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Mass Extinctions and Its Role in Evolution are mass extinctions?How do we detect mass extinctions in the fossil record?What role do mass extinctions play in evolution?What are some common misconceptions about mass extinctions?How do scientists study mass extinctions?What evidence do scientists use to study mass extinctions?What causes mass extinctions?How could volcanic activity cause mass extinctions?Are we experiencing a mass extinction now? INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET ?printMass extinctionby the Understanding Evolution team INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET Extinction?is a recurring theme over the history of life on Earth. Ninety-nine percent of?species?that have ever lived on Earth have gone extinct,1?and more than one third of the plant and animal species alive today are threatened with extinction.2?Usually, extinction operates at a fairly constant rate, culling some species while?speciation?generates new ones. However, at a few points in life's history — and likely today — the humdrum of regular extinction has been interrupted by?mass extinctions. During these catastrophic periods, extinction levels rose far above background rates and huge swathes of life's?diversity disappeared forever.What are mass extinctions? INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/mass-extinction-02-graph.png" \* MERGEFORMATINET ?Mass extinctions occur when global extinction rates rise significantly above background levels in a geologically short period of time. You can see these spikes in extinction rates in the graph shown at right. This graph shows extinction rates among families of marine animals over the past 600 million years. While background extinction levels hover around five families per million years, during mass extinctions, these rates shoot up.Mass extinctions can also be observed by looking at diversity levels over time. The graph below shows number of marine?genera?alive at different points in life's history. While diversity levels generally increase over time, mass extinctions cause sudden drop-offs in diversity. The largest mass extinctions in Earth's history are marked on this graph. Here, we will refer to each mass extinction by the name of the geologic period that it ended (e.g., the end-Ordovician extinction marks the end of the Ordovician period around 440 million years ago). During several of these events (notably, the Devonian and Triassic extinctions), low speciation rates also contributed to the loss of diversity. Biologists estimate that each of these mass extinctions correspond to a loss of ? of the species on Earth at the time!3Detecting mass extinctions in the fossil recordMass extinctions were first identified by the obvious traces they left in the?fossil?record. In the?strata?corresponding to these time periods, the lower, older rock layer contains a great diversity of fossil life forms, while the younger layer immediately above is depauperate in comparison. Often, the rock layers bookending the mass extinction are noticeably different in their compositions. These changes in the rocks show the effects of environmental disturbances that triggered the mass extinction and sometimes hint at the catastrophic cause of the extinction.right000 INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET The Cretaceous-Tertiary boundary, as recorded in the rocks at Gubbio, Italy. At left, the later Tertiary rocks appear darker — almost orange — and the earlier Cretaceous rocks appear lighter when viewed with the naked eye. At right, magnification reveals few different sorts of microfossils in the later Tertiary layers (top), but a wide variety in the earlier Cretaceous sample (bottom).Such dramatic changes in adjacent rock layers make it clear that mass extinctions were geologically rapid and suggest that they were caused by catastrophic events (e.g., a period of intense volcanic activity). Exactly what do we mean by "geologically rapid?" Even in cases where a mass extinction seems to have been triggered by a near instantaneous event (e.g., a massive asteroid colliding with Earth), the impact of this event on Earth's systems and biota may have taken much longer to play out. Figuring out exactly how much real time passed during a mass extinction is difficult and requires the use of state-of-the-art radioisotopic dating techniques. Using such techniques, geologists estimate that some of these massive extinctions took place in 200,000 years or less. Current techniques for dating such ancient rocks cannot pinpoint dates more specifically than this — so we can't be sure if some mass extinctions took place in 150 years or 150,000 years. Either way, this represents a sudden event when compared to life's 3.5 billion year history. For example, if you compressed the entire history of life into a human lifespan of 80 years, a mass extinction would zip by in less than a day!The role of mass extinction in evolution INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/mass-extinction-04-mammal_phylogeny.png" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET At the most basic level, mass extinctions reduce diversity by killing off specific?lineages, and with them, any descendent species they might have given rise to. In this way, mass extinction prunes whole branches off the tree of life. But mass extinction can also play a creative role in evolution, stimulating the growth of other branches.The sudden disappearance of plants and animals that occupy a specific habitat creates new opportunities for surviving