Mass Extinction

A mass extinction or extinction event is the phenomenon in which a large number of species of life on Earth become extinct in a relatively short period of time. In general usage by scientists, "mass extinction" refers to an extinction affecting a great many different groups of organisms occupying diverse and wide-spread environments. Extinction of species, genera, families, and even orders of organisms has occurred throughout the history of life on Earth, but mass extinctions are those events that greatly exceed the normal or background extinction rate.

Based on the fossil record, the background rate of extinctions is about two to five taxonomic families of marine invertebrates and vertebrates every million years. In contrast, during a mass extinction event some 20 to 50 percent of all genera on Earth at that time may become extinct over a period of one million years or less.

At least five major and global mass extinction events have occurred during the past 542 million years in which there have been sufficient bones, shells, and other hard parts to produce a fossil record supporting a systematic study of extinction patterns. Given the lack of a precise definition of mass extinction, some authorities argue for as many as 20 mass extinctions.

The concept of mass extinction has occasioned relatively little stress with religion and theology because religions in the Euro-American cultural sphere in the nineteenth century had already been forced by the strong fossil evidence of single species extinction to accept that the Creator must have permitted some of His creations to become extinct.

Many scientists believe that the earth is presently undergoing another mass extinction, the "Sixth Extinction" (or the "Holocene extinction event") tied to the arrival of human beings and their dispersal over the globe. As humans become aware of this ongoing and accelerating extinction and of human culpability for it, the human species has a choice as to whether it will continue to decrease species diversity or to reverse direction and begin to conserve biodiversity. The new school of Christian environmental theology aims to relate theology to responsible stewardship of the environment, including preservation of biodiversity.

Extinction events

The classical "Big Five" mass extinctions identified by Raup and Sepkoski (1982) are widely agreed upon as some of the most significant: (1) End Ordovician (Ordovician-Silurian extinction), (2) Late Devonian (Late Devonian extinction), (3) End Permian (Permian-Triassic extinction), (4) End Triassic (Triassic-Jurassic extinction), and (5) End Cretaceous (Cretaceous-Tertiary extinction). (See geologic time scale for an overview of these time periods.)

These and a pair of other extinction events acting as "book ends" for the Big Five are highlighted below:

1. End Ordovician extinction (about 444 million years ago). Two Ordovician-Silurian extinction events occurred, probably as the result of a period of glaciation. Marine habitats changed drastically as sea levels decreased causing the first die-off, and then, when sea levels rose rapidly between five hundred thousand to a million years later, a second great die-off occurred. On hypothesis is that a gamma ray burst may have triggered this extinction (Jha 2005).
2. Late Devonian extinction (about 360 million years ago). Near the Devonian-Carboniferous transition a prolonged series of extinctions led to the elimination of about 70 percent of all species. This was not a sudden event, with the period of decline lasting perhaps as long as 20 million years. However, there is evidence for a series of extinction pulses within this period.
3. End Permian extinction (about 251 million years ago). At the Permian-Triassic transition (the Permian-Triassic extinction event) about 95 percent of all marine species went extinct. This catastrophe was Earth's worst mass extinction, killing 53 percent of marine families, 84 percent of marine genera, and an estimated 70 percent of land species (including plants, insects, and vertebrate animals.)
4. End Triassic extinction (about 200 million years ago). At the time of the Triassic-Jurassic transition, about 20 percent of all marine families as well as most non-dinosaurian archosaurs, most therapsids (except the order from which mammals descended), and the last of the large amphibians were eliminated.
5. End Cretaceous extinction (about 65 million years ago). At the time of the Cretaceous-Paleogene transition (the Cretaceous-Tertiary extinction event) about 50 percent of all species became extinct (including all non-avian dinosaurs). This extinction is widely believed to have resulted from an asteroid or comet impact event, although there is not a consensus on this theory.
6. End Cambrian extinction (about 488 million years ago). A series of mass extinctions at the Cambrian-Ordovician transition eliminated many brachiopods and conodonts (a group of small eel-like vertebrates characterized by multiple pairs of bony toothplates) and severely reduced the number of trilobite species.
7. Holocene extinction (Present day). A 1998 survey by the American Museum of Natural History found that 70 percent of biologists view the present era as part of a mass extinction event, the Holocene extinction event. The extinction of many megafauna near the end of the most recent ice age is also sometimes considered a part of the Holocene extinction event.

Causes for mass extinction

Some of the hypotheses for the causes of mass extinction events are:.

