Complex sentence: Perhaps the most significant ecological disaster was the rapid depletion of the bison population, a vital food source, bison numbers had been dwindling since the 1760s due to overhunting by the English, Canadian and aboriginal inhabitants of the prairies.
Split into simpler sentences: Perhaps the most significant ecological disaster was the rapid depletion of the bison population. A vital food source, bison numbers had been dwindling since the 1760s due to overhunting by the British, Canadian and aboriginal inhabitants of the prairies.