While there are examples of remarkable natural long-range dispersal of plants (Ridley, 1930), human-mediated trans-global movement of a wide range of organisms has resulted in an extraordinary dispersal of alien species. A very large number of species have been, and are being, accidentally or deliberately transported vast distances to new regions across the world, well beyond the limits of natural dispersal. Some of these have proved to be winners: others have been less successful.

There are a number of important reviews of the introduced species, including invasive species (Gibbs & Meischke, 1985; Drake et al., 1989; Williamson, 1996; Mooney & Hobbs, 2000; Sandlund, Schei & Viken, 2001; Sax, Stachowicz & Gains, 2006). These sources provide information on a wide range of issues, including the economic damage wrought by introductions, and provide detailed information on the attempts to control invasions. The control of introduced plant species by pesticides or biological means will be discussed in some detail in Chapter 15.

Here, our main focus is on plants and considers microevolutionary issues relating to success or failure of introduced taxa. However, reference will also be made to introduced animals, fungi and micro-organisms, as these influence the native plants in many ecosystems. The following questions are considered: by what means have plant species been introduced to new territories, and how many of these have become established in functioning populations? What factors are important in determining whether or not species are successful as introductions, and what factors determine whether introductions become invasive?