Extinction occurs when all individuals of a species have died, as opposed to being extinct in the wild (but still extant in captivity) or functionally extinct, where breeding no longer occurs, but individuals persist. Species often become functionally extinct prior to full extinction. The dodo (Raphus cucullatus) is an example of an extinct species, while the Hawaiian crow (Corvus hawaiiensis) is an example of species that is extinct in the wild. The Yangtze river dolphin (Lipotes vexillifer) of China, on the other hand, may be functionally extinct.
Of the species that have ever lived, over 99% of them are now extinct. The number of extinct species is estimated to be more than five billion. Currently, nearly nine million eukaryotic (e.g., animals and plants) species are estimated to be living on Earth, along with many more prokaryotes (e.g., bacteria).
The following article will cover the causes of species extinction, in addition to providing examples of extinction throughout history, and the role of humans in current extinction.
Causes of species extinction
There are numerous causes of species extinction, both natural and anthropogenic. Natural causes of extinction include both environmental and genetic factors. For example, if a species does not evolve quickly enough to adapt to a changing environment, extinction may occur. Some species may have a restricted geographic range, thus rendering them vulnerable to extinction due to any number of causes, including natural disasters and hybridization with closely related species, resulting its eventual disappearance as a unique species.
The Cuban crocodile (Crocodylus rhombifer) (Figure 1A) has a geographic range that is currently restricted entirely to a portion of the Zapata Swamp, along the southern coast of Cuba's Matanzas province (Figure 1B). While the population at this location is healthy, numbering in the thousands, the species is at risk of extinction in the wild due to its limited geographic range.
One major threat involves hybridization with the sympatric and wide-ranging American crocodile (C. acutus), as high levels of hybridization between the two species has been confirmed. This would not have been a problem historically, when the Cuban crocodile was more widespread, possibly within inland habitat without the presence of American crocodiles. In addition to the threat posed by hybridization, any significant loss of habitat in the Zapata Swamp, due to natural or anthropogenic causes, could also render the species extinct in the wild.
Figure 1A: The Cuban crocodile. Figure 1B: The restricted range of the Cuban crocodile within Cuba leaves it vulnerable to extinction. Source: Mark Garrison, Hakai Magazine
Other natural causes of extinction include excessive predation (dubbed the "overkill hypothesis), interspecific competition, and disease. This can be of particular concern when a non-native species is introduced to a new environment. In such cases, the native species may not have the adaptations necessary to survive the arrival of this new species.
For example, herbivores living on an island who have never had a natural predator would not have developed any physical or behavioral adaptations to prevent predation, thus the introduction of a predator could result in extinction. While most recent conservation issues involving the introduction of non-native species are anthropogenic in origin, species can be introduced to new areas through natural causes (e.g., driftwood has been hypothesized as a means by which some lizards and snakes have colonized islands).
Cause of the extinction of the dinosaurs
Natural disasters are, of course, another natural cause of extinction. Perhaps the most well-known natural disaster resulting in extinction is the meteor that is believed to have rendered non-avian dinosaurs extinct around 66 million years ago, during the Cretaceous-Paleogene (K-Pg) extinction event at the end of the Mesozoic Era. You may notice the term "non-avian", and that is because most paleontologists now consider birds to be actual dinosaurs (of the Coelurosauria clade), not merely related. Thus, only non-avian dinosaurs are extinct and avian dinosaurs continue to be widely distributed! Birds, non-avian dinosaurs, and crocodilians are all members of the same clade- Archosauria.
All non-avian dinosaurs (as well as numerous avian species) are believed to have become extinct when a meteor some 6-9 miles in width impacted an area on what is now Chicxulub in Mexico's Yucatan Peninsula (Fig. 2). This is commonly known as the Alvarez hypothesis, named after the physicist Luis Alvarez, who developed the theory. The impact is believed to have been more powerful than over a billion atomic bombs. An event of this size, which results in the extinction of so many species in a short period of time and a massive decrease in global biodiversity levels, is known as a mass extinction.
While most evidence supports the Alvarez hypothesis, some dissenting opinions have suggested that extinction occurred more gradually and was the result of volcanic eruptions in the Deccan Traps of what is now the Western Ghats region of India, which also occurred 65-66 millions years ago (Fig. 3). One bit of evidence proposed for the more gradual extinction theory is the presence of a small amount of non-avian dinosaur bones found in units of rock from hundreds of thousands of years into the Cenozoic Era (over 64 million years ago), suggesting that some dinosaurs may have persisted following the extinction event. Many scientists, however, believe that the bones may have simply been redeposited or washed into these units of rock, rather than originating from that period of time.
