Feedback
My knowledge of nomadic pastoralists is meagre, to say the least, and previous posts have made it clear that the vision of peaceful sharing in the Mongolian steppes could not be further from the truth. The repeated invasions of Europe from the East* were more likely to have been caused by resource depletion than mindless aggression.
That said, I suspect that in practice the tragedy of the commons must depend on the efficientcy of the feedback loop. If shortage of resources culls the population of depleters fast enough, then a stable state can be achieved. The problem is that although homo sapiens has fairly low fertility, the resilience and adaptability of the species are such that it can usually survive exhaustion of one resource and go on to deplete another.
Lemmings have a large 3-4 year population cycle that might be due to this kind of feedback. I suppose we have yet to find if there's an equivalent cyle for us.
* Huns and possibly Goths, as well as Mongols.