Weeds are one of the leading threats to conservation of native species.
People have always moved plants around the planet, for agriculture, horticulture, or by accident. In the last century, the rate of introductions has increased dramatically. That is, they are able to live on their own and spread rapidly, changing the character of natural areas by altering native communities and ecological processes. Some plants are very aggressive and displace native plant species- transforming diverse plant communties into single species stands with limited value as wildlife habitat or forage. Well known examples include spotted knapweed, leafy spurge, kudzu and salt cedar.