The Once-ler represents people who first explore an area and found human settlements. They for the most part work with the surrounding land and animals. They will cut down some trees and usually use some animals for food, but they will leave most of the area untouched except the part they need to live. As the Once-ler is, they are often fascinated with and appreciative of the local plants, animals and trees, but often react in an exploitative way, to see how they can benefit. An example is when the Once-ler cuts down the Truffula tree to make a sweater out of its wool.
The Lorax represents environmentalists. He speaks for the trees, and tries to convince the Once-ler not to cut them down. The Once-ler is only interested in making profit though. The Lorax is cute but harmless until it becomes a nuisance, which incidentally is how most companies see environmentalists.
Eventually, the reasons the first settlers come to an area are no longer recognizable. Trees, rivers, wildlife and geographical features no longer exist as the inhabitants adapt the land to fit their profiteering needs.
People today can find resources to help them that are like The Lorax. We may not have cuddly Loraxes around to educate us, but we can educate ourselves before we buy by using the internet and information from environmental activist groups. There are resources. People need to take advantage of them!