Finite resources
Exponential population growth
Think about it
Tied closely to overpopulation is the phenomenon of species degeneration. The most successful members of the race (those living by chance or by skill in the areas most able to support more people) are the ones having the fewest children. The vast majority of the species population growth is in areas where the newly arrived children have a fairly poor chance of having a successful life. Before you flame me: "successful" doesn't mean rich. It means safety, food, comfort, and knowledge.
Ideas so far:
Other ideas? Let me know. [an error occurred while processing this directive]