Download Notes towards Biodiversity Chapter 6

Notes towards Biodiversity Chapter 6
Introductory/Title slide (1)
Hello. This is Gwen Raitt. I will be presenting this chapter on extinction over time.
The picture shows a mammoth (genus Mammuthus).
What is extinction?
Extinction is the process through which a species or higher taxonomic category ceases to
exist (Fiedler & Jain 1992, Wikipedia Contributors 2006a). This definition may be
applied within selected geographical boundaries (Fiedler & Jain 1992). Under this
definition, extinct species may or may not have descendants (Wikipedia Contributors
2006a).
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 (Cox 1997, Broswimmer 2002). 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 (Futuyma 1998, Wikipedia
Contributors 2006a). Pseudoextinction is hard to prove (Wikipedia Contributors 2006a).
The new and original species are known as chronospecies (Futuyma 1998).
Extinction may be regarded as the result of failing to adapt to environmental changes
(Futuyma 1998). Extinction is a natural process that has occurred throughout the earth’s
history. Every species will become extinct (Cox 1997, Freeman & Herron 1998, Lévêque
& Mounolou 2001). Calculations from the fossil record show that most genera have a
relatively short lifespan of less than 50 million years. The average lifespan of a genus is
5—10 million years (Gaston & Spicer 1998) while a species has an average lifespan of 4
million years (Groombridge 1992, Barbault & Sastrapradja 1995).
The picture shows a representation of the extinct Steller’s sea cow (Hydrodamalis gigas).
Steller’s sea cow was native to the Bering Sea. The last individual was killed in 1767
(Caughley & Gunn 1996).
The geologic time scale
The geologic time scale illustrates the magnitude of the time period in which fossils
occur. The oldest fossil dates from 3.5 billion years ago. Older deposits, from 3.8 billion
years ago, containing lots of carbon are regarded as the earliest evidence of life on earth
(Wikipedia Contributors 2006b). The 5 recognised mass extinction events in prehistoric
time and the present mass extinction event are marked with red arrows.
The fossil record – key to the past
The occurrence of fossil-bearing rocks
Fossils are usually found in sedimentary rocks. (The picture shows a piece of limestone
containing fossils.) Fossils in metamorphic rocks are normally changed beyond
recognition. Fossils never occur in igneous rocks (Futuyma 1998, 2005). Sedimentary
deposits are most likely to occur in low-lying areas such as aquatic environments.
Marine sediments are the most common (Freeman & Herron 1998, Futuyma 1998). Each
site may have fossils representing a limited fraction of geological time because sediment
deposition was not continuous and sedimentary rocks weather and erode or
metamorphose (Futuyma 1998). The further back in time, the fewer the sedimentary
deposits that are available because there has been more time for weathering and erosion
or metamorphosis to occur (Futuyma 1998).
Additional Notes
Extract from Wikipedia (2006A), http://en.wikipedia.org/wiki/. “Igneous rocks are
formed from molten magma and are divided into two main categories: plutonic rock and
volcanic. Plutonic or intrusive rocks result when magma cools and crystallises slowly
within the Earth’s crust, while volcanic or extrusive rocks result from magma reaching
the surface either as lava or fragmental ejecta. Intrusive igneous rocks usually take a few
thousand years to cool whereas extrusive igneous rocks take only a few days or weeks to
cool and solidify.
Sedimentary rocks are formed by deposition of clastic sediments, organic matter, or
chemical precipitates (evaporites), followed by compaction of the particulate matter and
cementation during diagenesis. Sedimentary rocks form at or near the Earth’s surface.
Metamorphic rocks are formed by subjecting any rock type (including previouslyformed metamorphic rock) to different temperature and pressure conditions than those in
which the original rock was formed. These temperatures and pressures are always higher
than those at the Earth’s surface and must be sufficiently high so as to change the original
minerals into other mineral types or else into other forms of the same minerals (e.g. by
recrystallisation).”
Extract from Wikipedia (2006B), http://en.wikipedia.org/wiki/. “Weathering is the
process of disintegration of rocks, soils and their minerals through direct, or indirect
contact with the atmosphere. Weathering occurs in situ, or ‘without movement’, and thus
should not to be confused with erosion, which involves the movement and disintegration
of rocks and minerals by processes such as water, wind, ice or gravity.”
