THE EXTINCTION OF SPECIES – PART I

Human actions have resulted in the widespread loss of natural habitats, fragmentation of the remaining habitats, pollution of many areas, displacement of uniquely adapted species by exotics, etc. Highly endangered species face at least two or three of the above-mentioned threats. These threats develop so rapidly and on such a large scale that species are not able to adapt genetically to these sudden changes or to move to favourable locations.

Extinction is a natural process, which occurs at the end of the evolutionary life of a species, a family, or a group of organisms. Species arise through the natural process of mutation and natural selection and also disappear the same way. Life evolved on earth more than 600 million years ago. Many species have evolved and many others have gone extinct over millennia. Extinctions at the species level occur constantly and give way for new species to evolve. But it will take thousands of years for the species to develop. As a result of this biodiversity is intact and is on the increase.

Mass extinctions

Fossil evidence reveals that the biodiversity of the earth was severely affected on 5 occasions. These are called mass extinctions. The best-studied of these events occurred at the end of the cretaceous period when dinosaurs disappeared, along with at least 50% of existing genera and 15% of marine animal families.

There is no convincing proof or evidence to support any particular pattern to mass extinctions. The documentation of extinction is uneven both geographically and taxonomically. Incomplete fossil records and problems inaccurate dating of fossils are the two main reasons for incorrect information about the rate of extinctions.

12-02-22-01

Manmade extinctions

Manmade extinctions may have occurred 15,000 to 25,000 years ago. Excessive hunting of large animals contributed to significant extinctions in three continents namely Australia, North and South America. Colonization of new countries particularly oceanic islands substantially affected the diversity of unique animals and plant species. Experts believe that these extinctions occurred as a result of the conversion of wildlands to pastures and the conversion of lowland forests for agriculture. Population growth is also responsible for species extinction.

Humanity is contributing in a variety of ways to global extinctions. The current rate of extinction is around 1000 times greater than natural rates of extinction in the past. In an undisturbed ecosystem, the rate of extinction appears to be about one species in every decade. Between A.D 1600 and 1850 human activities appear to have been responsible for the extermination of two or three species per decade. Since 1600 only 465 animals and 584 plant species are listed as extinct. Harvard entomologist E.O. Wilson estimated that we are now pushing 20,000 species a year into extinction.

Causes for manmade extinctions

  • Ecologists have listed the main causes for manmade extinctions as
  • Habitat destruction
  • Habitat fragmentation
  • Habitat degradation (pollution)
  • Overexploitation of species for human consumption and through hunting
  • Introduction of exotic species
  • Increased spread of diseases.

Human actions have resulted in the widespread loss of natural habitats to human habitation, agriculture and building dams and roads, fragmentation of the remaining habitats for the same reasons, pollution of many areas, displacement of uniquely adapted species by exotics, etc. Highly endangered species face at least two or three of the above-mentioned threats. These threats develop so rapidly and on such a large scale that species are not able to adapt genetically to these sudden changes or to move to favorable locations.

The extinction of mammals or bird species is more likely to be recognized than a plant or amphibian species. Some scientists estimate that at the current rate of habitat destruction we may lose up to one-third of the total wild species within the next few decades, that is an astounding 45,000 known species and probably many more unrecorded ones.

Toxic pollutants can have disastrous effects on local populations of organisms. Pesticide linked declines of birds were documented as early as in the 1960′s. Marine mammals, alligators, fish and other declining populations suggest complex interrelations between pollution and health. In the North Atlantic region, large-scale death of seals was attributed to the effect of chlorinated hydrocarbons, such as DDT, PCB and dioxins in fat, causing a weakened immune system that is vulnerable to infections.

The trade-in wild species for pets is an enormous business. Worldwide some 5 million birds are sold each year as pets, mostly to Europe and North America. It is generally estimated that fifty animals are caught or killed for every live animal that reaches the market. Buyers, many of whom say they love animals, keep this trade going. Plant species are also threatened by over-harvesting with prices ranging as high as Rs. 45,000.00 for a rare specimen. So it is not surprising that many are now endangered. The United States imports 99% of all live cacti and 75% of all orchids sold each year.

A growing threat is the spread of alien species, which is very difficult to control by any means. Thousands of alien species are moved by people deliberately or accidentally around the globe. Both animals and plants when introduced to areas and habitats where they do not naturally occur displace native species through predation, competition, disease and hybridization. Well-established exotic species may be impossible to remove from communities. In some cases, it is extraordinarily difficult and expensive. Overharvesting of both animal and plant species, for either economic or cultural reasons is a universal threat.

Manmade extinctions are not restricted to wild species alone. Thirteen to seventeen principal fishing zones around the world are now reported to be commercially exhausted. Fish stocks have been seriously depleted by over-harvesting in many parts of the world. Zoos house about 5,00,000 animals but only about 900 species in total. Bats, whales and many reptiles rarely reproduce in captivity.

…to be continued

J. Murali
C.P.R. Environmental Education Centre

 Source: Eco News, Vol.7, No.3 (October – December), 2001.

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