Invasive species have been regarded as one of the most important yet subtle threats to biological diversity (Cronk and Fuller 1995; Humphries et al. 1991; Luken and Thieret 1997; Schmitz et al. 1997; Simberloff 1998; Vitousek et al. 1997). In fact, according to the Convention on Biological Diversity (CBD), invasive species are defined as those that result in considerable damage to ecosystem processes (Secretariat of the Convention on Biological Diversity 2005). It is frequently claimed that the impact of invasive species on biological diversity is the second largest threat next only to that due to habitat destruction and climate change, especially in the Islands (D’Antonio and Kark 2002; Vitousek et al. 1997; Walker and Steffen 1997). Free from their native habitats, invasive species are no longer reined in by ecological forces that may have otherwise kept their population in check. Consequently, through their path of invasion, invasive species compete, marginalize, and frequently usurp native biological diversity (Mooney and Hobbs 2000). In extreme cases, invasive species can also lead to the extinction of native species (Atkinson et al. 1995, 2000; Gurevitch and Padilla 2004; Jenkins et al. 1989; Van Riper et al. 1986; Warner 1968; Wikelski et al. 2004; Williamson 1996 and references there in; Work et al. 2000).