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Geological time scale extinction. converted

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Awais Bakshy
Awais Bakshy

Extinction can occur at the population level, called local extinction, or at the species level, called true extinction. True extinction involves the loss of all populations of a species globally. Mass extinctions have occurred throughout history due to various causes like asteroid impacts, volcanic activity, climate change, and more recently, human activity. Currently most extinctions are caused by human interference with habitats and ecosystems. International organizations monitor extinction rates and endangered species to try and control further biodiversity loss.

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Extinction. By Awais Bakshy.
 What Does Extinction Mean: The simple answer to this question is “the end of a species” literally the word is cognate with “extinguish” that is to say the point at which the last member of a species dies or is unable to breed. In practice, however, we need to broaden and vary this definition some what to make it more useful, both as a description of events and as a mechanism to use for prediction.  Extinction definitions are fundamentally scale dependent. Most species of organisms consist of several separate breeding populations occupying multiple habitat locations. If a small population of an organism in one habitat is wiped out by a catastrophic event, we refer to the loss as a local extinction (or extirpation). Although local extinction can have a dramatic local effect, disrupting that particular ecosystem, the problem is manageable for two reasons: 1) The area may receive colonizing individuals of the species from other populations nearby, and thus the extirpated population may re-establish.2) The species is not lost, since other populations are likely to persist outside the range of the extinction causing event. (Human activity can sometimes deliberately reintroduce the organism, not waiting for natural decolonization to occur.)
 True extinction, sometimes called species extinction or just “extinction”, involves either the loss of a species which existed in a single population instead of several, or the loss of all populations of a species to a larger causal event. Once an entire species is gone, there is no recovery. Species loss can have a major destabilizing effect (if a top predator species disappears, for example), or a more subtle effect. For example, when the dodo (a large flightless species of pigeon) was driven extinct by hunting on the Indian Ocean island of Mauritius, a steady decline in the breeding success of some tree species (such as Calvaria) began to be noticed. Today, some 350 years after the last dodos were killed, few stands of these trees younger than 350 years are observed. Though many other forces (such as deforestation) have also been at work in Mauritius, the role of the dodo remains clear: dodos used to eat the fruits of these trees, digesting the thin flesh on the surface and passing (only slightly scarred-up) the large seeds. Probably giant tortoises also had this effect on seeds. The trees are adapted to herbivory, and in fact “expect” it, and the lack of herbivores has led to reduced natural rates of germination, and a change in forest structure. You can demonstrate the “dodo effect” by scraping seeds yourself, or force-feeding them to dodo-analogue turkeys (the experiment actually was done!); germination rates are enhanced.
 Pseudo extinction (or phyletic extinction): of a species occurs when all members of the species are extinct, but members of a daughter species remain alive. The term pseudo extinction refers to the evolution of a species into a new form, with the resultant disappearance of the ancestral form. Pseudo extinction results in the relationship between ancestor and descendant still existing even though the ancestor species no longer exists.  The classic example is that of the non-avian dinosaurs. While the non-avian dinosaurs of the Mesozoic died out, their descendants, birds, live on today. As of now it is accepted that modern birds evolved from Archaeopteryx, a small dinosaur with flight feathers from the Mesozoic period. Many other families of bird-like dinosaurs also died out as the heirs of the dinosaurs continued to evolve, but because their birds continue to thrive in the world today their ancestors are only pseudo extinct.  Example: Horse evolution while the earliest ancestor became extinct its descendant survived to produce the modern ‘Equus’.
 The Role Of Extinction Through Long Term History:  Extinction has been a steady and influential force over evolutionary time, and few species escape it for more than 10 million years or so – mammal species, for instance, tend to persist for only 1-3 million years. New species are arising all the time, and it’s possible for older species to be outcompeted by new types, or eaten by enhanced new predators, or wiped out by newly-evolved diseases. Probably most extinctions are caused not by such biotic changes, but rather by a biotic ones: subtle climate-changes, drying-up of lakes and ponds, forests transitioning into grasslands or grasslands into deserts, all may leave specialized local organisms with no options. New organisms flood into the spaces created by extinction, and resources seldom go unused for very long – life is enterprising and attentive to opportunity.  Several times since the origin of life, the landscape of the biosphere has been modified almost overnight by massive forces of several different kinds: the result in each case has been a mass extinction event.
