Biodiversity - 'Extinction is forever'

Sunday article by Mohan Pai

Extinction is forever !

Over 99% of the species that ever lived are now extinct.

Mass extinction exhibits a cyclic nature. 5 major extinctions that occurred during the last 540 million years of earth history wiped out most living species.

Mass extinction is a sharp decrease in the number of species in a relatively short period of time. Mass extinctions affect most major taxonomic groups present at the time — birds, mammals, reptiles, amphibians, fish, invertebrates and other simpler life forms. They may be caused by one or both of:

*extinction of an unusually large number of species in a short period.

*a sharp drop in the rate of speciation.

Over 99% of species that ever lived are now extinct, but extinction occurs at an uneven rate. Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine invertebrates and vertebrates every million years. Marine fossils are mostly used to measure extinction rates because they are more plentiful and cover a longer time span than fossils of land organisms. Since life began on earth, several major mass extinctions have significantly exceeded the background extinction rate. The most recent, the Cretaceous–Tertiary extinction event, occurred 65 million years ago, and has attracted more attention than all others as it marks the extinction of nearly all dinosaur species, which were the dominant animal class of the period. In the past 540 million years there have been five major events when over 50% of animal species died. There probably were mass extinctions in the Archean and Proterozoic Eons, but before the Phanerozoic there were no animals with hard body parts to leave a significant fossil record.

Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity.

Extinction cycles

It has been suggested by several sources that biodiversity and/or extinction events may be influenced by cyclic processes. The best-known hypothesis of extinction events by a cyclic process is the 26M to 30M year cycle in extinctions proposed by Raup and Sepkoski (1986). More recently, Rohde and Muller (2005) have suggested that biodiversity fluctuates primarily on 62 ± 3 million year cycles.Much early work in this area also suffered from the poor accuracy of geological dating, where errors often exceed 10M years. However, improvements in radiometric dating have reduced the scale of uncertainty to at most 4M years - theoretically adequate for studying these processes.

The concept of periodicity has important implications for determining which factors cause extinction. Hypotheses invoking catastrophism have particularly been advanced utilizing this concept, which imply extra-terrestrial forces as extinction-causing agents. This is because only astronomical forces are known to operate on such a precise periotic time schedule. Contrary to catastrophism are hypotheses which focus on gradualism. These gradualistic hypotheses invoke various terrestrial extinction mechanisms including volcanism, glaciation, global climatic change, and changes in sea level. Most recently hypotheses centered on the new non-linear science of complexity have emerged. Under these hypotheses species-species interactions lead to occasional instability resulting in cascades which may ripple through entire ecosystems, with potentially devastating results.
Major extinction events

The classical "Big Five" mass extinctions: End Ordovician, Late Devonian, End Permian, End Triassic, and End Cretaceous. The Holocene extinction event is referred to as the Sixth Extinction.

Cretaceous-Tertiary. 65 million years ago, the dinosaurs were wiped out in a ma extinction that killed nearly a fifth of land vertebrate families, 16% of marine families and nearly half of all marine animals.
End of Triassic. About 200 million years ago, lava floods erupting from the central Atlantic are thought to have created lethal global warming, killing off more than a fifth of all marine families and half of marine genera.

Permian-Triassic. The worst mass extinction took place 250 million years ago, killing 95% of all species.
Late Devonian. About 360 million years ago, a fifth of marine families were wiped out, alongside more than half of all marine genera.
Ordovician-Silurian. About 440 million years ago, a quarter of all marine families were wiped out.

Most widely supported Causes

The most often cited as causes of mass extinctions are:

*Flood basalt events: 11 occurrences, all associated with significant extinctions.

*Sea-level falls: 12, of which 7 were associated with significant extinctions

*Asteroid impacts producing craters over 100km wide: one, associated with one mass extinction. *Asteroid impacts producing craters less than 100km wide: over 50, the great majority not associated with significant extinctions.