1. Impact events. The impact of a sufficiently large asteroid or comet could create large tsunamis, global forest fires, and reduction of incoming sunlight due to large amounts of dust and smoke in the atmosphere. Taken together, it is not surprising that these and other related effects from an impact event might be sufficiently severe as to disrupt the global ecosystem and cause extinctions. Only for the End Cretaceous extinction (about 65 mya) is there strong evidence of such an impact. Circumstantial evidence of such events is also given for the End Ordovician extinction (about 444 mya), End Permian extinction (about 251 mya), End Jurassic extinction (about 145 mya), and End Eocene extinction (about 40 mya).
2. Climate change. Rapid transitions in climate may be capable of stressing the environment to the point of extinction. However, it is worth observing that the recent cycles of ice ages are believed to have had only very mild impacts on biodiversity. Extinctions suggested to have this cause include: End Ordovician (about 444 mya), End Permian (about 251 mya), and Late Devonian (about 360 mya).
3. Volcanism. The formation of large igneous provinces through the outflow of up to millions of cubic kilometers of lava in a short duration is likely to poison the atmosphere and oceans in a way that may cause extinctions. This cause has been proposed for the End Cretaceous extinction (about 65 mya), End Permian extinction (about 251 mya), End Triassic extinction (about 200 mya), and End Jurassic extinction (about 145 mya).
4. Gamma ray burst. A nearby gamma ray burst (less than 6,000 light years distance) could destroy the ozone layer and sufficiently irradiate the surface of the Earth to kill organisms living there. From statistical arguments, approximately 1 gamma ray burst would be expected to occur in close proximity to Earth in the last 540 million years. This has been suggested as a possible explanation for the End Ordovician extinction (about 444 mya). However, a recent study by leading gamma ray burst researchers says that gamma ray bursts are not possible in metal rich galaxies like our own (Stanek et al. 2006).
5. Plate tectonics. The opening and closing of seaways and land bridges may play a role in extinction events as previously isolated populations are brought into contact and new dynamics are established in the ecosystem. This is most frequently discussed in relation to the End Permian extinction (about 251 mya).

Other hypotheses, such as the spread of a new disease or simple competition following an especially successful biological innovation are also considered. However, it is often thought that the major mass extinctions in Earth's history are too sudden and too extensive to have resulted solely from biological events.

The Big Five extinction events

Ordovician-Silurian extinction

The Ordovician-Silurian extinction (about 444 mya), which may have comprised several closely spaced events, was the second largest of the five major extinction events in Earth history in terms of percentage of genera that went extinct. (The only larger one was the Permian-Triassic extinction (about 251 mya).)

The End Ordovician extinctions occurred approximately 447 to 444 million years ago and mark the boundary between the Ordovician period and the following Silurian period. During this extinction event, there were several marked changes in the isotopic ratios of the biologically responsive elements carbon and oxygen. These changes in the isotopic ratios may indicate distinct events or particular phases within one event. At that time, all complex multicellular organisms lived in the sea, and of them, about 100 marine families covering about 49 percent of genera (a more reliable estimate than species) of fauna became extinct (Rohde 2005). The bi-valve brachiopods and the tiny, colonial bryozoans were decimated, along with many of the families of trilobites, conodonts, and graptolites (small, marine colonial animals).

The most commonly accepted theory is that they were triggered by the onset of a long ice age, perhaps the most severe glacial age of the Phanerozoic eon, which ended the long, stable greenhouse conditions typical of the Ordovician period. The event was preceded by a fall in atmospheric CO₂, which selectively affected the shallow seas where most organisms lived. As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it. Evidence of these has been detected in late Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time. Glaciation locks up water from the oceans, and the interglacials free it, causing sea levels repeatedly to drop and rise. During the glaciation, the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches, then returned carrying diminished founder populations lacking many whole families of organisms, then withdrew again with the next pulse of glaciation, eliminating biological diversity at each change (Emiliani 1992).

The shifting in and out of glaciation stages incurred a shift in the location of bottom water formation—from low latitudes, characteristic of greenhouse conditions, to high latitudes, characteristic of icehouse conditions, which was accompanied by increased deep-ocean currents and oxygenation of the bottom water. An opportunistic fauna briefly thrived there, before anoxic conditions returned. The breakdown in the oceanic circulation patterns brought up nutrients from the abyssal waters. Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.

The end of the second event occurred when melting glaciers caused the sea level to rise and stabilize once more.

Scientists from the University of Kansas and NASA have suggested that the initial extinctions could have been caused by a gamma ray burst originating from an exploding star within 6,000 light years of Earth (within a nearby arm of the Milky Way Galaxy). A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, causing surface-dwelling organisms, including those responsible for planetary photosynthesis, to be exposed to high levels of ultraviolet radiation. This would have killed many species and caused a drop in temperatures. While plausible, there is no unambiguous evidence that such a nearby gamma ray burst has ever actually occurred.

The rebound of life's diversity with the permanent re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving orders.