species. Over many generations of?natural selection, these lineages and their descendent lineages may evolve specializations suited to the newly freed up resources and may take over ecological roles previously held by other species, or may evolve brand new ecological strategies. In this way, mass extinction can level the evolutionary playing field for a brief time, allowing lineages that were formerly minor players to diversify and become more prevalent. By removing so many species from their ecosystems in a short period of time, mass extinctions reduce competition for resources and leave behind many vacant?niches, which surviving lineages can evolve into. For example, mammals have been around for more than 200 million years — but for most of that time, they've remained a small group of rodent-like organisms. It was only when the non-avian dinosaurs went extinct 65 million years ago in the end-Cretaceous mass extinction, that mammals really diversified. In less than 20 million years, they evolved into the great variety of mammals we know today — forms that play many of the same roles in terrestrial ecosystems that their dinosaur predecessors had.Correcting common misconceptions about mass extinctionsBecause of their catastrophic outcomes, it's tempting to think of mass extinctions as sudden apocalypses — an asteroid strikes, the Earth is engulfed in flames, and when the smoke clears, half of all life is dead. However, evidence suggests that mass extinctions are more complicated than this: INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/mass-extinction-05-Causes.png" \* MERGEFORMATINET Mass extinctions are complex and may have multiple causes.?For example, the end-Cretaceous mass extinction?doesseem to have been triggered by an asteroid impact, but continental-scale volcanic activity, climate change, and changes in atmospheric and oceanic chemistry likely also played a part.The intensity of mass extinctions can be amplified by the interplay among the atmosphere, oceans, and geologic activity.?For example, large scale volcanic activity at the end of the Permian may have killed some organisms directly through eruption events and through the poisonous gases released — but many more extinctions probably resulted from the effect of the eruptions on other Earth systems. Greenhouse gases released by the volcanic activity would have caused global warming, and this climate change would have caused shifts in ocean chemistry and circulation. The changes in ocean chemistry could have prevented reefs from being formed, and changes in circulation could have caused huge areas of oxygen-depleted waters, which could, in turn, imbalance ecosystems the world over resulting extinction on a global scale.Some organisms may be more vulnerable during mass extinctions than others.?For example, during the end-Permian mass extinction, marine invertebrates with shells made of calcium carbonate — like clams — were more likely to go extinct than animals that don't depend on this compound — such as certain kinds of sponges.4?That's because the end-Permian mass extinction likely involved ocean acidification, which makes less calcium carbonate available for organisms that need it to build their shells. INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/giantclam.jpg" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/hexactinellids.jpg" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET At left, the shell of the giant clam (Tridacna?sp.) is based on calcium carbonate, putting it at risk when oceans acidify. But the skeleton of the hexactinellid sponge?Staurocalyptus?at right is based on silicon, protecting it from that particular environmental disturbance.Although mass extinctions occur in geologically short periods of time, they are not instantaneous. Mass extinctions involve complex processes that may play out over many thousands of years. Figuring out what processes occurred during a mass extinction is a challenge for scientists.Studying mass extinctionsPaleontologists and geologists try to answer all sorts of questions about mass extinctions:Which species went extinct and which survived?What geographic areas and ecosystems were most affected?When and over what period of time did the mass extinction occur?These questions may seem simple enough, but they can be tricky to answer.Establishing snapshots of life before and after a mass extinction is challenging for many reasons. We have access to only a small subset of all the fossils that might be preserved in fossil record. And for the fossils we?do?have, it is often difficult to identify a species and?genus, let alone figure out whether it had any descendents that survived to later time periods. Adding a further challenge, whole groups of organisms do not have body parts that fossilize well and so are largely absent from the fossil record.Radioactive Isotope Dating (Absolute Dating)1. As certain minerals form, radioactive elements (like uranium and potassium) may be bound up in the structure of the mineral. Around the time of their formation, these minerals became part of a rock (for example, in volcanic rocks).2. Over time, the radioactive elements decay. For example, over many millions of years, uranium decays into lead and potassium decays into argon. Scientists have determined the rate at which this happens by studying these elements experimentally.3. Scientists collect samples of the rock, extract the minerals containing the radioactive elements, and determine the ratio of the radioactive element to its decay product present in the mineral.4. Based on this ratio and the known