Evolutionary importance

Mass extinctions have sometimes accelerated the evolution of life on earth. When dominance of particular ecological niches passes from one group of organisms to another, it is rarely because the new dominant group is "superior" to the old and usually because an extinction event eliminates the old dominant group and makes way for the new one.For example mammaliformes ("almost mammals") and then mammals existed throughout the reign of the dinosaurs, but could not compete for the large terrestrial vertebrate niches which dinosaurs monopolized. The end-Cretaceous mass extinction removed the non-avian dinosaurs and made it possible for mammals to expand into the large terrestrial vertebrate niches. Many groups which survive mass extinctions do not recover in numbers or diversity, and many of these go into long-term decline.

Sixth Mass Extinction is NOW !

There is little doubt left in the minds of professional biologists that Earth is currently faced with a mounting loss of species that threatens to rival the five great mass extinctions of the geological past.

The classical "Big Five" mass extinctions are End Ordovician, Late Devonian, End Permian, End Triassic, and End Cretaceous. The Holocene extinction event is referred to as the Sixth Extinction, that is the extinction event that is taking place NOW !
A study published in the international journal Conservation Biology reveals a sorry and worsening picture of habitat destruction and species loss. It also describes the deficiencies of and opportunities for governmental action to lessen this mounting regional and global problem. The review highlights destruction and degradation of ecosystems as the main threat.

A study published in the international journal Conservation Biology reveals a sorry and worsening picture of habitat destruction and species loss. It also describes the deficiencies of and opportunities for governmental action to lessen this mounting regional and global problem. The review highlights destruction and degradation of ecosystems as the main threat.

According to the IUCN Red List of Threatened Species, 2008

*Nearly 17,000 of the world's 45,000 assessed species are threatened with extinction (38 percent). Of these, 3,246 are in the highest category of threat, Critically Endangered, 4,770 are Endangered and 8,912 are Vulnerable to extinction.

*Nearly 5,500 animal species are known to be threatened with extinction and at least 1,141 of the 5,487 known mammal species are threatened worldwide.

*In 2008, nearly 450 mammals were listed as Endangered, including the Tasmanian Devil (Sarcophilus harrisii), after the global population declined by more than 60 percent in the last 10 years.

*Scientists have catalogued relatively little about the rest of the world's fauna: only 5 percent of fish, 6 percent of reptiles, and 7 percent of amphibians have been evaluated. Of those studied, at least 750 fish species, 290 reptiles, and 150 amphibians are at risk.

*The average extinction rate is now some 1,000 to 10,000 times faster than the rate that prevailed over the past 60 million years.

The Passenger Pigeon
In Michigan during a single hunt in 1878 an estimated 1,000 million birds were destroyed at nesting sites. On September 1, 1914 the last Passenger Pigeon named Martha died in captivity in the Cincinnati Zoo.

Indian Vultures
In India, the White backed vulture population was estimated at 30 million birds in 1992. Today, it is a mere 11,000 birds and falling due to Diclofenac poisoning.

Extinction is irreversible.

This has been part of the evolutionary process which has produced more advanced forms of life - a process that has occurred over a vast span of time over millions of years. The greatest contribution of Charles Darwin, who propounded the Theory of Evolution, in his logical explanation for evolutionary changes and appearance of new form of life - natural selection - the success of those organisms that are capable of adapting to the environment, to survive and reproduce. One of the world’s rarest birds and an almost extinct species, today lessthan 200 birds survive!
In India, the Cheetah, the lesser one-horned rhinoceros, the pink- headed duck and the mountain quail have become extinct in the last one century.

The Sangai, the brow-antlered deer is found only in Manipur and only 162 animals survive.

Many mammals and birds have become rare and endangered and many a natural range diminished in size with increasing deforestation, often confining the animals to small territories.

The Golden Toad of Monteverde, Costa Rica was among the first casualties of amphibian decline. Formerly abundant, it was last seen in 1989.

References: Wikipedia, J. C. Daniel (’Extinction is for ever’), IUCN Red List, park.org, Mohan Pai.

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