Late Devonian extinction

The Late Devonian extinction was one of the five major extinction events in the history of the Earth's biota. A major extinction occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), about 364 million years ago, when all the fossil agnathan fishes (the jawless fishes) suddenly disappeared. A second strong pulse closed the Devonian period.

Although it is clear that there was a massive loss of biodiversity toward the end of the Devonian, the extent of time during which these events took place is still unclear, with estimates as brief as 500 thousand years or as extended as 15 million years, the full length of the Famennian. Nor is it clear whether it concerned two sharp mass extinctions or a cumulative sequence of several smaller extinctions.

Anoxic conditions in the seabed of late Devonian ocean basins produced some oil shale. The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef-systems, including the coral-like stromatoporoids, and the rugose and tabulate corals. The reef system collapse was so severe that major reef-building (effected by new families of carbonate-excreting organisms, the modern scleractinian corals) did not recover until the Mesozoic era.

The late Devonian crash in biodiversity was more drastic than the familiar extinction event that closed the Cretaceous: A recent survey (McGhee 1996) estimates that 22 percent of all the families of marine animals (largely invertebrates) were eliminated, the category of families offering a broad range of real structural diversity. Some 57 percent of the genera went extinct, and—the estimate most likely to be adjusted—at least 75 percent of the species did not survive into the following Carboniferous. The estimates of species loss depend on surveys of marine taxa that are perhaps not known well enough to assess their true rate of losses, and for the Devonian it is not easy to allow for possible effects of differential preservation and sampling biases. Among the severely affected marine groups were the brachiopods, trilobites, ammonites, conodonts, and acritarchs, as well as jawless fish, and all placoderms (armored fishes). Freshwater species, including our tetrapod (four-legged vertebrates) ancestors, were less affected.

Reasons for the late Devonian extinctions are still speculative. Bolide (asteroids, meteorites) impacts could be dramatic triggers of mass extinctions. In 1969, Canadian paleontologist Digby McLaren suggested that an asteroid impact was the prime cause of this faunal turnover, supported by McGhee (1996), but no secure evidence of a specific extraterrestrial impact has been identified in this case.

The "greening" of the continents occurred during Devonian time: By the end of the Devonian, complex branch and root systems supported trees 30 m (98 ft) tall, and the deposits of organic matter that would become Earth's earliest coal deposits accumulated. But the mass extinction at the Frasnian-Famennian boundary did not affect land plants. The covering of the planet's continents with photosynthesizing land plants may have reduced carbon dioxide levels in the atmosphere, and since CO₂ is a greenhouse gas, reduced levels may have helped produce a chillier climate. A cause of the extinctions may have been an episode of global cooling, following the mild climate of the Devonian period. Evidence, such as glacial deposits in northern Brazil (located near the Devonian South Pole), suggest widespread glaciation at the end of the Devonian, as a large continental mass covered the polar region. Massive glaciation tends to lower eustatic sea-levels, which may have exacerbated the late Devonian crisis. Because glaciation appears only toward the very end of the Devonian, it is more likely to be a result, rather than a cause of the drop in global temperatures.

McGhee (1996) has detected some trends that lead to his conclusion that survivors generally represent more primitive or ancestral morphologies. In other words, the conservative generalists are more likely to survive an ecological crisis than species that have evolved as specialists.

Permian-Triassic extinction

The Permian-Triassic (P-T or PT) extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 251 million years ago, defining the boundary between the Permian and Triassic periods. It was the Earth's most severe extinction event, with about 90 percent of all marine species and 70 percent of terrestrial vertebrate species going extinct.

For some time after the event, fungal species were the dominant form of terrestrial life. Though they only made up approximately 10 percent of remains found before and just after the extinction horizon, fungal species subsequently grew rapidly to make up nearly 100 percent of the available fossil record (Eshet et al. 1995). However, some researchers argue that fungal species did not dominate terrestrial life, as their remains have only been found in shallow marine deposits (Wignall 1996). Alternatively, others argue that fungal hypha (long, branching filament) are simply better suited for preservation and survival in the environment, creating an inaccurate representation of certain species in the fossil record (Erwin 1993).

At one time, this die-off was assumed to have been a gradual reduction over several million years. Now, however, it is commonly accepted that the event lasted less than a million years, from 252.3 to 251.4 million years ago (both numbers ±300,000 years), a very brief period of time in geological terms. Organisms throughout the world, regardless of habitat, suffered similar rates of extinction, suggesting that the cause of the event was a global, not local, occurrence, and that it was a sudden event, not a gradual change. New evidence from strata in Greenland shows evidence of a double extinction, with a separate, less dramatic extinction occurring 9 million years before the Permian-Triassic (P-T) boundary, at the end of the Guadalupian epoch. Confusion of these two events is likely to have influenced the early view that the extinction was extended.

