Jarod Diamond in 1984 listed four principal mechanisms for the loss of biodiversity. It was known as the 'Evil quartet'. It was also known as ‘four horsemen of the ecological apocalypse. The four mechanisms were:-
a) overhunting (or overexploitation),
b) introduced species,
c) habitat destruction and
d) chains of linked extinctions (trophic cascades, or co-extinctions).