Notes towards Biodiversity Chapter 6
Introductory/Title Slide (1)
Hello. My name is Gwen Raitt. I will be presenting this chapter on extinction over time.
What is Extinction?
Extinction is the process through which a species or higher taxonomic category ceases to
exist (Fiedler and Jain 1992, Wikipedia Contributors 2006a). This definition may be
applied within selected geographical boundaries (Fiedler and 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 show (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
and Herron 1998, Lévêque and 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 and Spicer 1998) while a
species has an average lifespan of 4 million years (Groombridge 1992, Barbault and
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 and Gunn 1996).
The Geologic Time Scale
The geologic time scale illustrates the magnitude of time period in which fossils occur.
The oldest fossil dates from 3.5 billion year 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 five recognized 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 (Futuyma 1998, 2005). 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 in low-lying areas such as aquatic
environments. Marine sediments are most common (Freeman and 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 erode (Futuyma
1998). The further back in time, the fewer the sedimentary deposits that are available
because there has been more time for erosion to act or metamorphosis to occur (Futuyma
1998).
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, USA.), 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 and 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 (higher
elevations) or they did not live/die during a period of sedimentation (Freeman and Herron
1998, Futuyma 1998, 2005).
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 (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. For some groups, living species can be
differentiated by skeletal features so the fossil species are probably also skeletally unique
(Futuyma 1998). 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 characterizes 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 and Herron 1998). Background extinction rates may be declining (Raup and
Sepkoski 1982). Interspecific competition is a possible cause of background extinction
(Barbault and Sastrapradja 1995). Extinction events are relatively short (in terms of
geological time) periods with greatly increased extinction rates (Leakey and 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 recognized as mass extinctions varies from five to more than
twenty (Leakey and Lewin 1995, Wikipedia Contributors 2006c). To qualify as a mass
extinction event, the extinction event must eliminate >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 and Lewin 1995, Freeman and Herron
1998, Broswimmer 2002). While mass extinctions remove >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 and Spicer
1998). Examination of extinction patterns is only possible as a result of the development
of such attributes as bones and teeth forming 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 and Sepkoski 1982, Leakey and
Lewin 1995, Raup 1998, Futuyma 1998). Raup and 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 Skeletonized Marine Invertebrates of the ‘Big Five’ Mass
Extinctions (modified from p713, Futuyma 1998). The modifications come from
Anderson (1999), Lévêque and Mounolou (2001), Broswimmer (2002), Futuyma (2005)
and Wikipedia Contributors (2006c). Time periods are given for the older mass
extinctions because the literature gives variable dates. A median 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 and Lewin 1995, Broswimmer 2002), for example, 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 and Lewin 1995). The picture shows Perito Morena Glacier in Argentina.
Consider also that proximate causes of extinctions, for example 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 and 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
and Lewin 1995, Freeman and Herron 1998). The aforementioned consequences would
have caused ecological disruption leading to further extinctions (Freeman and Herron
1998). Some previously postulated causes of mass extinctions may be unlikely or even
impossible. A supernova explosion is considered very unlikely (Leakey and Lewin 1995)
and recent research shows that a nearby gamma ray burst is impossible because our
galaxy is metal rich (Wikipedia Contributors 2006c). Mass extinctions are considered
too extensive and too sudden to be the result of biological causes such as disease or
competition (Wikipedia Contributors 2006c).
End Ordovician Mass Extinction
This is the earliest of the five mass extinctions (Raup and Sepkoski 1982). Of the five, it
had the second largest impact (Futuyma 1998). It may have occurred as three peaks
spread over 500 000 years (Groombridge 1992) or two peaks separated by one to two
million years (Caughley and 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 and 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
(Futuyma 1998, Anderson 1999, Wikipedia Contributors 2006c), asteroid or comet
impacts and a gamma ray burst (Wikipedia Contributors 2006c).
Late Devonian Mass Extinction
This is the second of the five mass extinctions. It may have occurred in two stages over
about 15 million years (Raup and Sepkoski 1982) or as two or more peaks over 3 million
years (Caughley and 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 and Lewin 1995, Lévêque and 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 and Gunn 1996,
Dobson 1996, Primack 1998, Lévêque and Mounolou 2001). The picture shows a
Devonian example of a trilobite. Suggested causes include climate change (a drop in
temperature) (Caughley and Gunn 1996, Wikipedia Contributors 2006c) and multiple
asteroid impacts (Anderson 1999).
End Permian Mass Extinction
This is the third and biggest of the five mass extinctions (Groombridge 1992, Barbault
and Sastrapradja 1995, Futuyma 1998). It may have taken place over 5—8 million years
(Groombridge 1992, Caughley and Gunn 1996, Dobson 1996) or been a single event that
took less than 10 000 years or occurred as two peaks over 5 million years (Barbault and
Sastrapradja 1995). This extinction event took place about 245 million years ago
(Barbault and Sastrapradja 1995, Gaston and Spicer 1998, Lévêque and Mounolou 2001).