 Most people, even if they know nothing else about the history of biodiversity, are aware that a mass extinction 65 million years ago spelled the end of the “Age of Dinosaurs” (the Mesozoic Era), and ushered in the “Age of Mammals” (the Cenozoic), through the agency of an extraterrestrial impact from an asteroid. Fewer people know that this event also wiped out many other types of organisms, while leaving an unusual set of survivors standing… or that it was far from being the worst such event known  The “dinosaur-ending” event, marking the “K-T Boundary” (the dividing line between the Cretaceous period (K) of the Mesozoic Era and the Tertiary period (T) of the Cenozoic era) is now well-characterized. We know from a convergence of data that an asteroid (or perhaps a more comet-like body) of about 10km diameter, an object roughly the size of Mount Everest, struck the surface of the Earth at some 40 times the speed of sound on the north coast of the Yucutan Peninsula, near the modern town of Chicxulub, Mexico, leaving behind as it vaporized a crater of about 150km diameter (which is today partly under the sea, and partly buried by sediments, but detectable using geologic methods). Billions of tones of material,
 from the object and from the Earth, were hurled into the upper atmosphere by the force of the impact, and circulated globally within a few days to blanket the planet in a dense veil of dust, which circulation-models suggest remained thick for years if not decades. We know this occurred because all over the world there is a clay layer of that age containing extraordinarily high concentrations of the element iridium, and element almost unknown in the Earth’s crust but relatively abundant in asteroids and meteorites, deposited in dust fallout following the impact. Huge tsunamis would have devastated the Earth’s coastlines for weeks, and in fact extraterrestrial impacts striking the sea may have more devastating effects than impacts on land for just this reason. No doubt global photosynthetic production was dramatically curtailed by the dust, and many species disappeared through starvation.
 Temperature also took some hits, first having been pretty hot from global-scale fires triggered by the extreme radiant heat of the impact, then chilled by the reduced light- inputs from the smoke added to the atmospheric dust. Species-losses from these forces directly probably also triggered further indirect losses, like a chain of dominoes falling when one is struck. Overall, about 70% of species known at the time had disappeared in what must be described as a geologic “blink of an eye”, and probably the abundance of the remaining species was much reduced – the biosphere would have seemed pretty empty by our standards. Although some recent data suggest that the Chicxulub crater was a little too early to be the direct culprit for the K-T mass extinction, the occurrence of a global-scale impact is nevertheless clearly marked in the geologic record; possibly a string of impacting objects reached the Earth over a few hundred millennia, much as the comet Shoemaker-Levy 9 sent a string of impacting pieces at Jupiter recently.
 The largest mass extinction event, however, preceded the K-T event. At the boundary between the Permian period (the last part of the Palaeozoic era) and the Triassic period (the first part of the Mesozoic), a little over 250 million years ago, about 90% of known species perished in another very rapid phase. Recently evidence has come to light for a crater of about 200km diameter off the NW coast of Australia, which appears to be just the right age for a suitable impactor – representing an object of perhaps 10km or a little more in diameter, but there is not widespread agreement about this case. Around the same time as the Permo-Triassic event, there was a period of extensive volcanic activity in modern Siberia – perhaps this was a partial driving force for atmospheric, and thus biotic, change. Although extraterrestrial impacts play a part in mass extinctions, they are not the only factor involved; the older the event, that harder it is to pin down a specific cause, of course.
