[glenra]

That's an odd take on it. No, I'd mainly say there's something about being invaded that can kill a civilization. A great many somethings. Though certainly some of them are environmental. It'd be harder to live there the same way after an invader used the land to graze sheep. Or after an invader brought in smallpox or rats or any number of other unwanted interlopers.

What's special about "un-invaded land" is that it has a bunch of people surviving on it living in a certain way and if they've been living there long enough, their way of life probably works pretty well for them. If some important resource they need is getting scarcer it's NOT going to disappear instantaneously all at once as per the poetic image of a "guy cutting down the last tree" catching everyone by surprise. As any given resource gets scarcer it gradually gets harder and harder to rely on access to it, which means people first have a small, then a moderate, then a large, and eventually a MASSIVE incentive to find substitutes. Other ways of living. Other things to eat. Other forms of shelter. Humans are a creative bunch; if there are other POSSIBLE ways to survive, we're pretty likely to find them given the existence of hundreds or thousands of people in an area whose lives depend on it and who have some ability to share info, try out ideas, and copy what works.

Invasion is a massive sudden shock to the system; some massive shocks aren't survivable.

[mikeash]

So what you're saying is, resource depletion is a problem if it happens fast enough?

I don't see the practical difference between losing your land to invaders and losing your land to, say, sea level rise, in terms of one's ability to cope with it and survive.

In any case, this is a lot of argument over what was mostly a throwaway argument. The precise nature of the Easter Island collapse doesn't much matter since it's not an isolated example anyway.

[glenra]

In a market economy with strong property rights, complete resource depletion is extremely unlikely because prices telegraph in advance the information that the resource is becoming scarce. But what I'm saying is that even in a non-market economy something similar can happen. Suppose the resource is food. When there's a big herd of dumb animals on the land it might not be worth anyone's while to try fishing instead. As the animals get fewer and smarter, it gets harder to get enough to eat; that is an incentive for individuals or families to try fishing or farming or moving away even if there's no trade or prices or futures market.

Losing land to sea level rise - if it happens - happens so slowly and incrementally that there's plenty of time to move elsewhere or build levees or put things up on stilts. On a timescale of decades and centuries, things fall down and break; when you rebuild you do it a bit further inland or somewhere else.

We might be having problems because an ecologist's definition of "resources" and "consumption" is different from an economist's; I tend towards the latter. I fear that if we listen to the ecologists we might inadvertently create the very scarcity they fear.

(Matt Ridley briefly discussed the difference between the two in a WSJ article this week: http://online.wsj.com/news/articles/SB1000142405270230427990... )