Invasive species pose a significant challenge to the National Wildlife Refuge System and have been identified as the single most important threat to habitat management on refuges. At present, it is estimated that over 2 million acres of refuge lands are invaded by invasive plants. The current and potential costs of controlling invasive plants, as well as monitoring and restoring refuge lands, are significant both financially and ecologically. Budgetary expenditures for invasive species projects in FY 2009 totaled $18.4 million. A number of strategies are used to confront this threat and have resulted in success on a variety of levels. The Refuge System utilizes key partnerships, invasive species strike teams, and a dedicated cadre of volunteers to implement projects that incorporate mechanical, chemical and biological control methods.