Have you ever heard the joke about the nasty green house-guest who never left? Actually, there is no such joke, because the story is true and no laughing matter. If trees in the national parks could talk, they would describe how many of the parks, which usually welcome human guests every day, are now being overrun by some most unwelcome visitors of another kind. Plant experts call these green pests invasive non-native, meaning aggressive, transplanted plant species that are not original to the particular area they now occupy. They may have come from as far away as Asia or as near as the next state. Such visitor plants, free of the natural disease and insect predators from their native turf, go wild and with their new freedom, behave terribly. They crowd the native plants out of their homes, eat up their food, usurp their water and sunlight, and frequently kill them to boot.
Eventually these unwelcome visitor plants may terrorize the entire neighborhood, even driving away the native animals that live there and further threatening plants and animals already listed by the government as endangered species. Such a complete takeover by one type of plant is called a monoculture--- a very unhealthy thing. Natural areas in the national parks- or anywhere, for that matter- need balance and diversity to thrive.
Now, not all non-native plants are invasive- some are downright friendly and helpful- wheat, for instance. First brought to America by English settlers in the early 17th century, this grain now thrives all over the globe. It has fed and employed billions of people over many centuries and has contributed to world's agricultural diversity. It is a good example of human's intelligent transplantation of noninvasive, non-native plants.
Unfortunately, human beings are also almost always responsible for introducing invasive non-native plants to unprotected areas. They may transport them totally unawares, as when weed seeds "hitchhike" on people's clothes, pets, or cars en route to a park. Worse, people who aren't familiar with nuisance species often bring them home from garden suppliers or even from trips to plant in their gardens. Then it's usually only a matter of time until those plants escape and spread to unprotected natural areas. Well-meaning folks who live near park property and think that transplanting garden cuttings or composting yard clippings there would be helpful, actually may do harm. Loose seeds and trimmings from such yard waste frequently take root, multiply, and overwhelm native plants.