1. Oddly enough, population is uncountable, in the same way that money is uncountable, so 'less than' is correct.
2. Much is correct, as range is uncountable. More is a comparative. As your example does not provide another range from another time, there is no comparison.
The time frame of this example writing is of significance. "The loss of beavers from much of their original range is also of importance for other species" requires that this writing be contemporary with the event (the stated loss of beavers). If this is a modern writing about history (as I suspect), then was is required.
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