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? :-)