Causes of biodiversity losses

by  SIDDHARTH SANGHVI SIR

The accelerated rates of species extinctions that the world is facing now are largely due to human activities.  There are four major causes  ‘ The Evil Quartet’ is the sobriquet

 (a nickname or an unofficial title given to somebody/something) used to describe them.

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1. Habitat loss and fragmentation

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Habitat destruction is the single greatest threat to biodiversity. 

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The International Union for Conservation of Nature and Natural Resources attributes habitat destruction for 73% of the species that have become extinct, endangered, vulnerable, or rare during modern history. 

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 Urbanization, agriculture, mining, pollution, and other human land use have resulted in wildlife habitats becoming smaller and farther apart. This is called habitat fragmentation. 

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Habitat fragmentation limits animals' access to resources such as food and potential mates. Animals in fragmented habitats are often unable to migrate to other regions with necessary resources when those resources become locally scarce. 

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 In general, habitat fragmentation decreases a species' ability to cope with stresses like resource limitation and disease. As a result, species in fragmented habitats are more likely to become extinct.

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The most dramatic examples of habitat loss come from tropical rain forests. Once covering more than 14 per cent of the earth’s land surface, these rain forests now cover no more than 6 per cent. They are being destroyed fast.

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By the time you finish reading this chapter, 1000 more hectares of rain forest would have been lost. The Amazon rain forest (it is so huge that it is called the ‘lungs of the planet’)

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harbouring probably millions of species is being cut and cleared for cultivating soya beans or for conversion to grasslands for raising beef cattle.  

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Besides total loss, the degradation of many habitats by pollution also threatens the survival of many species. 

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 When large habitats are broken up into small fragments due to various human activities, mammals and birds requiring large territories and certain animals with migratory habits are badly affected, leading to population declines.

OVERexploitation

Humans have always depended on nature for food and shelter, but when ‘need’ turns to ‘greed’, it leads to  over -exploitation of natural resources.

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OVERexploitation

 Many species extinctions in the last 500 years (Steller’s sea cow, passenger pigeon) were due to overexploitation by humans. 

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OVERexploitation

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Passenger pigeons (Ectopistes migratorius), museum specimens.

OVERexploitation

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The extinction of the passenger pigeon is unparalleled in recorded history: Due to our  relentless killing, these iconic birds’ population crashed from billions to zero in just fifty years.

OVERexploitation

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Adult male Passenger pigeons (Ectopistes migratorius), museum specimens.

OVERexploitation

Presently many marine fish populations around the world are over harvested, endangering the continued existence of some commercially important species.

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Alien   species invasions

When alien species are introduced unintentionally or deliberately for whatever purpose, some of them turn invasive, and cause decline or extinction of indigenous species. 

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Alien   species invasions

The Nile perch introduced into Lake Victoria in east Africa led eventually to the extinction of an ecologically unique assemblage of more than 200 species of cichlid fish in the lake

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Alien   species invasions

Nile perch (Lates niloticus) were introduced to Lake Victoria in the 1950s to boost the fishing industry. Though the introduction of Nile perch resulted in an economic boom, it almost caused cichlids, a native fish, to go extinct.

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Nile perch are some of the biggest freshwater fish in the world. Only these fish are consistently larger:

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You must be familiar with the environmental damage caused and threat posed to our native species by invasive weed species like  carrot grass (Parthenium)

 Lantana and water hyacinth (Eicchornia). 

 The recent illegal introduction of the African catfish Clarias gariepinus for aquaculture purposes is posing a threat to the indigenous catfishes in our rivers.

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The African catfish (Clarias gariepinus) was introduced to east India from Bangladesh in the early 1990s as a source of food. Its breeding and import are now banned by India’s agriculture ministry because it harms aquatic biodiversity. 

These hardy fishes thrive in polluted water and feed on native fish. However, they continue to be reared across the country because of their low-cost meat. These fish frequently escape unregulated aquaculture farms and enter waterbodies.

Co-extinctions

 When a species becomes extinct, the plant and animal species associated with it in an obligatory way also become extinct.

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Co-extinctions

 When a host fish species becomes extinct, its unique assemblage of parasites also meets the same fate. Another example is the case of a coevolved plant-pollinator mutualism where extinction of one invariably leads to the extinction of the other.