How can overcrowding contribute to the collapse of a civilation

I was just learning about China's one-child policy. They did this because Deng Xiaoping feared that China's farms wouldn't be able to produce enough food. Therefore, overcrowding can cause shortages of food, jobs, homes, etc. Now imagine an area that has a type of animal and because of overcrowding there are fights among the animals for food and anything that will keep them alive. Overcrowding can eventually lead to the extinction of a species because everyone and everything wants to survive. I hope this helps 

Sheet stain on pee wee's sheets