Most organisms do not form fossils because:

They do not have hard skeletal parts,

They get eaten,

They occur where decay is rapid or deposition does not occur,

They did not live/die during a period of sedimentation.
The Fossil Record – Key to the Past
Problems with Interpretation and Classification
 Determining fossil’s age is difficult because:

Radiometric methods cannot be used directly on the fossil,

Fossils deposited over a brief time interval
are often mixed before the sediment becomes
rock,
 Identifying fossils may be difficult because the
nature of the fossil may hide the diagnostic traits.
 For palaeontology, a species is a morphologically identifiable form.

Some living species cannot be morphologically separated by
skeletal features so a single fossil “species” may consist of
more than one biological species.

For some groups, living species can be differentiated by skeletal
features so fossil species are probably also skeletally unique.
 Species representation in the fossil record is poor so
palaeontologists tend to consider genera and higher taxa.
The Geologic Time Scale
Background Extinction and Extinction Events
 Extinction is natural (Freeman & Herron 1998). The normal
extinction rate is known as background extinction or the
background extinction rate (Futuyma 1998).
 Background extinction rates are constant within clades but
vary greatly between clades (Freeman & Herron 1998).
 Extinction events were used to demarcate the geological time
periods (Leakey & Levin 1995).
 gg
 Raup & Sepkoski (1984) suggest that mass extinction events
occur periodically at about 26 million year intervals.
Some Quantified Effects of Mass Extinctions
Table 6.1: The Effects on Skeletonized Marine Invertebrates of the ‘Big
Five’ Mass Extinctions (modifieda from p713, Futuyma 1998)
Extinction Event
Families
(%)
Genera (%)
Species
(%)c
65.0
16—17
47—50
76 ± 5
End Triassic
200.0—220.0
22—23
48—53
80 ± 4
End Permian
245.0—251.0
51—57
82—84
95 ± 2
Late Devonian
360.0—370.0
19—22
50—57
83 ± 4
End Cretaceous
Age
(x106 years)b
End Ordovician
435.0—444.0
26—27
57—60
85 ± 3
a Modifications come from Anderson (1999), Lévêque & Mounolou
(2001), Broswimmer (2002), Futuyma (2005) and Wikipedia
Contributors (2006c).
b Time periods are given for the older mass extinctions because the
literature gives variable dates.
c The species percentages are estimated from statistical analyses of
the numbers of species per genus.
Causes of Mass Extinctions
 Extinction events were used to demarcate the geological time
periods (Leakey & Levin 1995).
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 Raup & Sepkoski (1984) suggest that mass extinction events
occur periodically at about 26 million year intervals.
End Ordovician Mass Extinction
 The earliest of the five mass extinctions.
 Happened about 439 million years ago.
 Impacts on life forms:

Plants, insects and tetrapods had not yet developed so
they were not affected.

Marine organisms affected: brachiopods, cephalopods,
echinoderms, graptolites, solitary
corals and trilobites.
 Suggested causes include:

Climate change,

A drop in sea level,

Asteroid or comet impacts,

A gamma ray burst.
Late Devonian Mass Extinction
 The second of the five mass extinctions.
 Happened about 365 million years ago.
 Impacts on life forms:

Insects and tetrapods had not yet developed so they were not
affected.

Plants: the rhyniophytes decreased.

Marine organisms affected: ammonoids, brachiopods, corals,
agnathan fish, placoderm fish,
ostracods and trilobites.
 Suggested causes include:

Climate change,

Multiple asteroid impacts.
End Permian Mass Extinction
 The third and biggest of the five mass extinctions happened about
245 million years ago.
 Impacts on life forms:

Plants: the previously dominant Ottokariales (glossopterids)
became extinct.

Insects: about two thirds of the insect families became extinct
and six insect orders disappeared.

Tetrapods affected: amphibians and mammal-like reptiles

Marine organisms affected: benthic foraminifera, brachiopods,
bryozoans, echinoderms, 44% of fish families, all graptolites,
solitary corals and all trilobites.
 Suggested causes include: climate change, a drop in sea level,
massive carbon dioxide (CO2) poisoning, oceanic anoxia, the
explosion of a supernova, asteroid or comet impacts, plate
tectonics during the formation of Pangea and high volcanic activity.
End Triassic Mass Extinction
 The fourth of the five mass extinctions.
 Happened about 210 million years ago.
 Impacts on life forms:

Plants: several orders of gymnosperms were lost and the Umkomasiales (Dicroidium) became extinct.

Insects: not severely affected.

Tetrapods affected: some reptile lineages – the mammal-like
reptiles (therapsids) especially.

Marine organisms affected: ammonites, ammonoids, bivalves
(Molluscs), brachiopods, corals, gastropods and sponges.
 Suggested causes include: one or more asteroid/comet impacts,
climate change and volcanic activity.
End Cretaceous Mass Extinction
 The final and best known of the five mass
extinctions.
 Happened about 65 million years ago.
 Impacts on life forms:

Plants: debatably up to 75% of species.

Insects: not severely affected.

Tetrapods affected: 36 families from 3 groups (dinosaurs (all
non-avian), plesiosaurs and pterosaurs.

Marine organisms affected: ammonites, ammonoids,
cephalopods, bivalves, foraminifera, icthyosaurs, mosasaurs,
plackton and rudists.
 Suggested causes include: asteroid/comet impact, climate change
and volcanic activity.
 The occurrence of an impact event has been verified.
Present Mass Extinction
 There is evidence that the extinctions on New Zealand and the
Pacific Islands after human colonization were ultimately caused by
humans (Caughley & Gunn 1996).
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Human Extinction?
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Conclusions – the Future?
 If mass extinctions do occur periodically, the next natural mass
extinction should occur in the next 10 million years.
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Chapter 1 Biodiversity: what is it?
Chapter 2 The evolution of biodiversity
Links
to Other Chapters
Chapter 3 Biodiversity: why is it important?
Chapter 4 Global biodiversity and its decline
Chapter 5 Biodiversity: why are we losing it?
Chapter 6 Extinction: past, present, future.
I hope that you found chapter 6 informative.