Human activities are the basic cause for it.
Causes – 4 major causes (The evil Quartet along with suitable examples of each:
i) Habitat loss and fragmentation-decline in covered forest area from 14% to 6%.
ii) Overexploitation- extinction of steller’s sea cow
iii) Alien species invasion-Nile perch introduction into Lake Victoria resulted in extinction of 200 species cichlid fish.
iv) Co-extinction-coevolved plant pollinator mutualism.
Harms: (a) Decline in plant production
b) Lowered resistance to Env. Perturbations like drought.
c) Increased variability in certain ecosystem processes such as plant productivity, water use etc.

(i) A = Dissolved Oxygen
(ii) B=Biochemical oxygen demand(BOD)

(i) (a) N₂O (nitrous oxide) (b) CH₄(Methane)

(a) In West Bengal, ground water is contaminated with arsenic.Chronic exposure to arsenic causes black foot disease in humans.
(b) Presence of excess nitrates in drinking water are harmful for human health and may be fatal for infants.In infants, excess nitrates react with haemoglobin to form non-functional methamoglobin that impairs oxygen transport.This condition is termed as blue baby syndrome or methaemoglobinemia.

An electrostatic precipitator is a device to remove particulate matter present in the foul air. In situations when it stops functioning in a thermal power plant, it may lead to breathing and respiratory problems, irritation in the throat, inflammation, and damage to lungs in people working in it or living around.