If you have to ask, explanations will likely be pointless. Look up 'carrying capacity'.
(Standard disclaimer- I am not one of those wacko 'childfree' zealots. I kids, and replacements have to come from somewhere. But now that most of us do NOT live on farms and need to breed our own help, and now that 1 kid in 3 or so DOESN'T die by age five, well, maybe 'replacements' should be the term people keep in mind. 2.3 kids per breeding couple or so will assure population doesn't go down. Maybe a little higher to take into account the people like me that never succeeded in reproducing. I am one kid out of 8, with 7 surviving. Families that large make no economic or ecological sense any more. My sisters that did have kids had 2 each, so the genetic line will go on.)
aem sends...