Figure 3: An artist's rendition of the Deccan Traps volcanic eruptions. Source: Wiki Commons
Human causes of extinction
In modern times, most causes of extinction are due to anthropogenic (human-driven) factors. In fact, most scientists agree that we are currently in the midst of a mass extinction event, referred to as the Holocene extinction. This is considered the sixth mass extinction event and the first since the end of the aforementioned Cretaceous-Paleogene extinction event (Fig. 4).
Since the beginning of the 20th Century, species have been going extinct at a rate of greater than 1000 times the normal, expected rate of extinction. This extinction event likely began with overhunting by humans but, during the 20th and 21st centuries, overpopulation and its resulting resource depletion, habitat destruction, climate change, and pollution, are the main drivers. Due to the massive amount of influence humans have had on species extinction rates and environmental change since the mid-20th Century, some scientists have suggested that the Holocene Epoch has ended and a new epoch, dubbed the Anthropocene Epoch, has begun.
Figure 4: The six mass extinction events. Source: Wiki Commons
Causes of extinction of biodiversity
This anthropogenic extinction event has caused a massive decrease in the levels of biodiversity on a global scale. Pressure from human populations, particularly within the last century or two, has resulted in widespread destruction of habitat, reducing the geographic range of numerous species. The continued expansion of communities into wildlife habitat, combined with the invasion of habitat by destructive industries (e.g., oil palm plantations, tin-mining, cattle ranching, etc.), destroyed large areas of forest, drained countless areas of swamp, and has polluted many coastal areas.
While some animals (e.g., coyotes and raccoons) are able to adapt to a human-dominated environment, most cannot. The results have been particularly dire for large apex predators, that are often incompatible to coexistence with human populations, which they may prey upon. In places like Indonesia, for example, conflict with saltwater crocodiles (C. porosus) and Sumatran tigers (Panthera tigris sondaica) in fringe habitats near human populations frequently results in the deaths of humans and retaliatory killings of predators. The ranges of both of these large predators have been drastically reduced from historical times. In Indonesia, two of the three tiger populations, on Bali and Java, became extinct during the 20th Century, leaving only the Sumatran population. The saltwater crocodile has been eradicated from much of its range on the Asian mainland (Cambodia, China, Thailand, and Vietnam), resulting in a significant shrinking of its range.
In addition to habitat destruction, wildlife populations are further threatened by the pollution of their habitat, changing environmental conditions due to anthropogenic climate change, and overhunting.
Brown bears (Ursus arctos), for example, were once widely distributed throughout North America, with large populations in California and Mexico. By the early 20th Century, massive hunting pressure had resulted in the eradication of the brown bear from most of these areas. Today, in the New World, the species is now restricted to only a few locations in the northern United States and throughout Canada.
Our closest relatives are also not immune from this extinction event, with around 60% of all living primate species threatened with extinction (Fig. 5), due to factors such as deforestation and the bushmeat trade.
Figure 5: The gorilla (Gorilla beringei) and the bonobo (Pan paniscus) are two of our closely related ape relatives that are threatened with extinction due to human activity.
Historical example: Cause of the extinction of the passenger pigeon
The passenger pigeon (Ectopistes migratorius) was a pigeon species native to the United States and Canada (Fig. 6). With a range that extended from the mid-west and southern United States to the eastern seaboard, the species was once quite abundant. Due to massive hunting pressure (for use as a cheap source of food) and the loss of forest habitat (deforestation), the passenger pigeon began to experience a population decline during the early 19th Century, followed by a massive decline during the later part of the century. The last wild passenger pigeons are believed to have been killed by the end of the 19th or early 20th Century.
Figure 6: A captive passenger pigeon, photographed in the late 1890s. Source: Wikipedia
Causes of Extinction - Key takeaways
- Extinction occurs when all individuals of a species have died. Of the species that have ever lived, over 99% of them are now extinct.
- There are numerous causes of species extinction, both natural and anthropogenic. Natural causes of extinction include both environmental and genetic factors.
- The Alvarez hypothesis proposes that non-avian dinosaurs became extinct following the impact of a meteor at Chicxulub in Mexico, causing the Cretaceous-Paleogene (K-Pg) extinction event. Most evidence suggests that the Alvarez hypothesis is correct.
- The Deccan Traps volcanic eruptions are another proposed cause for the Cretaceous-Paleogene extinction event, resulting in a more gradual extinction of non-avian dinosaurs.
- Most scientists agree that we are currently in the midst of a mass extinction event, referred to as the Holocene extinction. Since the beginning of the 20th Century, species have been going extinct at a rate of greater than 1000 times the normal, expected rate of extinction.
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