What problems could arise when too many people living in a small area?

Depending on the time frame and circumstances... a big one would be limited resources. People need to eat and need a lot of space to grow and maintain their own resources or be able to import all they need with a very healthy economy. Also any kind of health issues or epidemics would spread much quicker in a small area with a dense population. Those are just a couple off the top of my head.

1.)dr. natterson- horowitz came to realize that their isn't much of a difference between animals and humans. just like humans, animals can suffer from illnesses and diseases like breast cancer, self injury, anxiety, and many more. she also discovered how they even treated and performed surgical procedures on animals just as if the animal it self were human. the los angeles zoo called dr. natterson- horowitz because they knew that she was a doctor and that  she would be able to recognize what was wrong with the gorilla. they knew that humans and gorillas were similar, so instead of calling another veterinarian, they called a doctor.