Extinction: past, present, future Material: Gwen Raitt BCB 341: Principles of Conservation Biology
What is Extinction? • Extinction is the process through which a species or higher taxonomic category ceases to exist. • Extinction may also be defined as the disappearance of any evolutionary lineage (from populations to species to higher taxonomic categories) because of death or the genetic modification of every individual. • Where a lineage has changed such that a new (daughter) species is recognised, the extinction of the original (parent) species may also be called pseudoextinction. • The new and original species are known as chronospecies. • Extinction may be regarded as the result of failing to adapt to environmental changes. • Extinction is a natural process.
The Fossil Record – Key to the Past The Occurrence of Fossil-Bearing Rocks • Fossils are usually found in sedimentary rocks. • Sedimentary deposits are most likely in low-lying areas. • Each site may have fossils representing a limited fraction of geological time because: • Sediment deposition was not continuous, • Sedimentary rocks erode. • The further back in time, the fewer the sedimentary depo-sits 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) 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.
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 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.
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 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.
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.