Complex sentence: Prior to massive population loss, Decatur was the sixth-most populous city in Illinois, but is now listed as number fifteen, as Decatur lost 19% of it's population from 1980 to 2010.
Split into simpler sentences: Prior to massive population loss, Decatur was the sixth-most populous city in Illinois. Now Decatur is listed as number fifteen among Illinois cities, as it lost 19% of it's population from 1980 to 2010.