In the last 150 years, the rate at which species are disappearing is about thousands per decade; while natural extinction rate is only one or two species per decade.
Some of the main causes are as follows:
(i) Loss/degradation of Habitat:
A habitat is place where living beings find food, cloth and shelter and a safe place to reproduce and bring up their offspring. Thus, loss of habitat is the greatest threat to the world.
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(ii) Overexploitation of Resources:
A number of species like tigers, giant pandas etc. are on the verge of extinction because of overexploitation of resources.
(iii) Pollution:
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Pollution is responsible for global climatic changes and for the extinction of most of the species.
(iv) Extinction of weaker species due to aggressive non-native species:
It is responsible for extinction of almost 50% of species on islands all over the world since 1800 AD.
(v) Poaching of wildlife:
Poaching is the illegal killing of wildlife for sale in the internation trade market. The animals are killed due to following reasons.
Some wildlife species are killed for consumption (eating);
Elephants are killed to obtain their teeth for financial gains;
Tigers/lions are killed to extract their skin to be sold for decoration of the drawing rooms of some peoples.