Discuss the causes of biodiversity loss.

(N/A) The world is currently facing accelerated species extinction,primarily due to four major causes,collectively known as 'The Evil Quartet':
$(a)$ Habitat loss and fragmentation: $(i)$ Tropical rainforests,which once covered more than $14\%$ of the Earth's land surface,now cover no more than $6\%$. $(ii)$ The Amazon rainforest,often called the 'lungs of the planet',is being cut and cleared for cultivating soya beans or for conversion to grasslands for raising beef cattle. $(iii)$ Pollution and degradation of habitats also threaten the survival of many species. $(iv)$ When large habitats are broken into small fragments due to human activities,mammals and birds requiring large territories are badly affected,leading to population declines.
$(b)$ Over-exploitation: Humans have always depended on nature for food and shelter,but when 'need' turns to 'greed',it leads to over-exploitation. $(i)$ Many species extinctions in the last $500$ years (e.g.,Steller's sea cow,passenger pigeon) were due to over-exploitation by humans. $(ii)$ Currently,many marine fish populations around the world are being over-harvested,endangering the continued existence of some commercially important species.
$(c)$ 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. $(i)$ The Nile perch introduced into Lake Victoria in east Africa led to the extinction of an ecologically unique assemblage of more than $200$ species of cichlid fish. $(ii)$ Invasive weed species like carrot grass $(Parthenium)$,$Lantana$,and water hyacinth $(Eichhornia)$ cause environmental damage and pose a threat to our native species. $(iii)$ The illegal introduction of the African catfish $Clarias$ $gariepinus$ for aquaculture is posing a threat to the indigenous catfishes in our rivers.
$(d)$ Co-extinctions: When a species becomes extinct,the plant and animal species associated with it in an obligatory way also become extinct. For example,when a host fish species becomes extinct,its unique assemblage of parasites also meets the same fate. Another example is the case of a co-evolved plant-pollinator mutualism where extinction of one invariably leads to the extinction of the other.