 Cause Of Mass Extinction:  Asteroid impacts. How: by reducing sunlight penetration of the atmosphere, changing climate and reducing production. Definitely occurs, but probably not for every mass extinction.  Tectonic rearrangement. How: by bringing together isolated continents (or oceans, as the case may be) and exposing organisms on each to the diseases, predators etc. upon the others. We know this occurred when South America was invaded by North American species a few million years ago, and most southern animal species went extinct and were replaced by northern ones. We don’t know how general or rapid a phenomenon it may be. (And some loss of biodiversity is likely whenever formerly separate areas are joined up, simply because there are fewer geographically isolated places to hold variant species.)  Loss of shallow seas. How: partly an artefact; most well-fossilized organisms like mollusks prefer shallow seas, so when shallow seas disappear, we measure (notice) “species-loss”. Shallow seas disappear when continents coalesce, and this loss may also be reflected in changes in global climate, since water warmed in coastal areas is less available to be circulated to cooler regions.
 Volcanic activity. How: by releasing large quantities of sulphur-containing or carbon-dioxide-rich gases into the atmosphere, altering climate and often inducing severely acidic precipitation. Requires a very large (thousands of square kilometers at least) area of intense eruptive activity, persisting for many millennia. Such areas have occurred (Siberia, Deccan Plateau of India), but their role in mass extinctions is still debated.  Climate change for other reasons. How: by changing the “life-zones” available to organisms and driving some off the map, as it were. We don’t know in most cases why climate changes, but it has done many times.  Ecological “cascades”. How: when some species go extinct, they destabilize life for others, in a “chain reaction”. Not clear if this is a strong enough force on its own to account for really major losses, but likely locally significant, and/or in combination with other forces.  Human activity. How: by direct hunting in part, but mostly by environmental pollution, habitat destruction, etc. Somewhat questionable if the last century has seen enough damage to count as a true mass extinction, but we may have more dramatic impacts yet to come.
 A combination of reasons. How: one or more of the others all occurring at about the same time. They may easily interact to wipe out more species than any could alone.  At Present:  Mostly due to human interference.  The realization of such a cause lead to starting of several international voluntary agencies to monitor and control extinctions.  “The red-data book brought out regularly by W. W. F (World Wide Fund for nature, Formerly IUCN - international union for the conservation of nature and natural resources) provides a list of animals and plants that are endangered or have become extinct.
Geological time scale extinction. converted

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Geological time scale extinction. converted

  • 2.  What Does Extinction Mean: The simple answer to this question is “the end of a species” literally the word is cognate with “extinguish” that is to say the point at which the last member of a species dies or is unable to breed. In practice, however, we need to broaden and vary this definition some what to make it more useful, both as a description of events and as a mechanism to use for prediction.  Extinction definitions are fundamentally scale dependent. Most species of organisms consist of several separate breeding populations occupying multiple habitat locations. If a small population of an organism in one habitat is wiped out by a catastrophic event, we refer to the loss as a local extinction (or extirpation). Although local extinction can have a dramatic local effect, disrupting that particular ecosystem, the problem is manageable for two reasons: 1) The area may receive colonizing individuals of the species from other populations nearby, and thus the extirpated population may re-establish.2) The species is not lost, since other populations are likely to persist outside the range of the extinction causing event. (Human activity can sometimes deliberately reintroduce the organism, not waiting for natural decolonization to occur.)
  • 3.  True extinction, sometimes called species extinction or just “extinction”, involves either the loss of a species which existed in a single population instead of several, or the loss of all populations of a species to a larger causal event. Once an entire species is gone, there is no recovery. Species loss can have a major destabilizing effect (if a top predator species disappears, for example), or a more subtle effect. For example, when the dodo (a large flightless species of pigeon) was driven extinct by hunting on the Indian Ocean island of Mauritius, a steady decline in the breeding success of some tree species (such as Calvaria) began to be noticed. Today, some 350 years after the last dodos were killed, few stands of these trees younger than 350 years are observed. Though many other forces (such as deforestation) have also been at work in Mauritius, the role of the dodo remains clear: dodos used to eat the fruits of these trees, digesting the thin flesh on the surface and passing (only slightly scarred-up) the large seeds. Probably giant tortoises also had this effect on seeds. The trees are adapted to herbivory, and in fact “expect” it, and the lack of herbivores has led to reduced natural rates of germination, and a change in forest structure. You can demonstrate the “dodo effect” by scraping seeds yourself, or force-feeding them to dodo-analogue turkeys (the experiment actually was done!); germination rates are enhanced.
