An extinction event, or 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.
Major Extinction Events
Ordovician-Silurian Extinction Event
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 the genera, 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 Extinction Event
By the next great extinction event, the glaciers had melted and 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 hypothesis 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 ocean. Other 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 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. Scientist 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 though 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.
The causes of this extinction, like the ones before it, are heavily debated. While it has been shown that organisms sensitive to carbon dioxide were the most vulnerable, the source of that gas is debated. As with the extinction events before it asteroids, volcanoes, and greenhouse gases are the likely culprits. Several possible impact sites of asteroids have been identified, but it is hard to date them. Further, it is more likely that an asteroid would have hit the ocean, and evidence of it would be completely gone by now. Evidence of large-scale volcanic eruptions has been found. While it is clear that there were major chemical and physical changes happening the world at a time, it is difficult to pinpoint the exact cause of the extinction event.
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.
Holocene Extinction Event
The most recent extinction event is the one we are currently living through. Since our recorded history, we have witnessed, if not caused, the extinction of many species. Changes in climate, whether man-made or natural, are driving the same conditions of greenhouse gas build up and ocean acidification which have driven other extinction events in the past. According to data from organizations which monitor threatened and endangered species, we are on the precipice of another extinction event. While some dismiss this theory, we should be careful. If the food webs we are dependent on collapse, we will not be able to survive.
Quiz
1. What is the difference between extinction and an extinction event?
A. Nothing
B. Extinction happens to a single species, an event happens to many
C. An extinction event is when an extinction happens
2. How can scientists determine we are entering an extinction event?
A. By comparing recent trends to historical records
B. By monitoring the extinctions we see today
C. All of the above
3. Your friend argues that a mass extinction would not affect humans, because we are so technologically advanced. Is he right?
A. Yes
B. No
C. It depends
References
- Feldhamer, G. A., Drickamer, L. C., Vessey, S. H., Merritt, J. F., & Krajewski, C. (2007). Mammology: Adaptation, Diversity, Ecology (3rd ed.). Baltimore: The Johns Hopkins University Press.
- Pimiento, C., & Clements, C. (2014, October 22). When did Carcharocles megalodon become extinct? A new analysis of the fossil record. PLOS One. Retrieved from http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0111086
- Pough, F. H., Janis, C. M., & Heiser, J. B. (2009). Vertebrate Life. Boston: Pearson Benjamin Cummings.
Extinction Event
No comments:
Post a Comment