rate of decay for that element, scientists calculate the age of the rock — i.e., how much time must have passed for the mineral to contain the observed ratio of radioactive element and product. This is usually expressed as an age range — for example, 252.4 million years ago plus or minus 0.3 million years. This means that scientists can be very certain that the rock dates to sometime between 252.1 million years ago and 252.7 million years ago. INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET ?Radioisotopic dating in a nutshellTo date a rock, geologists study minerals in the rock that contain radioactive elements and that were formed very close to the time that the rock was deposited. These elements decay away at known rates and so can be used as a sort of clock to determine the age of the rocks. Geologists measure how much of these radioactive elements have decayed away and, from that, work backwards to figure out how old the rock must be. It works like this:1. As certain INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/mass-extinction-06-geologypuzzle-01.png" \* MERGEFORMATINET 2. Over time, experimentally. INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/mass-extinction-06-geologypuzzle-02.png" \* MERGEFORMATINET 3. INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/mass-extinction-06-geologypuzzle-03.png" \* MERGEFORMATINET 4. Based on INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/mass-extinction-06-geologypuzzle-04.png" \* MERGEFORMATINET Radiocarbon dating works using the same basic principles, but is used to date organic material and relies on a different element (carbon), whose radioactive form (known as carbon-14) decays much more quickly. This speedy decay rate means that carbon can only be used to estimate ages up to around 60,000 years.Gathering evidence to study mass extinctionsFor any mass extinction, the ultimate question is, of course,?how? What was the trigger and chain of events that caused such a massive upset of Earth's biota? Though this question may be the most compelling of the lot, it is also the most difficult to answer. To get a handle on it, scientists figure out what sorts of observations they would expect to make today if a particular?hypothesis?about the cause of a mass extinction were true and then see if those expectations are borne out. For an example, consider the idea that an asteroid impact contributed to the end-Cretaceous extinction. If this hypothesis were true, we'd expect:To observe dust from the meteorite deposited in the rocks of the end-CretaceousTo discover a crater from the impact dating to the end-CretaceousTo observe debris from the impact dating to the end-CretaceousTo find evidence of shockwaves from the impact dating to the end-CretaceousTo find glass particles that were produced by the heat of the impact dating to the end-CretaceousModels?to predict large-scale environmental disturbances resulting from the impact of a massive asteroid INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/chicxulub_mapx.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/ejectaflowx.jpg" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/glass_clayx.jpg" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET At left, a map showing the location of a crater dating to the end-Cretaceous. At right top, a wall of a Belize quarry in which the orange wavy line marks the base of an end-Cretaceous debris flow that may have been caused by an asteroid impact. At right bottom, an end-Cretaceous rock from Belize contains greenish clay fragments that were once glass shards.All of these expectations were borne out and constitute strong evidence that an asteroid impact occurred and was a cause of the end-Cretaceous mass extinction. When multiple lines of evidence (e.g., fossils and patterns of fossil preservations, trace chemicals in different strata, dating techniques, and models) all point to the same cause or causes, we gain confidence in these hypotheses and can work towards refining them.Of course, all the evidence doesn't always line up so neatly. For example, when one scientist found trapped helium (of a sort that is common in extraterrestrial objects) in sediments dating to the end of the Permian, she interpreted it as evidence that an asteroid had triggered the end-Permian extinction.5?Other scientists rushed to investigate, but they could not reproduce the finding6?— nor could they find a crater that could be reliably dated to the same time period.7?Though one line of evidence pointing towards an asteroid impact fails to stand up to scrutiny, this does not necessarily mean that the hypothesis is entirely flawed. Scientists are still investigating the causes of the end-Permian mass extinction, including the idea of an asteroid impact.It is worth noting that determining the dates of ancient events is critically important in establishing how and why a mass extinction happened. If an event that is thought to be a cause of a mass extinction (e.g., an asteroid impact) turns out to have occurred?after?the mass extinction began, it cannot have been the trigger for the extinction.What causes mass extinctions?Although the best-known cause of a mass extinction is the asteroid impact that killed off the non-avian dinosaurs, in fact, volcanic activity seems to have wreaked much more havoc on Earth's biota. Volcanic activity is implicated in at least four mass extinctions, while an asteroid is a suspect in just one. And even in that case, it's difficult to disentangle how much of the end-Cretaceous extinction was caused by the asteroid and how much was caused by the