  • 4.  Pseudo extinction (or phyletic extinction): of a species occurs when all members of the species are extinct, but members of a daughter species remain alive. The term pseudo extinction refers to the evolution of a species into a new form, with the resultant disappearance of the ancestral form. Pseudo extinction results in the relationship between ancestor and descendant still existing even though the ancestor species no longer exists.  The classic example is that of the non-avian dinosaurs. While the non-avian dinosaurs of the Mesozoic died out, their descendants, birds, live on today. As of now it is accepted that modern birds evolved from Archaeopteryx, a small dinosaur with flight feathers from the Mesozoic period. Many other families of bird-like dinosaurs also died out as the heirs of the dinosaurs continued to evolve, but because their birds continue to thrive in the world today their ancestors are only pseudo extinct.  Example: Horse evolution while the earliest ancestor became extinct its descendant survived to produce the modern ‘Equus’.
  • 5.  The Role Of Extinction Through Long Term History:  Extinction has been a steady and influential force over evolutionary time, and few species escape it for more than 10 million years or so – mammal species, for instance, tend to persist for only 1-3 million years. New species are arising all the time, and it’s possible for older species to be outcompeted by new types, or eaten by enhanced new predators, or wiped out by newly-evolved diseases. Probably most extinctions are caused not by such biotic changes, but rather by a biotic ones: subtle climate-changes, drying-up of lakes and ponds, forests transitioning into grasslands or grasslands into deserts, all may leave specialized local organisms with no options. New organisms flood into the spaces created by extinction, and resources seldom go unused for very long – life is enterprising and attentive to opportunity.  Several times since the origin of life, the landscape of the biosphere has been modified almost overnight by massive forces of several different kinds: the result in each case has been a mass extinction event.
  • 6.  Most people, even if they know nothing else about the history of biodiversity, are aware that a mass extinction 65 million years ago spelled the end of the “Age of Dinosaurs” (the Mesozoic Era), and ushered in the “Age of Mammals” (the Cenozoic), through the agency of an extraterrestrial impact from an asteroid. Fewer people know that this event also wiped out many other types of organisms, while leaving an unusual set of survivors standing… or that it was far from being the worst such event known  The “dinosaur-ending” event, marking the “K-T Boundary” (the dividing line between the Cretaceous period (K) of the Mesozoic Era and the Tertiary period (T) of the Cenozoic era) is now well-characterized. We know from a convergence of data that an asteroid (or perhaps a more comet-like body) of about 10km diameter, an object roughly the size of Mount Everest, struck the surface of the Earth at some 40 times the speed of sound on the north coast of the Yucutan Peninsula, near the modern town of Chicxulub, Mexico, leaving behind as it vaporized a crater of about 150km diameter (which is today partly under the sea, and partly buried by sediments, but detectable using geologic methods). Billions of tones of material,
  • 7.  from the object and from the Earth, were hurled into the upper atmosphere by the force of the impact, and circulated globally within a few days to blanket the planet in a dense veil of dust, which circulation-models suggest remained thick for years if not decades. We know this occurred because all over the world there is a clay layer of that age containing extraordinarily high concentrations of the element iridium, and element almost unknown in the Earth’s crust but relatively abundant in asteroids and meteorites, deposited in dust fallout following the impact. Huge tsunamis would have devastated the Earth’s coastlines for weeks, and in fact extraterrestrial impacts striking the sea may have more devastating effects than impacts on land for just this reason. No doubt global photosynthetic production was dramatically curtailed by the dust, and many species disappeared through starvation.
