Dr. Seuss's The Lorax is a warning, plain and simple, against ignoring the environment. The story shows how overutilizing natural resources, such as the Truffula Trees, destroys complete ecosystems and makes wildlife flee. The pollution from the Once-ler's factory represents real-life environmental damage caused by unchecked industry and overconsumption.
Ultimately, the message of the Lorax is simple: it is our responsibility to care for the planet.