Download Chapter 6 - Extinction

BCB 341: Principles of
Conservation Biology
EXTINCTION: PAST, PRESENT, FUTURE
Material: Gwen Raitt
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 GEOLOGICAL TIME SCALE
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.
BACKGROUND EXTINCTION AND EXTINCTION EVENTS
 The extinction rate that is normal in the fossil record is known
as background extinction.
 Background extinction rates are constant within clades but
vary greatly between clades.
 Extinction events are relatively short (in terms of geological
time) periods with greatly increased extinction rates.
 A mass extinction event must eliminate >60% of species in a
relatively short period of geological time with widespread
geographical and taxonomical impacts.
 Mass extinction events are important because of the disruptive
effect they have on the way biodiversity develops.
 The principle subdivisions of geologic time are identified by
distinctive fossils and major faunal breaks (extinction events)
were used as the boundaries.
 Mass extinction events may occur periodically.
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
Age
(x106
years)b
Families
(%)
Genera
(%)
Species
(%)c
End
Cretaceous
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
a
Modifications come from Anderson (1999), Lévêque & Mounolou
End
435.0—444.0
26—27
57—60
(2001), Broswimmer (2002), Futuyma (2005) and Wikipedia
Ordovician (2006c).
Contributors
b
85 ± 3
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
 Most of the extinction events are likely to have been caused by a
combination of factors.
 Proximate causes of extinctions are in turn caused by other events.
 Postulated consequences of the asteroid strike that caused the end
Cretaceous (K/T) mass extinction include acid rain, widespread fires,
climate cooling due to dust and smoke, earthquakes and increased
volcanic activity elsewhere in the world and a tsunami (an enormous
tidal wave). The aforementioned consequences
would have caused ecological disruption leading to further extinctions.
 Some previously postulated causes of mass
extinctions may be unlikely or even impossible:

A supernova explosion,

A nearby gamma ray burst,

Biological causes.
THE 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.
THE 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.
THE 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.
THE 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.
THE 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.
THE PRESENT MASS EXTINCTION – PHASE ONE
 This phase began with the dispersal of
modern humans over the earth about
100 000 years ago.
 The probable causes considered are
human impacts, climate change or a
combination of the two.
 Bolide impacts have also been
suggested as a cause.
 Human impact is difficult to prove.
Continental extinctions (Australia & the
Americas) coincided with human arrival
and archaeological sites prove that the
megafauna were hunted but the
evidence is circumstantial.
 There are arguments for and against
climate change as a cause.
THE PRESENT MASS EXTINCTION – PHASE TWO
 The second phase began with the development of agriculture about
10 000 years ago.
 Agriculture allowed humanity to live outside
the boundaries of local ecosystems.
 We are causing major environmental changes.
 The drivers for this sixth mass extinction are
agriculture and human overpopulation, overexploitation and
invasive species.
 This is seemingly the first mass extinction to have a biotic cause.
 The effects of this mass extinction are hidden by:

The ex situ populations of species that are extinct in the wild,

The existence in the wild of the remnant populations of several
species,

Extinction debt.
HUMAN EXTINCTION?
 If all species will become extinct, then human extinction is also
inevitable.
 The risks of human extinction are not considered very great by the
average person despite knowledge of many possible mechanisms of
extinction.
 The ‘Doomsday argument’ proposed by Brandon Carter suggests that we should be suspicious of low values for the probability of
human extinction.
 Lester Brown provides evidence that the current methods of food
production are unsustainable.
 Julian Simon believes that the present technology is enough to provide for a continuously expanding population for the next 7 billion
years.
 Both cannot be right. Logic and the ‘Doomsday argument’ suggest
that it would be sensible to act on Brown’s evidence.
THE FUTURE?
 The present extinction acts differently to previous mass extinctions.
 Extinction, excluding as a result of catastrophes, happens in stages.
 There is insufficient knowledge of the natural world to predict how
much extinction ecosystems can experience without loss of function.
 If the present extinction event continues unchecked, we could push
ecosystems beyond the threshold at which they can maintain their
functions and thus sustain themselves and us. This would result in
the demise of Homo sapiens.
 Biodiversity has recovered following each mass extinction but only
after the cause of the event had dissipated.
 To end the present mass extinction, we must
change our present behaviour.
 If mass extinctions do occur periodically, then the next natural mass
extinction should occur in the next 10 million years.