Since Man's earliest dominion of the Earth, he has depended on the land and its plants for his survival, and he has often altered the forest and field to enhance his success. He has removed some species, planted others, and encouraged growth of still others, both intentionally and inadvertently, to the extent that few areas of the Earth still remain covered by large expanses of totally undisturbed plant communities.
Among the inadvertent changes that have been wrought by Man in natural systems are those resulting from the input of the pollutants of technology: sulphur dioxide, herbicides, photochemical pollutants, pesticides, radiation, and much more. The presence of each provides a selective force favouring some genotypes, discouraging others, and eliminating species that lack sufficient genetic diversity to survive. Thus the occurrence and distribution of plants is influenced, and community composition is changed—usually for the worse. Where large numbers of species are killed and the land is laid bare to erosion, the impact is highly undesirable. In other instances, tolerant genotypes existed of a sufficiency of species to allow them to reproduce and fill vacated niches. In the occasional instance where only a few plant species are damaged, the character of the change will depend on the nature of the tolerant individuals and species. A shift from forest to more tolerant grasses and forbs is common.