Mass Extinction Definition
A mass extinction is a period of time in which a considerable portion of the world’s biodiversity is lost. An extinction event can have many causes and can vary in intensity. There have been at least 5 major extinction events since the Cambrian explosion, each taking a large portion of the biodiversity with it. As seen in the graph below, these extinction events punctuate the fossil record. The following graph shows the intensity of extinction over time, which is a gradual and constant process. The spikes represent significant extinction events.
The highest bar represents the Permian-Triassic extinction event, which wiped out almost half the species on Earth in less than a million years. One ultimate reason that an extinction event may occur is the interdependent nature of food webs. If one species suffers and goes extinct, it often means changes for other species. As more and more species fall prey to the extinction event, the food web collapses and must be rebuilt from the bottom up. Oftentimes, a change in the Earth such as weather patterns will cause the extinction event. Other times, a species or group of species will change the environment and drive the extinction event.
Five Mass Extinction Events
Ordovician-Silurian Extinction Events
One of the oldest mass extinctions, this extinction event occurred nearly 450 million years ago. At the time, many forms of multicellular life roamed the ocean. Just before this extinction event, many changes were happening. For instance, land plants had emerged and were likely changing the composition of the atmosphere. In doing so, they shifted the balance from a carbon dioxide rich atmosphere to an oxygen rich atmosphere. Theoretically, this could have dramatically cooled the planet.
As much of the diversity of life was found within the oceans at this time, it suffered greatly as the planet cooled. As glaciers formed, sea levels fell. Many habitats on coastal areas were assumed to be destroyed as this happened. The change in atmosphere and global weather patterns ended up killing off up to 50 percent of all genera in existence and eliminating many marine species. Those species on land and in the sea which did not go extinct to glaciation expanded greatly once the glaciers melted and temperatures stabilized. It was after this mass extinction that major families of land plants and animals exploded.
Late Devonian Mass Extinction
By the next great extinction event, the glaciers had melted and the land was heavily colonized by plants and insects. These two groups had expanded rapidly in the newly available niche. The marine fauna had also rebounded, becoming greatly diversified and building huge coral reefs, which we can find evidence of today. The event may, in fact, be a series of events so close in time that they are not well defined in the fossil record.
The causes of the Late Devonian event are not understood well, and many hypotheses abound. It is understood that marine, warm-water organisms and early jawed vertebrates were heavily affected. In fact, almost 97 percent of all vertebrate species disappeared. At least 75 percent of all species did not survive this era. One of the causes could have been an asteroid impact, which would change weather patterns and cause glaciation and sea levels to fall. Another theory presented involves the evolution of plants.
It suggests that the new forms of plants, complete with roots and mechanisms to extract nutrients, had caused a massive influx of these nutrients into the ocean. As with fertilizer running into the ocean today, the increase in nutrients would cause massive growths of algae. As these blooms expanded, they would deplete the oxygen from large portions of the ocean. Another fact supporting this is that many species of vertebrate got considerably smaller after the extinction event. This suggests that there was less oxygen and prey in the water. Other causes include volcanism, which may have added greenhouse gases to the atmosphere, changing its composition.
Permian-Triassic Mass Extinction Event
The Permian-Triassic extinction event is the largest and most severe extinction event in the fossil record. The extinction event, also called the Great Dying, is supposed to have happened around 252 million years ago. Scientists have estimated that during this time 96 percent of all marine species went extinct. Further, terrestrial vertebrates, which had just expanded for the first time, lost nearly 70 percent of living species. Over 80 percent of all the known genera disappeared after this event. In today’s equivalent, it would be like wiping out all life on Earth, minus the insects and other invertebrates. That includes plants and fungi!
In the marine environment at one archeological site in China, for instance, nearly 87 percent of all the known invertebrate marine genera disappeared. It is thought that ocean acidification, as a result of increasing carbon dioxide in the atmosphere, contributed greatly to this loss. On land, things were just as bad. At the end of the Permian, insects had grown and diversified on land. Some of the largest insects to walk or fly the Earth existed in this period. Nearly all of them would be extinct by the end of the extinction event. Plant communities, while they didn’t experience the same level of extinction, went through rapid periods of fluctuation. This likely cause the extinction of many of the terrestrial vertebrates alive at the time.
Triassic-Jurassic Extinction Event
This mass extinction event, while much small than the one preceding it, allowed for many niches to be cleared for the rise of the dinosaurs. This extinction event took around somewhere around 30 percent of marine species. Interestingly, this extinction event coincides with the breakup of Pangea, a supercontinent that had formed as the continents drifted together. As the continent broke apart, there were massive changes to the flora and fauna.
Unlike the other extinction events, one cause of this extinction event might have been a decline in speciation as opposed to an increase in extinction. Theoretically, there is a level of background extinction, which is always taking place. If speciation slows, because organisms can’t adapt or all the niches are full, extinction wins out. While many species were lost over this time, the causes aren’t clear. Again, asteroids and climate change are presumed to be the culprits.
Cretaceous-Paleogene Extinction Event
Probably the most well-known extinction event, the Cretaceous-Paleogene is the one which wiped out the dinosaurs and cleared the way for mammals and humans. Unlike other mass extinction events, this extinction event happened relatively recently, only 66 million years ago. Also unlike the other extinction events, scientists have a fairly good idea of what caused the massive extinction.
An asteroid crater in the Gulf of Mexico was found that dated to the time of the extinction. At over 100 miles wide, the asteroid would have been able to completely shift the global atmosphere. One of the largest extinction events known, the impact is probably responsible for the die-off of around 75 percent of all living species. The main effect of the asteroid was to produce an impact winter. Dust and debris from the impact would float in the atmosphere for years, blocking the sun. As photosynthetic organisms died off, so too would the herbivores that feed on them and the carnivores which feed on them. As such, entire food webs in both the terrestrial and marine environments were lost.
6th Mass Extinction
While we generally consider mass extinction events as historical events, many scientists argue that we are currently in the beginnings of another mass extinction event. In fact, from the species that we can measure and observe going extinct, we can estimate the overall rate of extinction. This rate is much higher than in most periods of history. Furthermore, this 6th mass extinction may be entirely caused by human actions.
Starting around 10,000 years ago, humans developed agriculture. While they gained the ability to grow and store food, our ancestors also started modifying the environment to give us more room for agriculture. Starting around 300 years ago, the Industrial Revolution gave us an increased ability to change the environment. We developed tractors and chainsaws, to clear out forests on which to raise crops or animals. Further, these machines emit carbon dioxide and our animals emit methane, both of which are greenhouse gases. The release of these gases is changing the composition of the atmosphere, which in turn is disrupting the climate.
Unfortunately, these changes are happening so fast that scientists are entirely sure we can now reverse them. The rate of change is important because animals can only adapt over long periods of time. If a change happens too quickly, many animals will go extinct because they are not able to adapt in time. We are already seeing major groups of animals suffering, such as amphibians and coral reefs, both of which rely on specific amounts of water and temperatures to survive.
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