Causes of biodiversity loss - Practice Questions & MCQ

Habitat loss and fragmentation

• This is the most important cause driving animals and plants to extinction. 
• The tropical rain forests once covering more than 14 percent of the earth’s land surface now cover no more than 6 percent. 
• The Amazon rainforest (it is so huge that it is called the ‘lungs of the planet’) harbouring probably millions of species is being cut and cleared for cultivating soybeans or for conversion to grasslands for raising beef cattle. 
• Besides total loss, the degradation of many habitats by pollution also threatens the survival of many species. 
• 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.

Over-exploitation:

• Many species extinctions in the last 500 years (Steller’s sea cow, passenger pigeon) were due to overexploitation by humans. 
• Presently many marine fish populations around the world are over harvested, endangering the continued existence of some commercially important species.

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.
• 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.
• The environmental damage caused and threat posed to Indian’s 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 catfish in Indian’s rivers.

Co-extinctions: 

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