Question

what would happen to the number of lions if all the deer suddenly disappeared from the forest?

Answer 1

Lions consume a wide variety of meat. They prey on deer, zebra, giraffe, wild hogs,buffaloes, rhinos and hippos. They also feed on smaller animals like birds, hares and reptiles. In times of food scarcity, lions are also known to prey on elephants.
If the deer population becomes extinct, the lions would begin looking for alternative sources for food. Hence, there would be a depletion in the population of other animals and also birds. Consequently, the population of these animals would go down. In case these animals are also difficult to find, the lion population could be affected as their requirement of staple food would not be available. In such an event, however, the lions could stray out of the jungle and attack human populations close to it.   

Answer 2

If all the dear suddenly disappear from a forest then,the lions would not be able to survive and would harm all the human beings.