Population collapse and why it happens.

Spotted this article on the ecologist website.
I read this through a couple of times today, and I think that there are some good points made. The trouble is that the rationale is all a bit 'airy-fairy', too much store is given to points which need some facts to back them up.

Such as;

Well, there have been four collapses in Northern Europe. The first such collapse was round about 3000BC, and that was due partly to climate change ­the temperature got a great deal warmer ­ but also to the fact that we had deforested the uplands, where we lived. By 3000BC we had completely changed the forest and the woodland nature of Britain, and the same is true for most parts of Northern Europe. The climate changed, the soil blew away, we had not kept the hedges or the shrubs or the tree cover to ensure the soil stayed.

Now I have a bit of knowledge on the subject of prehistoric forestation and the developement of hedges in particular, ( don't ask it's a long story), and I don't think that this is right.

And;

 The third collapse was the end of the Roman Empire. During the height of the Roman Empire, in the second century AD to the fifth century AD, we were pumping out as much air pollution as we were in the mid Victorian periods. 

Do we have any figures? There are no links or sources given in the article. 
I am not saying  it is wrong but it would be nice to have a bit more meat on the bones.This is all a bit science lite.