John Varley's Eight Worlds[0] series has an alien species called the Invaders. They invaded Earth to protect cetaceans from the effects of human civilization.

John Varley's Eight Worlds[0] series has an alien species called the Invaders. They invaded Earth to protect cetaceans from the effects of human civilization. The Invaders divide sentient life into three tiers - species like themselves that evolve in gas giants, cetaceans, and vermin; we're in the third category. [0] https://en.wikipedia.org/wiki/Eight_Worlds https://en.wikipedia.org/wiki/Eight_Worlds