  • 8.  Temperature also took some hits, first having been pretty hot from global-scale fires triggered by the extreme radiant heat of the impact, then chilled by the reduced light- inputs from the smoke added to the atmospheric dust. Species-losses from these forces directly probably also triggered further indirect losses, like a chain of dominoes falling when one is struck. Overall, about 70% of species known at the time had disappeared in what must be described as a geologic “blink of an eye”, and probably the abundance of the remaining species was much reduced – the biosphere would have seemed pretty empty by our standards. Although some recent data suggest that the Chicxulub crater was a little too early to be the direct culprit for the K-T mass extinction, the occurrence of a global-scale impact is nevertheless clearly marked in the geologic record; possibly a string of impacting objects reached the Earth over a few hundred millennia, much as the comet Shoemaker-Levy 9 sent a string of impacting pieces at Jupiter recently.
  • 9.  The largest mass extinction event, however, preceded the K-T event. At the boundary between the Permian period (the last part of the Palaeozoic era) and the Triassic period (the first part of the Mesozoic), a little over 250 million years ago, about 90% of known species perished in another very rapid phase. Recently evidence has come to light for a crater of about 200km diameter off the NW coast of Australia, which appears to be just the right age for a suitable impactor – representing an object of perhaps 10km or a little more in diameter, but there is not widespread agreement about this case. Around the same time as the Permo-Triassic event, there was a period of extensive volcanic activity in modern Siberia – perhaps this was a partial driving force for atmospheric, and thus biotic, change. Although extraterrestrial impacts play a part in mass extinctions, they are not the only factor involved; the older the event, that harder it is to pin down a specific cause, of course.
  • 10.  Cause Of Mass Extinction:  Asteroid impacts. How: by reducing sunlight penetration of the atmosphere, changing climate and reducing production. Definitely occurs, but probably not for every mass extinction.  Tectonic rearrangement. How: by bringing together isolated continents (or oceans, as the case may be) and exposing organisms on each to the diseases, predators etc. upon the others. We know this occurred when South America was invaded by North American species a few million years ago, and most southern animal species went extinct and were replaced by northern ones. We don’t know how general or rapid a phenomenon it may be. (And some loss of biodiversity is likely whenever formerly separate areas are joined up, simply because there are fewer geographically isolated places to hold variant species.)  Loss of shallow seas. How: partly an artefact; most well-fossilized organisms like mollusks prefer shallow seas, so when shallow seas disappear, we measure (notice) “species-loss”. Shallow seas disappear when continents coalesce, and this loss may also be reflected in changes in global climate, since water warmed in coastal areas is less available to be circulated to cooler regions.
  • 11.  Volcanic activity. How: by releasing large quantities of sulphur-containing or carbon-dioxide-rich gases into the atmosphere, altering climate and often inducing severely acidic precipitation. Requires a very large (thousands of square kilometers at least) area of intense eruptive activity, persisting for many millennia. Such areas have occurred (Siberia, Deccan Plateau of India), but their role in mass extinctions is still debated.  Climate change for other reasons. How: by changing the “life-zones” available to organisms and driving some off the map, as it were. We don’t know in most cases why climate changes, but it has done many times.  Ecological “cascades”. How: when some species go extinct, they destabilize life for others, in a “chain reaction”. Not clear if this is a strong enough force on its own to account for really major losses, but likely locally significant, and/or in combination with other forces.  Human activity. How: by direct hunting in part, but mostly by environmental pollution, habitat destruction, etc. Somewhat questionable if the last century has seen enough damage to count as a true mass extinction, but we may have more dramatic impacts yet to come.
  • 12.  A combination of reasons. How: one or more of the others all occurring at about the same time. They may easily interact to wipe out more species than any could alone.  At Present:  Mostly due to human interference.  The realization of such a cause lead to starting of several international voluntary agencies to monitor and control extinctions.  “The red-data book brought out regularly by W. W. F (World Wide Fund for nature, Formerly IUCN - international union for the conservation of nature and natural resources) provides a list of animals and plants that are endangered or have become extinct.