The Pleistocene Extinction is one of the lesser extinctions, and a recent one. It is well known that the North American, and to some degree Eurasian, megafauna, or large animals, disappeared toward the end of the last glaciation period.
The extinction appears to have happened in a relatively restricted time period of 10,000-12,000 years ago. In North America, the losses were quite dramatic and included the woolly mammoths, mastodon, giant beavers, giant ground sloths, saber-toothed cats, and the North American camel, just to name a few.
The possibility that the rapid extinction of these large animals was caused by over-hunting was first suggested in the 1900s. Research into this hypothesis continues today. It seems likely that over-hunting caused many pre-written history extinctions in many regions of the world.
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 40,000 to 50,000 years ago, just 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 10,000-12,000 years ago. All that are left are the smaller mammals such as bears, elk, moose, and cougars.
On many remote oceanic islands, the extinctions of many species occurred coincident with human arrivals.
Not all of the islands had large animals, but when there were large animals, they were lost. Madagascar was colonized about 2,000 years ago and the large mammals 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 of years ago with the origin of the species in Africa. It seems clear that even if climate played a role, in most cases human hunting precipitated the extinctions.
The sixth, or Holocene, mass extinction appears to have begun earlier than previously believed and has mostly to do with the activities of Homo sapiens.
Since the beginning of the Holocene period, there are numerous recent extinctions of individual species that are recorded in human writings.
Most of these are coincident with the expansion of the European colonies since the 1500s.
One of the earlier and popularly known examples is the dodo bird. The dodo bird lived in the forests of Mauritius, an island in the Indian Ocean.
The dodo bird became extinct around 1662. It was hunted for its meat by sailors and was easy prey because the dodo, which did not evolve with humans, would approach people without fear.
Steller's sea cow became extinct in 1768; it was related to the manatee and probably once lived along the northwest coast of North America.
Steller's sea cow was first discovered by Europeans in 1741 and was hunted for meat and oil.
The last sea cow was killed in 1768. That amounts to 27 years between the sea cow’s first contact with Europeans and extinction of the species.
The dodo and Steller’s sea cow are only a few of the recorded extinctions in the past 500 years.
The International Union for Conservation of Nature (IUCN) keeps a list of extinct and endangered species called the Red List. The list is not complete, but it describes 380 extinct species of vertebrates after 1500 AD, 86 of which were driven extinct by overhunting or overfishing.
In 1914, the last living passenger pigeon died in a zoo in Cincinnati, Ohio. This species had once darkened the skies of North America during its migrations, but it was hunted and suffered from habitat loss through the clearing of forests for farmland.
In 1918, the last living Carolina parakeet died in captivity. This species was once common in the eastern United States, but it suffered from habitat loss. The species was also hunted because it ate orchard fruit when its native foods were destroyed to make way for farmland.
The Japanese sea lion, which inhabited a broad area around Japan and the coast of Korea, became extinct in the 1950s due to fishermen. The Caribbean monk seal was distributed throughout the Caribbean Sea but was driven to extinction via hunting by 1952.
Estimates of extinction rates are hampered by the fact that most extinctions are probably happening without observation.
The background extinction rate is estimated to be about one per million species per year (E/MSY). 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 1500. For birds alone this method yields an estimate of 26 E/MSY.
However, this value may be underestimated for three reasons:
Many species may not have been recorded
Species are now are being described from skeletal remains
Some species are probably already extinct
Taking these factors into account raises the estimated extinction rate closer to 100 E/MSY.
Another approach to estimating present-time 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 (recorded), when the area surveyed is increased.
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. Species-area estimates have led to species extinction rate calculations of about 1000 E/MSY and higher.
In general, actual observations do not show this amount of loss and suggestions have been made 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.
Using this method would bring estimates down to around 500 E/MSY in the coming century. Note that this value is still 500 times the background rate.
It may not be clear why biologists are concerned about biodiversity loss.
When biodiversity loss is thought of as the extinction of the passenger pigeon, the dodo bird, and even the woolly mammoth, the loss may appear to be an emotional one.
But is the loss practically important for the welfare of the human species? From the perspective of evolution and ecology, the loss of a particular individual species is unimportant, however, the loss of a keystone species can lead to ecological disaster.
Extinction is a normal part of macroevolution. But the accelerated extinction rate means the loss of tens of thousands of species within our lifetimes.
It is also likely to have dramatic effects on human welfare through the collapse of ecosystems and in added costs to maintain food production, clean air and water, and human health. Agriculture began after early hunter-gatherer societies first settled in one place and modified their immediate environment. This cultural transition has made it difficult for humans to recognize their dependence on undomesticated living things on the planet.
adaptive radiation rapid branching through speciation of a phylogenetic tree into many closely related species
biodiversity hotspot concept originated by Norman Myers to describe a geographical region with a large number of endemic species and a large percentage of degraded habitat
biodiversity variety of a biological system, typically conceived as the number of species, but also applying to genes, biochemistry, and ecosystems
bush meat wild-caught animal used as food (typically mammals, birds, and reptiles); usually referring to hunting in the tropics of sub-Saharan Africa, Asia, and the Americas
chemical diversity variety of metabolic compounds in an ecosystem
chytridiomycosis disease of amphibians caused by the fungus Batrachochytrium dendrobatidis; thought to be a major cause of the global amphibian decline
DNA barcoding molecular genetic method for identifying a unique genetic sequence to associate with a species
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