Explanatory theories

Many theories have been presented for the cause of the extinction, including plate tectonics, an impact event, a supernova, extreme volcanism, and the release of frozen methane hydrate from the ocean beds to cause a greenhouse effect, or some combination of factors.

Plate tectonics. At the time of the Permian extinction, all the continents had recently joined to form the super-continent Pangaea and the super-ocean Panthalassa. This configuration radically decreased the extent and range of shallow aquatic environments and exposed formerly isolated organisms of the rich continental shelves to competition from invaders. As the planet's epicontinental systems coalesced, many marine ecosystems, especially ones that evolved in isolation, would not have survived those changes. Pangaea's formation would have altered both oceanic circulation and atmospheric weather patterns, creating seasonal monsoons. Pangaea seems to have formed millions of years before the great extinction, however, and very gradual changes like continental drift alone probably could not cause the sudden, simultaneous destruction of both terrestrial and oceanic life.

Impact event. When large bolides (asteroids or comets) impact Earth, the aftermath weakens or kills much of the life that thrived previously. Release of debris and carbon dioxide into the atmosphere reduces the productivity of life and causes both global warming and ozone depletion. Evidence of increased levels of atmospheric carbon dioxide exists in the fossil record. Material from the Earth's mantle released during volcanic eruption has also been shown to contain iridium, an element associated with meteorites. At present, there is only limited and disputed evidence of iridium and shocked quartz occurring with the Permian event, though such evidence has been very abundantly associated with an impact origin for the Cretaceous-Tertiary extinction event. If an extraterrestrial impact triggered the Permian extinction event, scientists ask, where is the impact crater? Part of the answer may lie in the fact that there is no Permian-age oceanic crust remaining; all of it has been subducted, so plate tectonics during the last 252 million years have erased any possible P-T seafloor crater. Others have claimed evidence of a possible impact site off the coast of present-day Australia.

Supernova. A supernova occurring within ten parsecs (33 light years) of Earth would produce enough gamma radiation to destroy the ozone layer for several years. The resulting direct ultraviolet radiation from the sun would weaken or kill nearly all existing species. Only those deep in the oceans would be unaffected. Statistical frequency of supernovas suggests that one at the P-T boundary would not be unlikely. A gamma ray burst (the most energetic explosions in the universe, believed to be caused by a very massive supernova or two objects as dense as neutron stars colliding) that occurred within approximately 6,000 light years would produce the same effect.

Volcanism. The P-T boundary was marked with many volcanic eruptions. In the Siberian Traps, now a sub-Arctic wilderness, over 200,000 square kilometers were covered in torrents of lava. The Siberian flood basalt eruption, the biggest volcanic effect on Earth, lasted for millions of years. The acid rain, brief initial global cooling with each of the bursts of volcanism, followed by longer-term global warming from released volcanic gases, and other weather effects associated with enormous eruptions could have globally threatened life. The theory is debated whether volcanic activity, over such a long time, could alter the climate enough to kill off 95 percent of life on Earth. There is evidence for this theory though. Fluctuations in air and water temperature are evident in the fossil record, and the uranium/thorium ratios of late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction. Numerous indicators of volcanic activity at the P-T boundary are present, though they are similar to bolide impact indicators, including iridium deposits. The volcanism theory has the advantage over the bolide theory, though, in that it is certain that an eruption of the Siberian Traps—the largest known eruption in the history of Earth—occurred at this time, while no direct evidence of bolide impact has been located.

Atmospheric hydrogen sulfide buildup. In 2005, geoscientist Dr. Lee R. Kump published a theory explaining a cascade of events leading to the Great Extinction. Several massive volcanic eruptions in Siberian Traps, described above, started a warming of the atmosphere. The warming itself did not seem to be large enough to cause such a massive extinction event. However, it could have interfered with the ocean flow. Cold water at the poles dissolves atmospheric oxygen, cools even more, and sinks to the bottom, slowly moving to the equator, carrying the dissolved oxygen. The warmer the water is, the less oxygen it can dissolve and the slower it circulates. The resulting lack of supply of dissolved oxygen would lead to depletion of aerobic marine life. The oceans would then become a realm of bacteria metabolizing sulfates, and producing hydrogen sulfide, which would then get released into the water and the atmosphere, killing oceanic plants and terrestrial life. Once such process gets underway, the atmosphere turns into a mix of methane and hydrogen sulfide. Terrestrial plants thrive on carbon dioxide, while hydrogen sulfide kills them. Increase of concentration of carbon dioxide would not cause extinction of plants, but according to the fossils, plants were massively affected as well. Hydrogen sulfide also damages the ozone layer, and fossil spores from the end-Permian era show deformities that could have been caused by ultraviolet radiation.