steady ooze of lava that was blanketing most of India at around the same time.While multiple causes may have contributed to many mass extinctions, all the hypothesized causes have two things in common: they cause major changes in Earth systems — its ecology, atmosphere, surface, and waters — at rapid rates. Here are some hypothesized causes for each of Earth's biggest mass extinctions:End-Ordovician:? Beginning of glacial cycles on Earth, and corresponding changes in sea level? Changes in atmospheric and oceanic chemistry relating to the rise of the Appalachian mountainsEnd-Devonian extinction:? Climate change, possibly linked to the diversification of land plants? Decrease in oxygen levels in the deep oceanEnd-Permian extinction:? Volcanic activity? Climate change? Decrease in oxygen levels in the deep ocean? Changes in atmospheric chemistry? Changes oceanic chemistry and circulationEnd-Triassic extinction:? Volcanic activityEnd-Cretaceous extinction:? Asteroid impact? Volcanic activity? Climate change? Changes in atmospheric and oceanic chemistry INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/mass-extinction-08-tables_vertical.png" \* MERGEFORMATINET What doesn't cause mass extinctions?It may not come as much surprise that powerful volcanic eruptions and massive asteroid impacts can trigger mass extinctions. After all, we'd expect such disasters to bring about death and destruction. However, as paleontologists and geologists have studied Earth's history, they've noticed something interesting: sometimes, major catastrophes pass with hardly a blip in extinction rates. For example, the Manicouagan crater in Canada is several miles wide and constitutes strong evidence that a huge asteroid struck Earth one and a half million years ago — yet, the fossil record indicates no major dip in diversity associated with this event. Similarly, 2.5 km3?of lava (called the Karoo-Ferrar volcanic province) covering what is now South Africa and Antarctica indicate extensive volcanic activity around 180 million years ago — yet despite this large-scale disruption, only a small rise in extinction rates occurred during that time period. INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/manicouagan.jpg" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/ferrar.jpg" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET At left, the Manicouagan crater in Quebec as seen from space. At right, a researcher near exposures of the Ferrar flood basalts (dark rocks in the background) in Antarctica.Why do some catastrophic events trigger mass extinctions and others do not? The devil seems to be in the details — particularly in the chain reaction of Earth systems disruptions that are triggered (or not) and in the rate at which those disruptions occur. Mass extinctions seem to occur when multiple Earth systems are thrown off kilter and when these changes happen rapidly — more quickly than organisms evolve and ecological connections adjust. For example, the asteroid that triggered the end-Cretaceous extinction happened to hit carbon-rich rocks, which probably led to ocean acidification, and hence the disruption of reef formation and the oceanic food web. However, the asteroid that caused the Manicouagan did not hit carbon-rich rocks and so did not set off this chain reaction or such a significant disruption of Earth systems. The Karoo-Ferrar volcanic activity, on the other hand, was so large that it would certainly have disrupted Earth's atmosphere and oceans; however, in this case, the changes came about very slowly. The volcanic activity was spread over millions of years. For comparison, the volcanic activity that may have caused the end-Triassic mass extinction likely occurred in less than 100,000 years, leaving no time for evolution to take place as habitats changed and leading to widespread extinction.Volcanic activity and mass extinctionVolcanic activity is now thought to be an important cause of several mass extinctions, but it may not be obvious exactly how this could trigger extinction on a global scale. After all, volcanoes like Vesuvius and Krakatoa were destructive, but didn't cause mass extinctions. These sort of explosive eruptions are the kind that we are most familiar with, but in fact, they can't happen on a large enough scale to cause a mass extinction. The sort of volcanic activity that scientists suspect leads to mass extinctions is not your iconic mountain erupting in a spew of lava. Imagine instead massive fissures and vents in the earth that ooze steady pulses of lava over hundreds of thousands of years. This sort of volcanic activity may not be as sensational as a top blown off a volcano, but it generates much more lava and affects vast areas, covering millions of square kilometers with lava, the bulk of which is released in a geologic instant — less than a few hundred thousand years. For example, the eruptions associated with the end-Permian extinction (the Siberian Traps) left behind lava that covers an area the size of Western Europe and is more than a kilometer thick! INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/siberian_traps_map.