How can overcrowding contribute to the collapse of a civilization?
Answer: The more people you have the more resources/food is necessary to keep them alive you have more people then resources/food they civilization will collapse, because its over populating the area more than the civilization can supply.
If more people came to a civilization, food and resources and space can become scarce. The lack of space and housing can cause a faster spread of disease, causing the end of a civilization.
