The Once-ler escalates production of Thneeds immediately. He calls up his relatives and asks them to come down to his shop, giving them instructions and telling them that it is their chance to get rich. Soon, the whole Once-ler family is working in a big factory. The illustration shows pairs of hands knitting many different colors of Thneeds, accompanied by the chopping-down sound of Truffula Trees. But as business grew, manual chopping becomes too slow. An illustration depicts a fancy red machine with blue wheels rolling in. The Once-ler's new Super-Axe-Hacker has the capacity chop down four trees at once.
Then, one day the following week the Lorax knocks on the Once-ler's office door. The illustration shows the rapid expansion of the Once-ler's production: his factory extends beyond where the eye can see in the landscape. Forlorn tree stumps dot the grass, and below the Lorax stand several pathetic-looking Brown Bar-ba-loots. Their eyes half-open, they clutch their stomachs and look as if they are about to faint. The Lorax informs the Once-ler that without enough Truffula Trees, the Brown Bar-ba-loots do not have enough Truffula fruits to eat and are now suffering from gas in their stomachs. The Lorax is forced to send the "boys" away to find food for their survival, even though they loved living here. The Once-ler feels momentarily downcast watching them go, but knows that business must go on regardless.
The next illustration shows the continued expansion of the Once-ler's business. The Thneed factory headquarters have four chimneys spewing black smoke into the sky, and purple vans with the words "You Need a Thneed" head off in multiple directions, presumably bringing Thneeds to the masses. Indeed, the Once-ler confides that he needs to get bigger, even though he wishes no harm on anyone. Soon, the Lorax is back, this time coughing and holding a bedraggled-looking Swomee-Swans. The Swomee-Swans, the Lorax informs the Once-ler, are now unable to sing because their throats are clogged with factory smoke. And so the Lorax sends the Swomee-Swans away, too—to where, he doesn't know. The illustration shows them flapping away in the sky with frowns on their faces.
But the Lorax doesn't stop there. He brings the Once-ler to look at the factory's waste goo, depicted as a toxic yellow and green and finally brown once it has been disposed of. The Once-ler disposes of this brown waste in a nearby pond, where the Humming Fish lived. They can no longer live there, because the goo is clogging their gills and they can't hum anymore. They are forced to leave the pond and waddle away on their short fins.
This is the last straw for the Once-ler. He yells at the Lorax, telling him that he has the right to keep going as he pleases, and he plans to get bigger and bigger. But suddenly, from far away, a sound can be heard. The accompanying illustration shows tree stump upon tree stump, extending across the hills that roll out in every direction from the Thneed factory. The sound is the "thwack" of the last Truffula tree being chopped down. Suddenly, the work at the factory is all over. With no trees left, the Once-ler's relatives pack up their things and drive away. The Once-ler is left alone in a big empty factory. Only the Lorax is left with him, until he lifts himself up the seat of his pants and hoists himself up and away into the sky, with a sad grimace on his face. The Lorax leaves only a pile of rocks with one word: 'unless.'
The Once-ler does not know what the Lorax means by 'unless.' He sits in that spot as time goes on, trying to figure it out. Only now does he understand. Only if someone like "you," the small boy depicted in the illustrations, cares enough to change the situation, nothing will get better. And with that, the Once-ler retracts the long cord of the telephone through which he has imparted his secrets.
The story concludes with the Once-ler throwing something down to the listener. It is a Truffula seed. According to the Once-ler, it is the last one. It is the listener's job to plant it and take care of it, give it fresh air and clean water, grow more, and protect the forest from axes. Only then will the Lorax and the rest of the animals come back.
Environmental catastrophe follows quickly on the heels of the Once-ler's drive for profit, establishing a clear moral critique of capitalism. Indeed, the consequences of this corporate expansion lead to an array of complicated ecological phenomena. For example, the ecosystem is highly interconnected: chopping down the Truffula Trees adversely affects the Brown Bar-ba-loots as their food supply of Truffula fruits diminishes. Without the trees, the creatures must leave, too. That interdependence is demonstrated by the suffering of other animals, as well, such as the Swomee-Swans and the Humming Fish, who are threatened by the smog and waste products from the Once-ler's factory. These cascading consequences are at the heart of Dr. Suess's conservationist message: disrupt an ecosystem for profit, and the whole landscape will change for the worse.
The Lorax's descriptions of the Once-ler's drive for expansion present a clear critique of capitalism, in addition to their environmental message. The Once-ler is portrayed as clearly and solely motivated by profit. He does not value a Thneed for anything other than its value to consumers. The business of selling Thneeds itself is portrayed as slightly ludicrous—who has ever heard of a Thneed?—and yet the Once-ler has clearly convinced the masses that they do, in fact, need a Thneed. And as the market for Thneeds grows, so does the Once-ler's factory. This phenomenon is, of course, that central tenet of market capitalism: supply and demand. But The Lorax seems to be asking, at what cost, and for what reason?
Unusual for the genre of children's literature in which Dr. Seuss writes, the book's dark tale is punctuated by only a slight sliver of hope. Much of the story is told in the past. The events of the present unfold in a dark, sinister landscape. This suggests to readers that the task of preserving the natural world will not be accomplished without their intervention. The status quo, according to The Lorax, is one of continued environmental devastation. Changing that status quo will require bold, decisive action—replanting an entire forest of Truffula Trees. In other words, according to The Lorax, environmental protection will not happen unless someone intervenes.