Excerpts from a Jared Diamond talk:
In trying to understand the collapses of ancient societies, I quickly realised that it�s not enough to look at the inadvertent impact of humans on their environment. It�s usually more complicated. Instead I�ve arrived at a checklist of five things that I look at to understand the collapses of societies, and in some cases all five of these things are operating. Usually several of them are.
The first of these factors is environmental damage, inadvertent damage to the environment through means such as deforestation, soil erosion, salinisation, over-hunting etc.
The second item on the checklist is climate change, such as cooling or increased aridity. People can hammer away at their environment and get away with it as long as the climate is benign, warm, wet, and the people are likely to get in trouble when the climate turns against them, getting colder or drier. So climate change and human environmental impact interact, not surprisingly.
Still a third consideration is that one has to look at a society�s relations with hostile neighbours. Most societies have chronic hostile relations with some of their neighbours and societies may succeed in fending off those hostile neighbours for a long time. They�re most likely to fail to hold off the hostile neighbours when the society itself gets weakened for environmental or any other reasons, and that�s given rise for example, to the long-standing debate about the fall of the Western Roman Empire. Was the conquest by Barbarians really a fundamental cause, or was it just that Barbarians were at the frontiers of the Roman Empire for many centuries? Rome succeeded in holding them off as long as Rome was strong, and then when Rome got weakened by other things, Rome failed, and fell to the Barbarians. And similarly, we know that there were military factors in the fall of Angkor Wat in Cambodia. So relations with hostiles interacts with environmental damage and climate change.
Similarly, relations with friendlies interacts. Almost all societies depend in part upon trade with neighbouring friendly societies, and if one of those friendly societies itself runs into environmental problems and collapses for environmental reasons, that collapse may then drag down their trade partners. It�s something that interests us today, given that we are dependent for oil upon imports from countries that have some political stability in a fragile environment.
And finally in addition to those four factors on the checklist, one always has to ask about people�s cultural response. Why is it that people failed to perceive the problems developing around them, or if they perceived them, why did they fail to solve the problems that would eventually do them in? Why did some peoples perceive and recognise their problems and others not?