The fossil record – key to the past
An incomplete record
The fossil record is known to be incomplete because some time periods are poorly
represented by sedimentary rock formations (Futuyma 1998, 2005) (The picture shows a
sedimentary rock formation in the Grand Canyon, U.S.A.); Lazarus taxa show up at
widely separated time intervals (Raup 1988, Futuyma 1998, 2005); many large extinct
species are only represented by a few specimens and the rate of description of new fossil
species is steady (Futuyma 1998, 2005). Fossil formation depends on the durability of
the specimen, burial and lack of oxygen (Freeman & Herron 1998). Most organisms do
not form fossils because: they do not have hard skeletal parts that will endure long
enough for fossilization to occur; they get eaten so nothing is left to become fossilized;
they occur in places where decay is rapid (e.g. tropical forests) or deposition does not
occur (e.g. higher elevations) or they did not live/die during a period of sedimentation
(Freeman & Herron 1998, Futuyma 1998, 2005).
Additional Notes
Extract from Wikipedia (2006C), http://en.wikipedia.org/wiki/. “In paleontology, a
Lazarus taxon (plural taxa) is a taxon that disappears from one or more periods of the
fossil record, only to appear again later. The term refers to the New Testament story of
Lazarus, whom Jesus miraculously raised from the dead. Lazarus taxa are observational
artifacts that appear to occur either because of (local) extinction, later resupplied, or as a
sampling artifact.”
Other notes. The fossil record is best for various groups of marine invertebrates
(Freeman & Herron 1998, Futuyma 1998). Marine organisms dominate the fossil record
whereas only about 10% of extant species are marine organisms (Leakey & Lewin 1995,
Freeman & Herron 1998).
The fossil record – key to the past
Problems with interpretation and classification
Determining a fossil’s age is difficult because: radiometric methods cannot be used
directly on the fossil and fossils deposited over a brief time interval are often mixed
before the sediment becomes rock (Futuyma 1998).
Identifying fossils may be difficult because the nature of the fossil may hide the
diagnostic traits because it has been crushed (e.g. insect imprint fossils) or fragmented
(e.g. different plant parts that cannot be matched or bone fragments) (Futuyma 1998).
The picture shows an example of an imprint fossil.
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 (Futuyma 1998). For some groups, living
species can be differentiated by skeletal features so the fossil species are probably also
skeletally unique. Species representation in the fossil record is poor so palaeontologists
tend to consider genera and higher taxa (Futuyma 1998).
Background extinction and extinction events
The extinction rate that characterises the greater part of the fossil record (i.e. is normal in
the fossil record) is known as background extinction (Futuyma 1998, Broswimmer 2002).
Background extinction rates are constant within clades but vary greatly between clades
(Freeman & Herron 1998). They may be declining (Raup & Sepkoski 1982).
Interspecific competition is a possible cause of background extinction (Barbault &
Sastrapradja 1995).
Extinction events are relatively short (in terms of geological time) periods with greatly
increased extinction rates (Leakey & Lewin 1995, Futuyma 1998, Futuyma 2005,
Wikipedia Contributors 2006c). Extinction events form peaks on the graph shown. The
definition of a mass extinction event depends on the threshold chosen as major and the
data chosen (Wikipedia Contributors 2006c). The number of extinction events
recognised as mass extinctions varies from 5 to more than 20 (Leakey & Lewin 1995,
Wikipedia Contributors 2006c). To qualify as a mass extinction event, the extinction
event must eliminate more than 60% of species in a relatively short period of geological
time with widespread geographical (implying all habitat types – shallow and deep water
and on land) and taxonomical (i.e. extinction of taxa from ecologically distant groups)
impacts (Leakey & Lewin 1995, Freeman & Herron 1998, Broswimmer 2002). While
mass extinctions remove more than 60% of species extant at the time, they account for
only 4% of all extinctions in the Phanerozoic Eon. They are important because of the
disruptive effect they have on the way biodiversity develops rather than because of the
contribution they make to total extinctions (Gaston & Spicer 1998).
