An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Mass extinction can also be described as the extinction of a large number of species within a relatively short period of geological time, thought to be due to factors such as a catastrophic global event or widespread environmental change that occurs too rapidly for most species to adapt. There have been five mass extinction events in Earth's history, "many scientists are now predicting that we are on track for a sixth mass extinction (Table-A). The world's species already seem to be vanishing at an unnaturally rapid rate” (Source: The Washington Post, February 11, 2014).
Table- A: The Mass Extinction Events on Earth
|Rank||Extinction Event||Approximate Time of Occurrence||Major Effects|
|1||Ordovician–Silurian Extinction||439 million years ago||
|2||Late Devonian Extinction.||364 million years ago||
|3||Permian-Triassic extinction||251 million years ago||
|4||Triassic-Jurassic extinction||Between 199 million to 214 million years ago.||
|5||Cretaceous-Paleogene extinction||65 million years ago||
A major of the American's biologists are convinced that, a "mass extinction" of plants and animals is underway that poses a major threat to humans in the 21st century. According to The Washington Post, April 21, 1998, "the rapid disappearance of species was ranked as one of the planet's gravest environmental worries, surpassing pollution, global warming and thinning of the Ozone layer, according to the servey of 400 scientists commissioned by New York's American Museum of Natural History" "The Earth is in the midst of the sixth mass extinction of both plants and animals, with nearly 50 percent of all species disappearing”, scientists say (Source: Science Daily, October 21, 2008).
Plants and animals in the seas and oceans are also at risk of mass extinction for various reason."The world's oceans are faced with an unprecedented loss of species comparable to the great mass extinctions of prehistory, a major report suggests. The seas are degenerating far faster than anyone has predicted, because of the cumulative impact of a number of severe individual stresses,ranging from climate warming and sea-water acidification, to widespread chemical pollution and gross over-fishing" (Source: Michael McCarthy, Environment Editor, The Independent, June 20, 2011).
"Carbon dioxide in the atmosphere is absorbed by the seas - at least a third of the carbon that humans have released has been dissolved in this way, and makes them more acidic” according to the Intergovernmental Panel on Climate Change. "The oceans are becoming more acidic at the fastest rate in 300 million years, due to carbon dioxide emissions from burning fossil fuels, and a mass extinction of key species may already be almost inevitable as a result” (Source: The Guardian, October 03, 2013).
Furthermore, recent studies revealed, “an increasing number of pollinating mammal and bird species are moving towards extinction”, according to the first study of its kind (Source: BBC, March 6, 2015).
"A decline in pollinator abundance and diversity can result in a loss of pollination services that could significantly affect the maintenance of wild plant diversity, wider ecosystem stability, crop production, food security, and human welfare” (Kremenet al. 2002; Garibaldi et al. 2013).
“On an average, 2.4 species per year have moved one red list category towards extinction in recent decades, representing a substantial increase in extinction risk across this set of species, report the scientist. This may be impacting the delivery of benefits that these species provide to people"(Source:"Global trends in the Status of Bird and Mammal Pollinators", Eugenie et al., Conservation Letters, March27, 2015).
BBC News, November 20, 2015 reported, "More than half of all tree species in the Amazon face extinction, warn international scientists”. According to new data, "up to 57% of all Amazonian trees may already fit the criteria of being globally threatened". "21% of all plants are at risk of extinction and face a broad range of threats” (Source: BBC, May 10, 2016).