Methane hydrate gasification. In 2002, a documentary, The Day the Earth Nearly Died, summarized some recent findings and speculation concerning the Permian extinction event. Paul Wignall examined Permian strata in Greenland, where the rock layers devoid of marine life are tens of meters thick. With such an expanded scale, he could judge the timing of deposition more accurately and ascertained that the entire extinction lasted merely 80,000 years and showed three distinctive phases in the plant and animal fossils they contained. The extinction appeared to kill land and marine life selectively at different times. Two periods of extinctions of terrestrial life were separated by a brief, sharp, almost total extinction of marine life. Such a process seemed too long, however, to be accounted for by a meteorite strike. His best clue was the carbon isotope balance in the rock, which showed an increase in carbon-12 over time. The standard explanation for such a spike—rotting vegetation—seemed insufficient. Geologist Gerry Dickens suggested that the increased carbon-12 could have been rapidly released by the upwelling of frozen methane hydrate from the seabed. Experiments to assess how large a rise in deep sea temperature would be required to sublimate solid methane hydrate suggested that a rise of 5°C would be sufficient. Released from the pressures of the ocean depths, methane hydrate expands to create huge volumes of methane gas, one of the most powerful of the greenhouse gases. The resulting additional 5°C rise in average temperatures would have been sufficient to kill off most of the life on earth.

A combination. The Permian extinction is unequaled; it is obviously not easy to destroy almost all life on Earth. The difficulty in imagining a single cause of such an event has led to an explanation humorously termed the "Murder on the Orient Express" theory: they all did it. A combination involving some or all of the following is postulated: Continental drift created a non-fatal but precariously balanced global environment, a supernova weakened the ozone layer, and then a large meteor impact triggered the eruption of the Siberian Traps. The resultant global warming eventually was enough to melt the methane hydrate deposits on continental shelves of the world-ocean.

Triassic-Jurassic extinction

The Triassic-Jurassic extinction event occurred 200 million years ago and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. Twenty percent of all marine families and all large Crurotarsi (non-dinosaurian archosaurs), some remaining therapsids, and many of the large amphibians were wiped out. At least half of the species now known to have been living on Earth at that time went extinct. This event opened an ecological niche allowing the dinosaurs to assume the dominant roles in the Jurassic period. This event happened in less than 10,000 years and occurred just before Pangea started to break apart.

Several explanations for this event have been suggested, but all have unanswered challenges.

• Gradual climate change or sea-level fluctuations during the late Triassic. However, this does not explain the suddenness of the extinctions in the marine realm.
• Asteroid impact. As yet, no impact crater can be dated to coincide with the Triassic-Jurassic boundary.
• Massive volcanic eruptions. Such eruptions, specifically the flood basalts of the Central Atlantic Magmatic Province, would release carbon dioxide or sulfur dioxide, which would cause either intense global warming (from the former) or cooling (from the latter). However, the isotopic composition of fossil soils of Late Triassic and Early Jurassic show no evidence of any change in the CO₂ composition of the atmosphere. More recently however, some evidence has been retrieved from near the Triassic-Jurassic boundary suggesting that there was a rise in atmospheric CO₂ and some researchers have suggested that the cause of this rise, and of the mass extinction itself, could have been a combination of volcanic CO₂ outgassing and catastrophic dissociation of gas hydrates. Gas hydrates have also been suggested as one possible cause of the largest mass extinction of all time; the so-called "Great Dying" at the end of the Permian era.

Cretaceous-Tertiary extinction

The Cretaceous-Tertiary extinction event was a period of massive extinction of species that occurred about 65.5 million years ago. It corresponds to the end of the Cretaceous period and the beginning of the Tertiary period.

The duration of this extinction event, like many others, is unknown. Many forms of life perished, encompassing approximately 50 percent of all plant and animal families, including the non-avian dinosaurs. Barnosky et al. (2011) and dos Reis et al. (2014) place the species lost at 76 percent. Many possible causes of the mass extinctions have been proposed. The most widely accepted current theory is that an object from space produced an impact event on Earth.

The extinction event is also known as the K-T extinction event and its geological signature is the KT boundary. ("K" is the traditional abbreviation for the Cretaceous period, named from the Latin for chalk, creta, which in German is kreide and in Greek is kreta. "K" is used to avoid confusion with the Carboniferous period, abbreviated as "C." "T" is the abbreviation for Tertiary a long-standing geological name for the period following the Cretaceous that has, in some scientific circles, been supplanted by the alternate name "Paleogene.")