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/laki.jpg" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET At left, extent of the Siberian tramps. At right, an example of a recently active fissure vent. This photo shows the Laki fissure vent in Iceland, which erupted between 1783 and 1784 releasing a total of 14.7 km3 of lava from a 25-km-long fissure.This type of slower, oozing volcanic activity seems to cause extinctions through secondary effects, not through the eruption itself. Although oozing eruptions?do?directly release gasses that poison animals and plants and contribute to acid rain and climate change, the real catastrophe is likely caused by the rock layers that the lava comes into contact with as it erupts.8If hot lava comes into contact with rocks that contain?organic?compounds (e.g., coal deposits) as it erupts, this releases huge amounts of greenhouse and toxic gases, like carbon dioxide, methane, and sulfur dioxide.The chain of events set off by such shifts in atmospheric chemistry could have been disastrous. Large amounts of sulfur dioxide lead to short-term cooling (on the scale of tens of years), while carbon dioxide and methane then cause long-term global warming (lasting hundreds of thousands of years).9?These climate changes may directly lead to the extinction of sensitive species and prompt others to shift their ranges, upsetting ecosystem dynamics and triggering additional extinctions. Furthermore, if many land plants die, this increases erosion and damages delicate marine environments as sediments are carried into the ocean. Global warming also has the potential to reduce circulation of water in the ocean. When combined with the fact that oxygen doesn't dissolve as well in warmer waters, this could lead to oceanic oxygen levels too low to support some species and could severely interrupt the flow of nutrients through the marine?food web. To make matters worse for marine life, an increase in carbon dioxide in the atmosphere also acidifies the ocean, which prevents corals from building reefs and disrupts marine ecosystems from the bottom up. Large-scale volcanic activity could impact organisms and their habitats at many different levels, ultimately leading to skyrocketing extinction rates. INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/mass-extinction-09-UGCStoryboard.png" \* MERGEFORMATINET Conceptual links among possible causes of mass extinction INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/bluebox_topleft.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/bluebox_topright.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/dot_clear.gif" \* MERGEFORMATINET ?January, 2018: The end-Cretaceous mass extinction — the event in which the non-avian dinosaurs, along with about 70% of all species in the fossil record went extinct — was probably caused by the Chicxulub meteor impact in Yucatán, México. However, scientists have long wondered about the massive volcanic eruptions that were occurring in northwestern India at about the same time, the Deccan Traps. Recent research suggests that the timing of these events is not coincidental. In fact, the meteor impact may have caused an increase in the intensity of Deccan volcanic activity. Keep up to date on this new field of investigation by following the researchers involved on?deccan.berkeley.edu.?The Earth's sixth mass extinction?As human populations have grown and become more technologically advanced, we've taken a larger toll on the rest of the natural world. We've encroached on (and sometimes wiped out) the habitats of other species; we've released pollutants into the air, soil, fresh water, and oceans; and we've even changed the atmosphere and climate. These changes are beginning to disrupt Earth's systems at global scales — and are occurring rapidly. Because of our burning of fossil fuels, carbon dioxide levels are rising faster than they ever have at any point in Earth's history.10?It's no surprise then that extinction rates have risen substantially in the last 500 years. INCLUDEPICTURE "/var/folders/38/y0rk10fn6p7b703m96_hbw6md3r328/T/com.microsoft.Word/WebArchiveCopyPasteTempFiles/mass-extinction-10-birds-03.png" \* MERGEFORMATINET The graph at left shows that rates of bird extinctions have increased over time due to human impacts.11?The graph at right shows that if extinctions continue at high rates, we will have officially caused a mass extinction.12In this module, we've seen that mass extinctions also involve a sharp increase in extinction rates over normal levels. So how bad is our current situation? Have humans pushed the Earth into its sixth mass extinction? The answer appears to be, "Not yet." But we are currently losing species at a rate far higher than normal background extinction rates, and the situation is dire. We are rapidly approaching a loss of diversity similar to that seen during mass extinctions. Biologists predict that unless we change course and begin preserving more species, within the next few hundred years, we will become the cause of Earth's sixth mass extinction.11The good news is that we can stop this mass extinction. While there's no way to deflect an unforeseen asteroid strike or put a plug in a volcanic eruption, the current extinction rate is being pushed ever higher by human activity — and that means that human activity can also reverse this trend. We can reduce the extinction rate through policy changes that increase conservation efforts and curb our production of greenhouse gases to slow climate change. Studies of the Earth's history may play a surprisingly important role in this effort. If we can understand the chain of events that led to past mass extinctions, we will be in a better position to break that chain today. And if we can understand what traits make a species particularly vulnerable during a mass extinction, we may be able to better focus our conservation efforts. Investigating Earth's past extinctions may be one of the keys to preserving biodiversity for the future. ................
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