Examination of extinction patterns is only possible as a result of the development of such
attributes as bones and teeth that made possible the formation of a fossil record adequate
for study (Wikipedia Contributors 2006c). This means that while extinction occurred in
the Archean and Proterozoic Eons of Precambrian time, it is only possible to investigate
the patterns of the Phanerozoic Eon (Wikipedia Contributors 2006c).
The principle subdivisions of geologic time are identified by distinctive fossils and major
faunal breaks (extinction events) were used as the boundaries (Raup & Sepkoski 1982,
Leakey & Lewin 1995, Raup 1998, Futuyma 1998).
Raup & Sepkoski (1984) suggest that mass extinction events may occur periodically at
about 26 million year intervals.
Some quantified effects of mass extinctions
Table 6.1: the effects on skeletonised marine invertebrates of the ‘Big Five’ mass
extinctions (modified from p713, Futuyma 1998).
Extinction Event
Age
Families (%)
Genera (%)
Species (%)c
6
b
(x10 years)
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
End Ordovician
435.0—444.0
26—27
57—60
85 ± 3
The modifications come from Anderson (1999), Lévêque & Mounolou (2001),
Broswimmer (2002), Futuyma (2005) and Wikipedia Contributors (2006c). Time periods
rather than fixed dates are given for the older mass extinctions because the literature
gives variable dates. (The mode time value has been selected (with references) for the
slides on each mass extinction event.) The species percentages are estimated from
statistical analyses of the numbers of species per genus (Futuyma 1998).
Causes of mass extinctions
Most of the extinction events are likely to have been caused by a combination of factors
(Leakey & Lewin 1995, Broswimmer 2002), e.g. extensive glaciation happened
periodically in Earth’s history but did not always coincide with extinction events so while
global cooling is important, it is not the primary cause in all extinction events (Leakey &
Lewin 1995). The picture shows Perito Morena Glacier in Argentina.
Consider also that proximate causes of extinctions, e.g. a drop in sea level, are in turn
caused by other events. In the case of a drop in sea level, the possible causes include
extensive polar glaciation and changes in continental configuration due to plate tectonics.
Possible ultimate causes of extinction events include the variation in the earth’s orbit, the
positions of the tectonic plates that form Earth’s crust and changes in mantle convection
(Leakey & Lewin 1995).
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 for which there is some evidence) (Leakey & Lewin 1995, Freeman
& Herron 1998). The aforementioned consequences would have caused ecological
disruption leading to further extinctions (Freeman & Herron 1998).
Some previously postulated causes of mass extinctions may be unlikely or even
impossible. A supernova explosion is considered very unlikely and recent research
shows that a nearby gamma ray burst is impossible because our galaxy is metal rich.
Mass extinctions are considered too extensive and too sudden to be the result of
biological causes such as disease or competition (Leakey & Lewin 1995, Wikipedia
Contributors 2006c).
The End Ordovician mass extinction
This is the earliest of the 5 mass extinctions (Raup & Sepkoski 1982). Of the 5, it had the
second largest impact (Futuyma 1998). It may have occurred as 3 peaks spread over
500 000 years (Groombridge 1992) or 2 peaks separated by 1—2 million years (Caughley
& Gunn 1996, Wikipedia Contributors 2006c). This extinction event took place about
439 million years ago coinciding with glaciation (Groombridge 1992, Futuyma 1998).
Plants, insects and tetrapods had not yet developed so they were not affected (Anderson
1999). The marine organisms affected included brachiopods, cephalopods, echinoderms,
graptolites, solitary corals and trilobites (Dobson 1996, Primack 1998, Lévêque &
Mounolou 2001). The picture shows an Ordovician brachiopod fossil assemblage
including bryozoan, coral, annelid and gastropod fossils.
Suggested causes include climate change (a drop in temperature), a drop in sea level,
asteroid or comet impacts and a gamma ray burst (Futuyma 1998, Anderson 1999,
Wikipedia Contributors 2006c).
