User blog comment:Kogath/Monster Hunter 4 Ultimate for the Nintendo 3DS/@comment-6050823-20141029004407/@comment-6050823-20141030010414

It's also described that the Dede Desert(Old Desert/Dunes)along with some of its inhabitants have changed/are starting change over the years, including the ecosystem itself and a lot of its terrain, which is the reason the Dede Desert looks different than before and also why some of the monsters have changed.

This is coming from in-game in MH4G by the way.