[glenra]

> whether the population declined precipitously (no dispute here either)

No, that is definitely one of the items under dispute. Lipo claims the population never reached that high a peak and did not substantially decline until after outsiders came.

> I can't see any support for the idea that the island was doing fine until outsiders bought war, disease, and slavery. As far as I can see, the war, disease, and slavery period followed European contact, but the initial population decline from a peak of around 15,000 down to 2-3,000 happened before European contact.

Lipo says that the claim that the population followed that sort of trajectory was essentially an early rough guess not based on actual data. A popular one, so you might find it in multiple sources, but a guess nonetheless. In response, quoting from that last link I gave before:

> Finally, Diamond ignores field research reporting dated domestic habitation sites (see Hunt and Lipo 2009 for discussion). When the habitations are plotted in fifty-year intervals, the number of those occupied clearly shows that the first and only sustained decline, as a relative measure of the population, began only in the first interval following European contact. Before contact the data show a population that is growing and stabilizing, as reflected in their habitations across the landscape. There is no evidence of population decline, let alone “collapse” until after European contact. Indeed, there is direct, abundant evidence that population numbers grew, stabilized, and then fell only after European contact beginning in 1722.

(source: http://www.evobeach.com/2011/10/diamond-attempts-to-defend-m... )

> Even if it was external, though, why is "land which isn't being overrun by invaders" different from any other resource?

That story would lead to a very different conclusion, especially if you want to turn the story into an analogy for how we should treat the earth as a whole. Should we worry more about building a Space Defense network to protect us from aliens and not so much about preserving forests? :-)

[mikeash]

You misunderstand my "un-invaded land as a resource" point.

I'm just pointing out that un-invaded land is a resource, and that these people weren't able to cope with the loss of that resource.

So, is there something about un-invaded land that makes it different from other resources, such that humans can cope with losing others but not that one? If so, what makes it different? If not, then it shows that resource exhaustion can kill civilizations.

If un-invaded land is unique somehow, then obviously we're not under threat in that respect. But if it's not, then it demonstrates that this class of problem exists, and shows that we need to take steps to ensure we don't run out of the resources we do have looming problems with.

[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... )