The Late Devonian mass extinction
This is the second of the 5 mass extinctions. It may have occurred in 2 stages over about
15 million years (Raup & Sepkoski 1982) or as two or more peaks over 3 million years
(Caughley & Gunn 1996) or as a series of peaks over 20 million years (Groombridge
1992, Wikipedia Contributors 2006c). This extinction event took place about 365 million
years ago (Leakey & Lewin 1995, Lévêque & Mounolou 2001).
Insects and tetrapods had yet to develop but of the plants, the rhyniophytes decreased
(Anderson 1999). The marine organisms affected include ammonoids, brachiopods,
corals, agnathan fish, placoderm fish, ostracods and trilobites (Caughley & Gunn 1996,
Dobson 1996, Primack 1998, Lévêque & Mounolou 2001). The picture shows a
Devonian example of a trilobite.
Suggested causes include climate change (a drop in temperature) and multiple asteroid
impacts (Caughley & Gunn 1996, Anderson 1999, Wikipedia Contributors 2006c).
The End Permian mass extinction
This is the third and biggest of the 5 mass extinctions (Groombridge 1992, Barbault &
Sastrapradja 1995, Futuyma 1998). It may have taken place over 5—8 million years
(Groombridge 1992, Caughley & Gunn 1996, Dobson 1996) or been a single event that
took less than 10 000 years or occurred as 2 peaks over 5 million years (Barbault &
Sastrapradja 1995). This extinction event took place about 245 million years ago
(Barbault & Sastrapradja 1995, Gaston & Spicer 1998, Lévêque & Mounolou 2001).
Plants were severely affected (Barbault & Sastrapradja 1995). The previously dominant
Ottokariales (glossopterids) became extinct (Futuyma 1998, Anderson 1999). About 2/3
of the insect families became extinct and 6 insect orders disappeared (Barbault &
Sastrapradja 1995, Anderson 1999, Lévêque & Mounolou 2001). About 70% of the
vertebrate families were lost (Lévêque & Mounolou 2001). The tetrapods that suffered
losses were the amphibians and the mammal-like reptiles (therapsids) (Dobson 1996,
Futuyma 1998, Anderson 1999). The picture shows a Permian amphibian
Branchiosaurus. Land extinctions were less severe than the marine extinctions (Futuyma
1998). The marine organism losses included benthic foraminifera, brachiopods,
bryozoans, echinoderms, 44% of fish families (possibly including freshwater fish), all
graptolites, solitary corals and all trilobites (Groombridge 1992, Barbault & Sastrapradja
1995, Dobson 1996, Lévêque & Mounolou 2001).
Suggested causes include a variable climate or climate change (cooling), 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 (Groombridge 1992, Barbault & Sastrapradja 1995, Dobson 1996,
Futuyma 1998, Gaston & Spicer 1998, Primack 1998, Anderson 1999, Broswimmer
2002, Wikipedia Contributors 2006c).
The End Triassic mass extinction
This is the fourth of the ‘Big Five’ mass extinctions (Raup & Sepkoski 1982). It may
have been a protracted event taking place over 15 million years (Groombridge 1992,
Caughley & Gunn 1996, Lévêque & Mounolou 2001) or it may have been caused by a
series of catastrophes within 100 000 years (Broswimmer 2002). This extinction event
took place about 210 million years ago (Leakey & Lewin 1995, Caughley & Gunn 1996).
From the plants, several orders of gymnosperms were lost and the Umkomasiales
(Dicroidium) became extinct. Insects were apparently not severely affected by this
extinction event (Anderson 1999). Some reptile lineages were affected – the mammallike reptiles (therapsids) especially (Caughley & Gunn 1996, Dobson 1996, Primack
1998, Anderson 1999, Broswimmer 2002). The picture shows Phytosaur teeth.
Phytosaurs became extinct during this extinction event (Humboldt State University,
Natural History Museum 2005). The marine organisms affected included ammonites,
ammonoids, bivalves (Molluscs), brachiopods, corals, gastropods and sponges (Caughley
& Gunn 1996, Dobson 1996, Futuyma 1998, Primack 1998, Lévêque & Mounolou 2001,
Broswimmer 2002).
Suggested causes include one or more asteroid/comet impacts, climate change and
volcanic activity (Anderson 1999, Broswimmer 2002, Wikipedia Contributors 2006c).
