Diamond's main theme is that societies have made choices about how they interact with the environment. Some choices are sustainable and some are not. Societies that survive make sustainable ecological choices. Diamond's theme is cautionary: he is arguing that our own culture's survival is not a given and that if we do not heed the environment, our society will sooner or later collapse.

Diamond, for example, points to the Norse settlers in Greenland. Their society lasted for hundreds of years, but ultimately was unsustainable because they refused to adapt to the ecology of Greenland. They imported their Scandinavian lifestyle and relied on cattle and other livestock. Eventually, they exhausted their natural resources and began to starve. However, rather than learn from the natives who had lived sustainably on Greenland for thousands of years through fishing in kayaks, the Norse, believing themselves innately superior, refused to adopt native ways. Eventually their society disappeared.

Diamond's book cries out against an arrogant approach to the environment. Humans can't simply dominate the environment forever in unsustainable ways and expect this to work because it's how they've "always" lived or because this is how they want it to be. Stupidity and willful blindness have sunk cultures before, and it can happen again. We need to recognize we are part of a bigger world of nature and understand that if we don't align ourselves with it, our culture will likely perish.

Definitely the central implication for us today is to pay attention to what we are doing to the environment and to respond accordingly, making sure that we learn from history rather than dwell ignorance and repeat the mistakes of past civilisations that have collapsed through the environment.

I would say that the main theme of this book is that damage to the environment can help cause socities to collapse but that such damage cannot be the only cause.  Diamond argues that environmental damage is often present in socities that collapse.  However, what is more important is how they respond to the environmental crises that arise.  The implication for this is that we need to respond to our current ecological problems in constructive ways.  We cannot be like the Norse on Greenland who keep trying to live in unsustainable ways.