Ax: <The Yeerks would take our world and make it as barren as their own. As they will to your planet unless they are stopped.>
Cassie: What... what are you saying? What do you mean about making the planet barren?
Ax: <The usual Yeerk pattern. Once a planet is under their control, they alter it to suit their own desires. They will leave enough plant and animal species to keep the host bodies fed - humans in the case of Earth - and the rest they eliminate.>
—Animorphs #4: The Message
By the end of the Dark Crusade, Kronus was a tomb world once more. Just as the marching Necron armies cut down all their opponents, so did other fiendish devices work on the planet's very ecosystem: scarab swarms scoured the lush jungles of the south and the fertile plains of the north, while pulses of loathsome energy killed even microscopic bacteria. The seas died and the air itself became rarefied and poisoned. Soon, sterile sand and dust swept across the Kronusian landscape, broken only by black monuments to the Nightbringer.
—The Necron Ending, Dawn of War: Dark Crusade