Invasive species are among the greatest threats to forested ecosystems globally (Liebhold et al.1995; Vitousek et al. 1996; Pimental et al. 2000), ranking behind only deforestation and land use conversion (Walker and Steffen 1997; Wilcove et al. 1998). Shifting patterns of trade, globalization of economies (e.g., Hulme et al. 2009; Meyerson et al. 2007), and climate change have ensured that even the most remote and pristine forests are not immune to this threat.
Recognition of the threat to forests posed by invasive species is universal among biologists and forestry professionals, yet despite the frequency with which the term invasive species is used, operational definitions vary widely among disciplines. Indeed, use of this term is so imprecise that some have advocated for its elimination from ecological literature (Colautti and MacIssac 2004). Here, the term is retained for continuity and defined as a non-indigenous species whose introduction was directly or indirectly facilitated by anthropogenic forces and causes or is likely to cause significant ecological or economic harm in natural and managed ecosystems. This definition separates a select group of species from a much larger pool of introduced and naturalized species that are relatively benign ecologically and economically, and also from those whose geographic distribution has changed in response to natural phenomena.