A broad range of organisms became extinct at the end of the Cretaceous, the most conspicuous being the dinosaurs. While dinosaur diversity appears to have declined in the last ten million years of the Cretaceous, at least in North America, many species are known from the Hell Creek, Lance Formation, and Scollard Formation, including six or seven families of theropods (the "lizard-hipped" dinosaurs that were also carniverous) and a similar number of Ornithischian ("bird-hipped") dinosaurs. Birds were the sole survivors among Dinosauria, but they also suffered heavy losses. A number of diverse groups became extinct, including Enantiornithes (primitive birds) and Hesperornithiformes (toothed and perhaps flightless diving birds). The last of the pterosaurs (flying reptiles that occurred in a great range of sizes) also vanished. Mammals suffered as well, with marsupials and multituberculates (rodent-like, tree-dwelling mammals) experiencing heavy losses; placentals were less affected. The great sea reptiles of the Cretaceous, the mosasaurs and plesiosaurs, also fell victim to extinction. Among mollusks, the ammonites, a diverse group of coiled cephalopods, were exterminated, as were the specialized rudist and inoceramid clams. Freshwater mussels and snails also suffered heavy losses in North America. As much as 57 percent of the plant species in North America may have become extinct as well. Much less is known about how the K-T event affected the rest of the world, due to the absence of good fossil records spanning the K-T boundary. It should be emphasized that the survival of a group does not mean that the group was unaffected: a species may be 99 percent annihilated, yet still manage to survive.

Darkness from an impact-generated dust cloud (Alvarez et al. 1980), one of the main theories for the extinction, would have resulted in reduction of photosynthesis both on land and in the oceans. On land, preferential survival may be closely tied to animals that were not in food chains directly dependent on plants. Dinosaurs, both herbivores and carnivores, were in plant-eating food chains. Mammals of the Late Cretaceous are not considered to have been herbivores. Many mammals fed on insects, larvae, worms, snails and so forth, which in turn fed on dead plant matter. During the crisis when green plants would have disappeared, mammals could have survived because they lived in "detritus-based" food chains. In stream communities, few groups of animals became extinct. Stream communities tend to be less reliant on food from living plants and are more dependent on detritus that washes in from land. The stream communities may also have been buffered from extinction by their reliance on detritus-based food chains. Similar, but more complex patterns have been found in the oceans. For example, animals living in the water column are almost entirely dependent on primary production from living phytoplankton. Many animals living on or in the ocean floor feed on detritus, or at least can switch to detritus feeding. Extinction was more severe among those animals living in the water column than among animals living on or in the sea floor.

Theories

Impact Theory (Alvarez hypothesis). In 1980, a team of researchers, led by Nobel Prize-winning physicist Luis Alvarez, discovered that fossilized sedimentary layers found all over the world at the Cretaceous-Tertiary boundary, 65.5 million years ago, contain a concentration of iridium hundreds of times greater than normal. They suggested that the dinosaurs had been killed off by an impact event from a ten-kilometer-wide asteroid. The theory is supported by the relative abundance of iridium in many asteroids and the similarity between the isotopic composition of iridium in asteroids and K-T layers, which differs from that of terrestrial iridium. Iridium is very rare on the Earth's surface, but is found more commonly in the Earth's interior and in extraterrestrial objects such as asteroids and comets. Furthermore, chromium isotopic anomalies found in Cretaceous-Tertiary boundary sediments strongly supports the impact theory and suggests that the impact object must have been an asteroid or a comet composed of material similar to carbonaceous chondrites.

The blast resulting from such an impact would have been hundreds of millions of times more devastating than the most powerful nuclear weapon ever detonated, may have created a hurricane of unimaginable fury, and certainly would have thrown massive amounts of dust and vapor into the upper atmosphere and even into space. A global firestorm may have resulted as the incendiary fragments from the blast fell back to Earth. Analyses of fluid inclusions in ancient amber suggest that the oxygen content of the atmosphere was very high (30–35 percent) during the late Cretaceous. This high O₂ level would have supported intense combustion. The level of atmospheric O₂ plummeted in the early Tertiary (Paleogene) period.

In addition, the worldwide cloud would have blocked sunlight for months, decreasing photosynthesis and thus depleting food resources. This period of reduced sunlight, a "long winter," may also have been a factor in the extinctions. Gradually skies would have cleared, but greenhouse gases from the impact would be assumed to cause an increase in temperature for many years.

Although further studies of the K-T layer consistently show the excess of iridium, the idea that the dinosaurs were exterminated by an asteroid remained a matter of controversy among geologists and paleontologists for more than a decade. The discovery of the Chicxulub Crater in the Yucatan, as well as various types of debris in North America and Haiti, has lent credibility to this theory. Most paleontologists now agree that an asteroid did hit the Earth 65 million years ago, but many dispute whether the impact was the sole cause of the extinctions. The age of the Chicxulub crater has been revised to approximately 300,000 years before the K-T boundary. This dating is based on evidence collected in northeast Mexico, detailing multiple stratigraphic layers containing impact spherules, the earliest of which occurs some 10 meters below the K-T boundary. This finding supports the theory that one or many impacts were contributory, but not causal, to the K-T boundary mass extinction.

