Extinction: past, present, future Gwen Raitt Biodiversity and Conservation Biology 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. 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 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 & 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 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. 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 depo- sits that are available because of: Erosion, Metamorphosis. 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 & 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. 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 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 & 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 & Herron 1998, Futuyma 1998, 2005). Additional Notes Extract from Wikipedia, 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 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. 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 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. 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 & Herron 1998). Background extinction rates 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 recognized as mass extinctions varies from five to more than twenty (Leakey & 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 & Lewin 1995, Freeman & 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 & 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 & 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 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 End Ordovician 435.0—444.0 26—27 57—60 85 ± 3 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 & 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). 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 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 lea- ding 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. Most of the extinction events are likely to have been caused by a combination of factors (Leakey & 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 & 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 & 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 (Leakey & 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).

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. This is the earliest of the five mass extinctions (Raup & 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 & 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 (Futuyma 1998, Anderson 1999, Wikipedia Contributors 2006c), asteroid or comet impacts and a gamma ray burst (Wikipedia Contributors 2006c).

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. This is the second of the five mass extinctions. It may have occurred in two 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) (Caughley & Gunn 1996, Wikipedia Contributors 2006c) and multiple asteroid impacts (Anderson 1999).

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. This is the third and biggest of the five 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 two 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 two thirds of the insect families became extinct and six 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 include benthic foraminifera, brachiopods, bryozoans, echinoderms, 44% of fish families (possibly including freshwater fish), all graptolites, solitary corals, all trilobites (Groombridge 1992, Barbault & Sastrapradja 1995, Dobson 1996, Lévêque & Mounolou 2001). Suggested causes include a variable climate (Futuyma 1998) or climate change (cooling (Dobson 1996)) (Groombridge 1992, Gaston & Spicer 1998, Broswimmer 2002, Wikipedia Contributors 2006c), a drop in sea level (Barbault & Sastrapradja 1995, Gaston & Spicer 1998, Futuyma 1998, Broswimmer 2002), massive carbon dioxide (CO2) poisoning (Anderson 1999), oceanic anoxia, the explosion of a supernova (Barbault & Sastrapradja 1995, Gaston & 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).

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 gymno- sperms were lost and the Umkoma- siales (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. This is the fourth mass extinction (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 mammal-like 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 (Anderson 1999, Broswimmer 2002), climate change (Broswimmer 2002) and volcanic activity (Broswimmer 2002, Wikipedia Contributors 2006c).

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. This is the final and best known of the five 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). Anderson (1999) indicates that plants were not seriously affected but Caughley & 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 & Herron 1998, Anderson 1999, Lévêque & Mounolou 2001, Broswimmer 2002, Wikipedia Contributors 2006c). The marine organisms affected include ammonites, ammonoids, cephalopods, bivalves, foraminifera, icthyosaurs, mosasaurs, plackton and rudists (Barbault & Sastrapradja 1995, Freeman & Herron 1998, Futuyma 1998, Lévêque & Mounolou 2001). The picture shows a juvenile Tyrannosaurus rex skeleton. Suggested causes include asteroid/comet impact (Groombridge 1992, Barbault & Sastrapradja 1995, Leakey & Lewin 1995, Freeman & 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 & Lewin 1995, Freeman & Herron 1998).

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. 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 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 & 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 on that continent (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 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 & Sastrapradja 1995, Leakey & 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 & 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, 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. 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. 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 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 & Lewin 1995).

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 Bran- don Carter suggests that we should be suspi- cious 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 pro-vide 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. 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). 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 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).

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. 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 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 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).

Links to Other Chapters Chapter 1 Biodiversity: what is it? Chapter 2 The evolution of biodiversity 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. I hope that you found chapter 6 informative.