The two negative effects of over population are :-
1)Water Shortages: About one billion people lack access to sufficient water for consumption, agriculture and sanitation.many people even do not get a single drop of water.
2)Food: one billion people, one out of every seven people alive, go to bed hungry.Every day, 25,000 people die of malnutrition and hunger-related diseases.
many people sleep everyday without eating a single bite of food.
hope helped you. ;D