Well, one man's "overpopulation" is another man's "Hurray, we are a large, and therefore, strong group." The invaders of Easter Island might NOT have felt too crowded where they came from, they might have simply seen a nice island while voyaging or fishing, and decided it would be nice to take what was there for themselves. You discount human desire to take stuff, in your analysis. If there is a market for slaves, some people will run around looking for people to enslave. If they can. If they can outnumber and out-mean them. We don't know the exact reasons for the invasion of Easter Island. Or I don't. I haven't read this book. Have you? In general, don't make assumptions by looking through the lens that everything has to be because of this one cause, overpopulation, simply because that is a subject that interests you so that's your go-to explanation. There are lots of other possible reasons why a numerous group overwhelms a smaller group. But as I say I haven't read the book. But SammyD1st is known to be a smart poster here. He may be a mod. So I am figuring he's right.

Have kids. You might need them. And they are fun anyway.