What's the Problem with Invasive Plants?
Invasive plants displace native plants by outcompeting them for water, light, nutrients, and space. As native plants are lost, native animals who rely on those plants for food and shelter are threatened. Invasive plants can also negatively affect ecosystems by changing fire intensity and frequency, soil moisture regimes, and by producing thickets of plants so dense that the stands are impassible for large animals. Yosemite's invasive plant management focuses on many non-natives, including: Himalayan blackberry, yellow star-thistle, bull thistle, common velvet grass, cheat grass, French broom, Italian thistle, perennial pepperweed, and spotted knapweed.
http://www.nps.gov/yose/naturescience/invasive-plants.htm