Thanks for bringing up the chronology. My long term memory may have blurred..
I was shown the eroded areas and told the story while crossing the Rockies (NYC to CA actually) with a naturalist in 1969. I think I remember him saying that the dynamite harvesting was near the end of the 19th century and well past the era of the mountan-men and Indian trappers and that it wasn't profitable any more to trap. There were old photographs of the area before it was eroded and desertified (if that is the right word) in a restaurant that we stopped at.
Whatever the beavers were used for 5 or more decades after the hat craze ended I have no idea. The original Astor was long dead and I can find no evidence that his heirs got back into the fur business. That is probably my memory confounding two different stories, or perhaps I was misinformed.
Anyway the idea of the furry engineer being a keystone species that supported a whole web of life stayed with me. On the Central CA coast I am told that there were very large beavers that were killed of at the approximate time of the Siberian invasion and like the areas in the West that I was shown the rainfall is all in the winter. I'm trying to get permits for ponds on farm.
An excellent, although PC-leaning book I read recently, 1491, is based on the premise that 90 to 95% of the Indians died off by 1600, and that they were the major keystone species in the Americas, having been such for 10,000 years or more.
The result was massive ecological disruption, even without the introduced species. One theory is the huge "virgin forests" of the eastern half of the country, the massive herds of bison and flocks of passenger pigeons were a consequence of this disruption.
IOW, the wilderness the white man found wasn't really wilderness at all. It was more like an overgrown graveyard.
I don't believe all of it, but much of it is interesting, and it blows giant holes in the "white man is bad" theory, often without meaning to. It appears that the natives weren't so inherently great at living in tune with the land. It's just that almost all of them were dead, which makes it a lot harder to be ecologically criminal. Almost a post-holocaust novel type of thing.