User blog comment:BannedLagiacrus/Monster Hunter Ecological and Lore Info Blog/@comment-2123858-20160512042625/@comment-4009941-20160513215148

MHF > MHF1 > MHFU > MHP3rd could transfer all character data to each game. Frontier timeline runs on same as main series with some delay I beleive, as they stated Brute Wyverns as a newly discovered monster type just like 3rd gen did and even mentioned the new world in 3rd gen. As for 4th gen games I have no idea, the enviroments are changing but ALL the NPC's are alive and well. With FU being able to transfer to MHP3rd, 3rd gen was not too far apart as brutes were being researched in MH3 and MHP3rd comes after that. Cha-Cha and Kayamba apparently are still children(?) in 4U and the same characters from MH1 and MH2(Which the ending scene still has me confused about it's timeline) even appear almost the same.

I suspect a semi rapid change of the dunes and other old areas. Elder Dragons alone change MH world rapidly so they can be blamed. Or you could blame everyone 2000+ hunts in every old area in MHFU causing damage to the land... The Dunes seems to have the most extreme change with all the sand but you could just as easy blame a monster like Odei or a random other dust storm and rock crushing/forming monster.