The End Cretaceous mass extinction
This is the final and best known of the 5 mass extinctions (Groombridge 1992, Barbault
& Sastrapradja 1995). It may have been a protracted event (Barbault & Sastrapradja
1995) or it may have occurred as a series of peaks over a relatively short period of time
(100 000—1 million years) (Groombridge 1992, Barbault & Sastrapradja 1995). This
extinction event took place about 65 million years ago (Barbault & Sastrapradja 1995,
Caughley & Gunn 1996, Freeman & Herron 1998, Lévêque & Mounolou 2001,
Wikipedia Contributors 2006c).
Caughley & Gunn (1996) indicate that up to 75% of plant species disappeared but
Anderson (1999) indicates that plants were not seriously affected. The impact on insects
was minor (Anderson 1999). Tetrapods lost 36 families from 3 groups (dinosaurs (all
non-avian), plesiosaurs and pterosaurs (Groombridge 1992, Freeman & Herron 1998,
Anderson 1999, Lévêque & Mounolou 2001, Broswimmer 2002, Wikipedia Contributors
2006c). The picture shows a juvenile Tyrannosaurus rex skeleton. The marine
organisms affected included ammonites, ammonoids, cephalopods, bivalves,
foraminifera, icthyosaurs, mosasaurs, plankton and rudists (Barbault & Sastrapradja
1995, Freeman & Herron 1998, Futuyma 1998, Lévêque & Mounolou 2001).
Suggested causes include asteroid/comet impact, climate change and volcanic activity
(Groombridge 1992, Barbault & Sastrapradja 1995, Leakey & Lewin 1995, Freeman &
Herron 1998, Anderson 1999, Broswimmer 2002, Wikipedia Contributors 2006c). The
occurrence of an impact event has been verified (Leakey & Lewin 1995, Freeman &
Herron 1998).
The present mass extinction –phase one
This mass extinction has two phases. The first phase began with the dispersal of modern
humans over the earth about 100 000 years ago in the Pleistocene (Eldredge 2001). The
causes of the extinctions of this phase are debated. The probable causes considered are
human impacts, climate change or a combination of the two (Barbault & Sastrapradja
1995, Leakey & Lewin 1995, Wikipedia Contributors 2006d). Bolide impacts have also
been suggested as a cause of the end Pleistocene extinctions (Wikipedia Contributors
2006d).
For this phase, the human impact is difficult to prove and therefore debated (Barbault &
Sastrapradja 1995, Leakey & Lewin 1995, Caughley & Gunn 1996, Wikipedia
Contributors 2006d) but the continental extinctions (in Australia and the Americas) did
coincide with human colonisation and population expansion and archaeological sites
prove that the extinct megafauna were hunted. This evidence is circumstantial. Neither
the coincidence of extinction nor the evidence of hunting prove that the advent of humans
caused the extinctions in this phase (Barbault & Sastrapradja 1995, Leakey & Lewin
1995). Against the idea of human hunting as a cause of extinction in the Americas is the
fact that some large mammals survived. A possible explanation for this is that these
species were relatively new arrivals there (Leakey & Lewin 1995). Extinctions have not
occurred on as large a scale in Africa where the fauna coevolved with humans (Leakey &
Lewin 1995, Wikipedia Contributors 2006d). This suggests that the spread of advanced
human hunters was responsible for the extinctions on other continents (Leakey & Lewin
1995). For Australia, the lack of fossil remains with weapons in them counts against
hunting being the cause of the megafaunal extinctions but this can be explained as being
the result of the short period between human arrival and spread and the extinction of
species being too short for geological time and therefore archaeological records (Leakey
& Lewin 1995).
There are arguments for and against climate change as a cause of the extinctions in this
phase. In support of climate change as the cause, earlier Cenozoic extinctions were
attributed to climate change and the retreat of glaciers would have caused great changes
that could have stressed faunal populations but against this is the fact that the spread and
retreat of glaciers happened repeatedly in the past million years yet no large extinction
events are evident and similar changes in the ocean during the terminal Pleistocene did
not cause large scale extinctions (Barbault & Sastrapradja 1995, Leakey & Lewin 1995).