Deccan traps. Several paleontologists remained skeptical about the impact theory, as their reading of the fossil record suggested that the mass extinctions did not take place over a period as short as a few years, but instead occurred gradually over about ten million years, a time frame more consistent with longer-term events such as massive volcanism. Several scientists think the extensive volcanic activity in India known as the Deccan Traps may have been responsible for, or contributed to, the extinction. Luis Alvarez, who died in 1988, replied that paleontologists were being misled by sparse data. His assertion did not go over well at first, but later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely, or at least partly, due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in sea level and massive volcanic eruptions in India (Deccan Traps sequence), and these may have contributed to the extinctions.

Multiple impact event. Several other craters also appear to have been formed at the K-T boundary. This suggests the possibility of near-simultaneous multiple impacts from perhaps a fragmented asteroidal object, similar to the Shoemaker-Levy 9 cometary impact with Jupiter.

Supernova hypothesis. Another proposed cause for the K-T extinction event was cosmic radiation from a relatively nearby supernova explosion. The iridium anomaly at the boundary could support this hypothesis. The fallout from a supernova explosion should contain the plutonium isotope Pu-244, the longest-lived plutonium isotope (half-life 81 million years) that is not found in earth rocks. However, analysis of the boundary layer sediments revealed the absence of Pu-244, thus essentially countering this hypothesis.

Overview of explanation. Although there is now general agreement that there was at least one huge impact at the end of the Cretaceous that led to the iridium enrichment of the K-T boundary layer, it is difficult to directly connect this to mass extinction, and in fact there is no clear linkage between an impact and any other incident of mass extinction, although research on other events also implicates impacts.

One interesting note about the K-T event is that most of the larger animals that survived were to some degree aquatic, implying that aquatic habitats may have remained more hospitable than land habitats.

The impact and volcanic theories can be labeled "fast extinction" theories. There are also a number of slow extinction theories. Studies of the diversity and population of species have shown that the [[[dinosaur]]s were in decline for a period of about 10 million years before the asteroid hit. (A study by Fastovsky & Sheehan (1995) counters that there is no evidence for a slow, 10-million-year decline of dinosaurs.) Slower mechanisms are needed to explain slow extinctions. Climatic change, a change in Earth's magnetic field, and disease have all been suggested as possible slow-extinction theories. As mentioned above, extensive volcanism such as the Deccan Traps could have been a long-term event lasting millions of years, still a brief period in geological time.

Holocene extinction event or the "Sixth Extinction"

The Holocene extinction event is a name customarily given to the widespread, ongoing extinction of species during the modern Holocene epoch. The extinctions vary from mammoths to dodos, to species in the rainforest dying every year. Because some believe the rate of this extinction event is comparable to the "Big Five" mass extinctions, it is also known as the Sixth Extinction, although the actual numbers of extinct species are not yet similar to the major mass extinctions of the geologic past.

The Holocene epoch extends from the present day to back about 11,500 years ago. An interglacial period, the Holocene starts late in the retreat of the Pleistocene glaciers. Human civilization dates entirely to the Holocene.

In broad usage, the Holocene extinction event includes the remarkable disappearance of large mammals, known as megafauna, by the end of the last ice age 9,000 to 13,000 years ago. Such disappearances have been considered as either a response to climate change, a result of the proliferation of modern humans, or both. These extinctions, occurring near the Pleistocene/Holocene boundary, are sometimes referred to as the Pleistocene extinction event or Ice Age extinction event.

The observed rate of extinction has risen dramatically in the last 50 years. There is no general agreement on whether to consider more recent extinctions as a distinct event or merely part of a single escalating process. Only during these most recent parts of the extinction have plants also suffered large losses.

The Pleistocene or Ice Age extinction

The Ice Age extinction event is characterized by the extinction of many large mammals weighing more than 40 kg (88 lb). In North America, around 33 of 45 genera of large mammals went extinct, in South America 46 of 58, in Australia 15 of 16, in Europe 7 of 23, and in sub-Saharan Africa only 2 of 44. Only in South America and Australia did the extinction occur at family levels or higher. The two main hypotheses concerning this extinction are: (1) the animals died off due to climate change (the retreat of the polar ice cap), and (2) the animals were exterminated as a result of human activity: The "prehistoric overkill hypothesis" (Martin 1967).

The prehistoric overkill hypothesis is not universally applicable and is imperfectly confirmed. For instance, there are ambiguities around the timing of sudden extinctions of marsupial Australian megafauna. Biologists note that comparable extinctions have not occurred in Africa, where the fauna evolved with hominids. Post-glacial megafaunal extinctions in Africa have been spaced over a longer interval. In North America, the culture that has been connected with the wave of extinctions is the paleo-Indian culture associated with the Clovis people, who were thought to throw spears to kill large animals. The chief opposition to the prehistoric overkill hypothesis has been that populations of humans, such as the Clovis culture, were too small to be ecologically significant.

