These little guys are cute, fuzzy, bear-like critters. They get sick when the Once-ler depletes their source of Truffula Fruits, and they're forced to leave the forest in search of a new food supply.
(Depleting the food supply? Check.)
These are the hardy creatures that last the longest in the Truffula forest. But when the waste from the Thneed factory turns their once clear lake into mucky, oozy mess, the Lorax has to send them on their way. They take to the land and walk/flop their way out of there. As with the other creatures, the possibility that they will die before they get to a better place hangs in the air. Seuss doesn't take it that far, but your little brainiacs might raise such issues themselves.
(Polluting the natural habitat? Check.)
These birds can't talk, but they can sing—at least until the Once-ler smogs up their air. Like the rest of the creatures, the Swans are forced to leave the land.
(Lowering the overall quality of life? Check.)