Further questions not answered by climate change are why the effects of climate change
were localised to one continent at a time and did not occur on all continents at the same
time and why, if climate change affected animals by devastating their food plants, were
important food plants for several extinct species abundant and widespread after the
animal species disappeared (Leakey & Lewin 1995)? Considered in favour of climate
change is the evidence that communities do not migrate as a unit, rather each plant
species migrates according to its own requirements meaning that animals would have to
adapt their feeding strategies (Leakey & Lewin 1995).
The diagram shows the pattern of extinctions following the advent of humans on different
continents and islands.
Additional Notes
Extract from Wikipedia (2006D), http://en.wikipedia.org/wiki/. “The term bolide (from
the Greek βολις, bolis, missile) can refer to either an extraterrestrial body that collides
with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it
ultimately impacts the surface.”
The present mass extinction –phase two
The second phase began with the development of agriculture about 10 000 years ago (the
beginning of the Holocene Epoch) (Eldredge 2001). The picture shows an example of
modern agriculture. Agriculture allowed humanity to live outside the boundaries of local
ecosystems and outside ecosystem carrying capacity (Eldredge 2001).
Extinctions in this phase are more clearly linked to human influence (Wikipedia
Contributors 2006d). There is evidence that the relatively recent migration of humans to
New Zealand and the Pacific Islands caused extinctions either directly (via hunting and
habitat destruction) or via the introduction of alien species (Leakey & Lewin 1995,
Caughley & Gunn 1996, Freeman & Herron 1998) which suggests that the Pleistocene
extinctions probably also resulted from the arrival of humans.
Humans are causing major environmental changes. The drivers for this sixth mass
extinction are agriculture, human overpopulation, overexploitation and invasive species.
This is seemingly the first mass extinction to have a biotic, not a physical, cause
(Eldredge 2001). 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 remnant
populations of several species (Wikipedia Contributors 2006d) and the phenomenon of
extinction debt whereby the majority of extinctions caused by habitat loss occur long
after the habitat was destroyed (Dobson 1996). Those who do not believe in the present
mass extinction either do not have access to the facts or are willfully ignorant (Leakey &
Lewin 1995).
Human extinction?
If all species will become extinct (Leakey & Lewin 1995, Freeman & Herron 1998), 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 (e.g. nuclear terrorism) (Leslie 1996, Wikipedia Contributors 2006e) but the
‘Doomsday argument’ proposed by Brandon Carter suggests that we should be suspicious
of low values for the probability of human extinction (Leslie1996). Human beings are
biased (against reason) in favour of the immortality of Homo sapiens (Wikipedia
Contributors 2006e).
Lester Brown, in his book ‘Eco-economy: building an economy for the earth’ (2001),
provides evidence that the current methods of food production are unsustainable (Brown
2001). However, Julian Simon believes that the present technology is enough to provide
for a continuously expanding population for the next 7 billion years (Leakey & Lewin
1995). Clearly, both cannot be right. Logic and the ‘Doomsday argument’ both suggest
that it would be sensible to act on Brown’s (2001) evidence regardless of whether we
believe it to be true or not. (Please note that I do not question the accuracy of Brown’s
(2001) evidence.)
Conclusions – the future?
Organisms with wide geographical ranges and smaller body size had a better chance of
surviving the past mass extinctions (Leakey & Lewin 1995, Freeman & Herron 1998) but
the present mass extinction acts differently to previous mass extinctions. The passenger
pigeon was widespread and abundant but it is extinct.
Extinction, excluding as a result of catastrophes, happens in stages (Leakey & Lewin
1995) starting with the reduction of habitat and population size. 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 (Leakey & Lewin 1995).
Biodiversity has recovered following each mass extinction event but only after the cause
of the event had dissipated. To end the present mass extinction, we must change our
present behaviour (Eldredge 2001).
If mass extinctions do occur periodically, then the next natural mass extinction should
occur in the next 10 million years (Wikipedia Contributors 2006c).
Last slide
I hope that you found chapter 6 informative and that you will enjoy chapter 7.
The picture shows a tightrope walker in Seoul.