Plants were severely affected (Barbault and Sastrapradja 1995). The previously dominant
Ottokariales (glossopterids) became extinct (Futuyma 1998, Anderson 1999). About two
thirds of the insect families became extinct and six insect orders disappeared (Barbault
and Sastrapradja 1995, Anderson 1999, Lévêque and Mounolou 2001). About 70% of
the vertebrate families were lost (Lévêque and 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 include benthic foraminifera, brachiopods,
bryozoans, echinoderms, 44% of fish families (possibly including freshwater fish), all
graptolites, solitary corals, all trilobites (Groombridge 1992, Barbault and Sastrapradja
1995, Dobson 1996, Lévêque and Mounolou 2001). Suggested causes include a variable
climate (Futuyma 1998) or climate change (cooling (Dobson 1996)) (Groombridge 1992,
Gaston and Spicer 1998, Broswimmer 2002, Wikipedia Contributors 2006c), a drop in
sea level (Barbault and Sastrapradja 1995, Gaston and Spicer 1998, Futuyma 1998,
Broswimmer 2002), massive carbon dioxide (CO2) poisoning (Anderson 1999), oceanic
anoxia, the explosion of a supernova (Barbault and Sastrapradja 1995, Gaston and Spicer
1998), asteroid or comet impacts (Primack 1998, Wikipedia Contributors 2006c), plate
tectonics during the formation of Pangea (Groombridge 1992, Broswimmer 2002,
Wikipedia Contributors 2006c) and high volcanic activity (Groombridge 1992, Futuyma
1998, Primack 1998, Wikipedia Contributors 2006c).
End Triassic Mass Extinction
This is the fourth mass extinction (Raup and Sepkoski 1982). It may have been a
protracted event taking place over 15 million years (Groombridge 1992, Caughley and
Gunn 1996, Lévêque and 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 and Lewin 1995, Caughley and 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 mammal-like reptiles
(therapsids) especially (Caughley and 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 and Gunn
1996, Dobson 1996, Futuyma 1998, Primack 1998, Lévêque and Mounolou 2001,
Broswimmer 2002). Suggested causes include one or more asteroid/comet impacts
(Anderson 1999, Broswimmer 2002), climate change (Broswimmer 2002) and volcanic
activity (Broswimmer 2002, Wikipedia Contributors 2006c).
End Cretaceous Mass Extinction
This is the final and best known of the five mass extinctions (Groombridge 1992,
Barbault and Sastrapradja 1995). It may have been a protracted event (Barbault and
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 and Sastrapradja
1995). This extinction event took place about 65 million years ago (Barbault and
Sastrapradja 1995, Caughley and Gunn 1996, Freeman and Herron 1998, Lévêque and
Mounolou 2001, Wikipedia Contributors 2006c). Anderson (1999) indicates that plants
were not seriously affected but Caughley and Gunn (1996) indicate that up to 75% of
plant species disappeared. The impact on insects was minor (Anderson 1999). Tetrapods
lost thirty-six families from three groups (dinosaurs (all non-avian), plesiosaurs and
pterosaurs (Groombridge 1992, Freeman and Herron 1998, Anderson 1999, Lévêque and
Mounolou 2001, Broswimmer 2002, Wikipedia Contributors 2006c). The marine
organisms affected include ammonites, ammonoids, cephalopods, bivalves, foraminifera,
icthyosaurs, mosasaurs, plankton and rudists (Barbault and Sastrapradja 1995, Freeman
and Herron 1998, Futuyma 1998, Lévêque and Mounolou 2001). The picture shows a
juvenile Tyrannosaurus rex skeleton. Suggested causes include asteroid/comet impact
(Groombridge 1992, Barbault and Sastrapradja 1995, Leakey and Lewin 1995, Freeman
and Herron 1998, Anderson 1999, Broswimmer 2002, Wikipedia Contributors 2006c),
climate change (Broswimmer 2002) and volcanic activity (Wikipedia Contributors
2006c). The occurrence of an impact event has been verified (Leakey and Lewin 1995,
Freeman and Herron 1998).
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 and Sastrapradja
1995, Leakey and 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 and Sastrapradja 1995, Leakey and Lewin 1995, Caughley and Gunn 1996,
Wikipedia Contributors 2006d) but the continental extinctions (in Australia and the
Americas) did coincide with human colonization and population expansion and
archaeological sites prove that the extinct megafauna were hunted but this evidence is
circumstantial, neither prove that the advent of humans caused the extinctions in this
phase (Barbault and Sastrapradja 1995, Leakey and 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 on that continent (Leakey and Lewin 1995). Extinctions have not occurred
on as large a scale in Africa where the fauna coevolved with humans (Leakey and Lewin
1995, Wikipedia Contributors 2006d). This suggests that the spread of advanced human
hunters was responsible for the extinctions on other continents (Leakey and 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 and 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 extinction were
attributed to climate change and the retreat of glaciers would have cause 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 extinctions
are evident and similar changes in the ocean during the terminal Pleistocene did not cause
extinctions (Barbault and Sastrapradja 1995, Leakey and Lewin 1995). Further questions
not answered by climate change are why the effects of climate change were localized 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 mammal species
disappeared (Leakey and 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 and Lewin 1995). The diagram shows the pattern of
extinctions following the advent of humans on different continents and islands.
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 and Lewin 1995, Caughley and Gunn 1996,
Freeman and 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 and 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, by the existence
in the wild of the remnant populations of several species (Wikipedia Contributors 2006d)
and by 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 and Lewin 1995).
Human Extinction?
If all species will become extinct (Leakey and Lewin 1995, Freeman and 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).
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 and Lewin
1995). Clearly, both cannot be right. Logic and the ‘Doomsday argument’ both suggest
that it would be sensible to act on Brown’s evidence regardless of whether we believe it
to be true or not. Please note that I do not question the accuracy of Brown’s evidence.
Human beings are biased (against reason) in favour of the immortality of Homo sapiens
(Wikipedia Contributors 2006e).
Conclusions – the Future?
Organisms with wide geographical ranges and smaller body size had a better chance of
surviving the past mass extinctions (Leakey and Lewin 1995, Freeman and Herron 1998)
but the present 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 and 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 and Lewin 1995). 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
(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.