When do you say an animal is endangered and General idea why organisms become extinct?

an animal is considered endangered when its population drop to a dangerous low number.Endangered species are in the environment but due to factors like hunting by man,haphazardness in temperature and climate as a whole,it's number becomes reduced and it comes to a level where it's occurrence or live is tempered and so it is faced to disappearance it measures are not taken.

Organisms become extinct will be answered later