Kingdom

If you were given a planet such as ours and you had to put all kinds of life on it. On this planet you had a specific amount of room for each group of life. 

How would you keep the populations down to that certain level? 

NO Religion in this please. 

I used to play a computer text game in the early to mid 90's that was called Kingdom. In the game you had to grow food for a certain amount of people, You were never given the number of the amount of people that you had to keep.  You had to harvest it. The game had things like Rats that would eat your stores and new people would come to your kingdom and some would leave. 

It was a game of thinking and of stragedy more than anything else.  

I played that for months and still never got it to be perfect or even close to perfect.  

I often killed all the people in my kingdom.

So how would you make yor world keep your life manageable.  

Would you need to kill off something or make one go extinct so that you could make a better species or lofe form?  

Would you grow more food for all or would you make a desaster strike so that it would keep the populations in check?  

Would you use disease to hold down a polulation?