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.