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Invasive species are having severe ecological (Mack et al. 2000) and economic (Pimentel et al. 2005) impacts on ecosystems around the world. Invasive species can alter many ecosystem processes (Crooks 2002, Walker & Smith 1997) including: water and nutrient availability, such as form and amount of N if the soil (Evans et al. 2001, Sperry et al. 2006); primary productivity, through shifts in growth rates or efficiency of resource use; disturbance regimes, including the type, frequency, and severity of disturbances such as fire (D’Antonio 2002); and community dynamics, such as species replacements (Alvarez & Cushman 2002). The economic losses and damages by invasive plants are estimated to be ~$34 billion in the US and ~$95 billion worldwide (Pimental et al. 2005). Although trade and human migrations are among the most important vectors for introducing invasive plants (Mack et al. 2000), similar consensus on the causal mechanism for invasiveness is lacking (Dietz & Edwards 2006). Many different hypotheses have been proposed to explain why species are invasive. Some hypotheses, such as the vacant niche hypothesis, are conceptually appealing but lack concrete evidence to support them (Mack et al. 2000). Others, such as the allelopathy hypothesis (Callaway & Aschehoug 2000, Bais et al. 2003), have strong evidence to support them for some specific cases, but are unlikely to be important for most plants. Understanding why a species is invasive is important because it provides insight into how to control the invasion. Because a causal mechanism that is universally applicable to all plants has not been identified to date, careful attention must be made to biological and ecological characteristics of the plants and communities of interest if control strategies are to be implemented.


CREES Agreement No. 2001-52103-11322