In Elemental, there are two types of mana that are not held within shards: Life and Death magic.
Life magic makes use of the energies created by living things.
Death magic takes something that is living and harvests the energies created from the transition from life to death.
The reason why the lands of the Empires look so mangled is because the natural ecosystems in their territories have been totally perverted.
Men can choose to take the path of death magic just as Fallen can choose the path of life magic. But the pre-made factions are based on the game's lore (canon) and only the men of Krax have taken the path of death magic and part of that is because it was an adapt or die situation during the final days of the war of the Titans (that led to the cataclysm).