Hypotheses of climate change, asteroid impact, and volcanic eruptions have been argued. The extinction event occurred just before the breakup of the supercontinent Pangaea; although, recent scholarship suggests that the extinctions may have occurred more gradually throughout the Triassic.
The causes of the end-Cretaceous extinction event are the ones that are best understood. It was during this extinction event, about 65 million years ago, that the dinosaurs, the dominant vertebrate group for millions of years, disappeared from the planet with the exception of a theropod clade that gave rise to birds. Indeed, every land animal that weighed more then 25 kg became extinct. The Cretaceous-Paleogene K-Pg boundary marked the disappearance of the dinosaurs in fossils, as well as many other taxa.
The researchers who discovered the iridium spike interpreted it as a rapid influx of iridium from space to the atmosphere in the form of a large asteroid , rather than a slowing in the deposition of sediments during that period. It was a radical explanation, but the report of an appropriately aged and sized impact crater in made the hypothesis more credible. Now, an abundance of geological evidence supports the hypothesis. Recovery times for biodiversity after the end-Cretaceous extinction were shorter, in geological time, than for the end-Permian extinction: on the order of 10 million years.
K-Pg mass extinction : In , Luis and Walter Alvarez, Frank Asaro, and Helen Michels discovered, across the world, a spike in the concentration of iridium within the sedimentary layer at the K—Pg boundary. These researchers hypothesized that this iridium spike was caused by an asteroid impact that resulted in the K—Pg mass extinction.
In the photo, the iridium layer is the light band. Biodiversity loss, especially the disappearance of megafauna, during the Pleistocene Extinction has been linked to the arrival of humans.
The Pleistocene Extinction is one of the lesser extinctions and a relatively-recent one. It is well known that the North American, and to some degree Eurasian, megafauna disappeared toward the end of the last glaciation period. The extinction appears to have happened in a relatively-restricted time period between 10,—12, years ago. In North America, the losses were quite dramatic and included the woolly mammoths last dated about 4, years ago in an isolated population , mastodons, giant beavers, giant ground sloths, saber-toothed cats, and the North American camel, to name just a few.
The possibility that the rapid extinction of these large animals was caused by over-hunting was first suggested in the s; research into this hypothesis continues today.
It seems probable that over-hunting was a factor in extinctions in many regions of the world. Giant ground sloth : Giant ground sloths, relatives of the living South American tree sloths, lived across much of North America. The giant sloths disappeared, along with the mammoths, mastodons, and many other large animals, at the end of the Pleistocene Epoch. In general, the timing of the Pleistocene extinctions correlated with the arrival of humans and not with climate -change events, which is the main competing hypothesis for these extinctions.
The extinctions began in Australia about 40, to 50, years ago, 10, to 20, years after the arrival of humans in the area. A marsupial lion, a giant one-ton wombat, and several giant kangaroo species disappeared. In North America, the extinctions of almost all of the large mammals occurred 10, to 12, years ago, several thousand years after the first evidence of humans in North America.
All that are left are the smaller mammals such as bears, elk, moose, and cougars. Finally, on many remote oceanic islands, the extinctions of many species occurred with the coincidence of human arrivals. Not all of the islands had large animals, but when there were large animals, they were lost.
Madagascar was colonized about 2, years ago; the large mammals prosimians that lived there became extinct. Eurasia and Africa do not show this pattern, but they also did not experience a recent arrival of humans.
Humans arrived in Eurasia hundreds of thousands to over one million years ago, after the origin of the species in Africa. This topic remains an area of active research and hypothesizing. It seems clear that even if climate played a role, human hunting was an additional factor in the extinctions. Human activities probably caused the Holocene mass extinctions; many methods have been employed to estimate these extinction rates. The sixth, or Holocene, mass extinction appears to have begun earlier than previously believed and is mostly due to the activities of Homo sapiens.
Since the beginning of the Holocene period, there have been numerous recent extinctions of individual species that are recorded in human writings. Most of these coincide with the expansion of the European colonies in the s. One of the earlier and popularly-known examples of extinction in this period is the dodo bird.
The dodo bird lived in the forests of Mauritius, an island in the Indian Ocean, but became extinct around It was hunted for its meat by sailors as it was easy prey because the dodo, which did not evolve with humans, would approach people without fear. Introduced pigs, rats, and dogs, brought to the island by European ships, also killed dodo young and eggs. The sea cow, first discovered by Europeans in , was hunted for meat and oil. In addition, the last living passenger pigeon died in a zoo in Cincinnati, Ohio in This species was hunted and suffered from habitat loss through the clearing of forests for farmland.
Furthermore, in , the last living Carolina parakeet died in captivity. This species, once common in the eastern United States, was a victim of habitat loss and hunting as well. Adding to the extinction list, the Japanese sea lion, which inhabited a broad area around Japan and the coast of Korea, became extinct in the s due to overfishing.
The Caribbean monk seal, found in the Caribbean Sea, was driven to extinction through hunting by These are only a few of the recorded extinctions in the past years.
The list is not complete, but it describes extinct species of vertebrates after AD, 86 of which were made extinct by over-hunting or overfishing. Estimates of extinction rates are hampered by the fact that most extinctions are probably happening without observation since there are many organisms that are of less interest to humans and many that are undescribed.
For example, assuming there are about ten million species in existence, the expectation is that ten species would become extinct each year. One contemporary extinction rate estimate uses the extinctions in the written record since the year However, this value may be underestimated for three reasons.
First, many species would not have been described until much later in the time period, so their loss would have gone unnoticed. Secondly, the number of recently-extinct species is increasing because extinct species now are being described from skeletal remains. Lastly, some species are probably already extinct even though conservationists are reluctant to name them as such. A second approach to estimating present-day extinction rates is to correlate species loss with habitat loss by measuring forest-area loss and understanding species-area relationships.
The species-area relationship is the rate at which new species are seen when the area surveyed is increased. Studies have shown that the number of species present increases as the size of the island increases. This phenomenon has also been shown to hold true in other habitats as well. Turning this relationship around, if the habitat area is reduced, the number of species living there will also decline.
Estimates of extinction rates based on habitat loss and species-area relationships have suggested that with about 90 percent habitat loss an expected 50 percent of species would become extinct. In general, actual observations do not show this amount of loss, suggesting that there is a delay in extinction. Recent work has also called into question the applicability of the species-area relationship when estimating the loss of species.
This work argues that the species-area relationship leads to an overestimate of extinction rates. A better relationship to use may be the endemics-area relationship. The destruction of environments by human activities is a cause of extinctions. Genetic collapse is another cause of extinction.
Species that are too closely interbreed can go extinct as mistakes in the DNA accumulate in the population. How does extinction affect biodiversity? David Drayer. Feb 5, Explanation: natural selection causes extinction. All forms of extinction cause reductions in the remaining biodiversity. Related questions What is the Law of Tolerance? Researchers have, so far, attributed this variation to differences in the strength of the forces that cause the differences.
The differences in risk depend on the organism's characteristics and on the environment they live in. One of the researchers' conclusions is that the characteristics of the species and their environment that increase their risk of dying out also results in fewer new species being formed. Another conclusion is that certain factors, which increase the chance that groups become more different and eventually evolve into separate species, can simultaneously increase the risk that the groups go extinct.
One such factor is poor dispersal ability. The new approach could change the way scientists look at how characteristics of organisms and environments affect species' tendency to multiply.
For example, scientists have recently drawn attention to how the impact of humans on the Galapagos Islands may have caused different groups of finches, which could have become new species, have instead mixed with each other.
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