An alternative to the theory of human responsibility is Tollmann's bolide theory, a more controversial hypothesis, which claims that the Holocene was initiated by an extinction event caused by bolide (asteroid or meteorite) impacts.

Among the major megafauna exterminated about 9,000 to 15,000 years ago were the woolly mammoth, the woolly rhinoceros, the Irish elk, the cave lion, the cave bear, and saber-toothed cats.

Recent extinctions

In more recent years, within the past 2,000 years, a large number of species have become extinct in ways more clearly linked to human dispersal or activity. Around 1500 C.E., several species became extinct in New Zealand after Polynesian settlers arrived, including ten species of Moa (giant flightless ratite birds). It is currently estimated that among the bird species of the Pacific, some 2,000 species have gone extinct since the arrival of humans (Steadman 1995). In Madagascar, starting with the arrival of humans about 2,000 years ago, nearly all of the island's megafauna became extinct, including the Aepyornism, or elephant bird (a giant flightless ratite bird); 17 of 50 species of lemur; and a giant tortoise. Starting about 500 years ago, a number of species became extinct upon human settlement of the Indian Ocean islands, including several species of giant tortoise on the Seychelles and the Macscarene islands. Notable examples of modern extinctions of mammal fauna include the Thylacine or Tasmanian tiger (Thylacinus cynocephalus); the Quagga (a zebra relative); the Dodo, the giant flightless pigeon of Mauritius; the Great Auk of islands in the north Atlantic; and the Passenger Pigeon of North America, which became extinct in 1914.

Human impacts

According to a report by the Center for Biodiversity and Conservation (1999), there is a general pattern that has emerged related to human activity in the past 50,000 years. After the emergence of modern humans, few known extinctions occur in those areas of longest human occupancy (Africa and Eurasia), and those that occur are spread out. But the migration of human beings into other areas is linked to the loss of many large vertebrate species.

For example, about 50,000 years ago, Indonesia lost about 50 percent of its large mammals when human beings migrated there, and the movement of human beings into Australia 60,000 to 40,000 years ago resulted in large mammals and other vertebrates disappearing. In North and South America, there was a loss of some 135 mammal species, including 70 percent of North America's large mammals, between 12,500 and 10,000 years ago, when humans migrated from Asia. The settlement of Madagascar (2,000 years ago), the West Indies (7,000 years ago), islands of the Mediterranean Sea (10,000 years ago), Hawaii (1,600 to 1,400 years ago), and New Zealand (1,200 to 800 years ago) all coincided with extinction episodes. Notably, all terrestrial vertebrates outside of Africa and Asia that weighed more than 1,000 kilograms have become extinct.

Among the human activities currently considered as impacting extinctions are overhunting (either directly, or indirectly by decimation of prey populations), introduction of infectious diseases (perhaps carried by associated animals such as rats or birds), increased interspecific competition, habitat destruction, and the introduction of exotic species. The destruction of large mammals could have had even wider impacts on the ecosystems of which they were part.

Many biologists believe that we are at this moment at the beginning of an accelerated anthropogenic mass extinction. Eldredge has stated "It is…well established that the earth is currently undergoing yet another mass extinction event…and is clear that the major agent for this current event is Homo sapiens” (Eldredge 1999). E.O. Wilson of Harvard, in The Future of Life (2002), estimates that at current rates of human destruction of the biosphere, one-half of all species will be extinct in 100 years.

Those who are skeptical about the current mass extinction argue that even if the current rate of extinction is comparable or higher than the rate during a great mass extinction event, as long as the current rate does not last more than a few thousand years, the overall effect will be small. There is still hope, argue some, that humanity can eventually slow the rate of extinction through proper ecological management. Current socio-political trends, others argue, indicate that this idea is overly optimistic. Many hopes are set on sustainable development.

Additional proposed factors

It has been suggested by several sources that biodiversity and/or extinction events may be influenced by cyclic processes. The best-known of these claims is the 26 to 30 million-year viral cycle in extinctions proposed by Raup and Sepkoski (1986). More recently, Rohde and Muller (2005) have suggested that biodiversity fluctuates primarily on 62± 3 million year cycles.

In 2005, Andrew Smith and Alistair McGowan of the Natural History Museum suggested that the apparent variations in marine biodiversity may actually be caused by changes in the quantity of rock available for sampling from different time periods. The diversity of the marine life appears to be proportional to the amount of rock available for study. Based on statistical studies, roughly 50 percent of the apparent diversity modification can be attributed to this effect.

References

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