Seventy plus isn't uncommon. Production drops of 5-10% are fairly common. I don't know what that would do to one of the Divide Mountains, considering their population.
Considering you can probably get max pop from 1 peasant in about a day for these regions thanks to emigration, I'd say it wouldn't really do much at all to them.
And I concur with egamma: if you can't defend a region, don't stash your food there.
