Invasive species present major challenges to the sustainability of biologically based economic systems, including apiculture, forestry, agriculture, horticulture and aquaculture, and the preservation of valued plants and animals (Liebhold et al., 1995; Williamson, 1996; Mack et al., 2000, 2002; Simberloff et al., 2005). In the USA, Executive Order 13112 defines an invasive species as any organism that is not native to a particular ecosystem and is causing or is likely to cause economic or environmental harm, or harm to human health (The White House, 1999). Invasive species cause these harms by directly consuming or infecting resident species, out competing resident species for resources, hybridizing with resident species, altering disturbance regimes, vectoring native or exotic pathogens and/or creating refuges for other pests (Mack et al., 2000). In production-oriented systems, infection, predation and competition are often the primary concerns (Pimentel et al., 2000).
The White House's definition of an invasive species is useful but assumes that the historic composition of ecosystems is well known and that harm can be objectively described. Plants, insects, vertebrates and certain fungi that had not been reported in the USA can be classified as non-native with some confidence. However, the nativity of microflora and microfauna can be more difficult to determine (Kurdyla et al., 1995). Additionally, a species may be exotic to an ecosystem but still native to that country. Why does the designation of a species as a native or exotic matter?