The question, shanshee, is actually a very simple one. But that doesn't mean the answer is simple. I certainly don't know it, and I don't expect you (or anyone else) to come up with it.

If your environment (your island, patch of forest, planet, etc.) can only support X number of people without malnutrition setting in, then what happens when baby X + 1 is born?

Historically, those cultures we refer to as "sustainable" tended to have methods for dealing with that situation that are unacceptable from our liberal western standpoint (infanticide and warfare being the two obvious ones, as already mentioned here).

Here's the thing though. All societies will eventually become sustainable or collapse and die. It's just physics. You can't support 100 people on a piece of land that only grows enough food for 50. Fifty people will eventually have to die, or move elsewhere.

But of course, if we're talking about a planet containing 7 billion people that can only support 2 billion, then there's really no "elsewhere" to move to. Right now we keep the extra people alive by drawing down our reserves of non-renewable resources. Once they are gone (or once they cease to be produced fast enough to support the additional people) then we are faced with a very real problem.

There's a French anthropologist (name escapes me at the moment) who studied several Amazonian villages and concluded that they were capable of remaining sustainable only through maintaining a constant state of low-level warfare between one another. This prevented populations expanding beyond sustainable limits, but more importantly, it prevented the aggregation of villages into towns and cities which -- arguably -- tend to become engines of unsustainability.