Introductions of organisms in general
Introductions of organisms into new environments can be intentional, accidental, or migratory and may encompass practically any kind of living material. Intentional introductions fall into two broad categories: crop plants and domestic animals, and agents for the biological control of pests or other beneficial purposes, such as microbes to degrade toxic compounds. Benefits from the introduction of crop plants and domestic animals are obvious, but good economic assessments of these introductions are rare. Rigorous evaluations have been done for some classical biological control programmes (see Cullen and Whitten, Chapter 26; Greathead, Chapter 5; and Pimentel, Chapter 2), and overall, a return to investment of 30:1 has been estimated. An example of outstanding economic success is the biological control of Rhodes grass scale in Texas, which cost only US$ 0.2 million, but from which annual economic benefit has been estimated at about US$ 200 million. Other benefits of introducing biocontrol agents include the avoidance of unwanted side-effects associated with chemical pesticides, important from environmental, occupational health and social points of view.
Introductions of living material are always associated with a risk of unwanted side-effects (see Pimentel, Chapter 2; and van Lenteren, Chapter 3). Thus, of all intentional introductions of species some 25–68% result in permanent establishment, and 0–2% of these organisms become pests. In contrast, only about 5% of unintentional introductions result in species establishment, but 7% of those become pests in their new environment. In total, 128 intentionally introduced crop and ornamental plants have become weeds in the USA (2.2% of all introductions of plants), including Johnson grass, Missisippi chick corn, goatsrue and crotalaria (crop plants); and multiflora rose, water hyacinth and lantana (ornamentals).