Invasive Species

Invasive species are characterized by their substantial population growth and spread outside their native ranges. Humans originally introduced most of today’s invasive species, and all regions on Earth have been affected by invasive species to some extent. Curiously, 90% or more of introduced species do not become invasive pests; rather, they die out or rely on direct human assistance to achieve limited persistence outside their native ranges (Williamson 1996). Of course, many introduced species benefi t humans by providing sources of food or fi ber, but those that become invasive are extremely costly to society (Ewel et al. 1999). One recent study estimated that invasive species are costing as much as $137 billion per year in losses in the United States alone (Pimentel et al. 2000). Furthermore, invasive species threaten native species with extinction (Wilcove et al. 1998) and alter crucial ecosystem functions such as freshwater fl ow in streams (vanWilgen et al. 1996). Although it is diffi cult to assign a monetary cost to these impacts, they have serious implications for humans because we rely on a diversity of species and ecosystem services for our survival.