If humans did kill them off they had good reason too and if the idiots that abound today can't see that they also would have been either throwing spears or being eaten then they are even dumber than they act!
I doubt that humans had much to do with killing off the big predators, however, as the american indians had a hard time with grizzlies let alone the huge meat gulping critters of the ice age.
The fact is, after the ice the water of the plains eventurally went away, as rain fall wasn't that great, the prey annimals died and then the big predators died from lack of food. This is known to many people but apparently facts don't stand in the way of a good story blaming humans for everything. Pretty soon we will have somehow made the dinos die off.
"Well to tell you the truth, if I was alive at that time I would have been getting all the men in the tribe together to figure out the best way to exterminate animals like the short faced bear, a bear who makes the grizzly look like a teddy bear, the sabre tooth and other predators that must have made life for humans just peachy."
Besides, they tasted great at a barbecue!!
A good deal of the central plains was actually woodland at the time.. At least that south of the glacial lakes..
When the glacial dams ( I think they were called Morains ) broke under the pressure, great floods swept south destroying much of those woodlands..
They weren't that heavy, but much more forested than the plains of today..
Those woodlands provided cover for predators, allowing them to close in on their prey before a quick, rushing attack to dispatch the prey..
Additionally the glacial lakes would have been fairly heavily forested along the shorelines..
In fact, as the glacial melt began, water levels would have risen right into the surrounding forestlands..
Big Cats especially, do a great deal of their hunting along the shores and banks of "watering holes"..
That is where the meat is..
Big predators need big protein..
Open plains mean plenty of running room for buffalo, antelope, etc..
Running down prey on an open plain requires even more protein..
Somewhere along the line, you find a point of no return, benefit less than effort required..
End result: Starvation..
I think you are essentially correct, but the actual emergence of wide open plains actually occurred at the end of the Ice age as well as all those other factors..
Climate change, (warming) glacial melt causing flooding, ( Mega Flash Floods ) The sudden loss of possibly thousands of small lakes and of course, the big glacial lakes, the loss of grazing land scoured by the floods, probably the deaths of millions of herbivores as well as thousands of carnivores..
All these factors would have contributed to the loss of not only mega fauna but taken a major chunk out of any other animal or human inhabitants throughout the Midwest, upper midwest, the northwest, large chunks of mid-southern Canada, etc...( Not positive about the eastern continent, but I seem to recall much of the same flooding, etc. took place there as well..
The arrival of a large number of new human inhabitants would have eventually contributed as well, ( I believe there were other, earlier, smaller groups scattered about along the east and gulf coast, as well as central and south american coast ) but I believe much of the "damage" was natural..
As others have pointed out, I don't think there were enough humans here at that point to have had that much effect on ALL of the Mega Fauna of America..
Just doesn't make sense..
Nasty little pygmies eating thunder lizard omlettes.....