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Naturalization and invasion of alien plants: concepts and definitions

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Much confusion exists in the English-language literature on plant invasions concerning the terms ‘naturalized’ and ‘invasive’ and their associated concepts. Several authors have used these terms in proposing schemes for conceptualizing the sequence of events from introduction to invasion, but often imprecisely, erroneously or in contradictory ways. This greatly complicates the formulation of robust generalizations in invasion ecology.

Based on an extensive and critical survey of the literature we defined a minimum set of key terms related to a graphic scheme which conceptualizes the naturalization/invasion process. Introduction means that the plant (or its propagule) has been transported by humans across a major geographical barrier. Naturalization starts when abiotic and biotic barriers to survival are surmounted and when various barriers to regular reproduction are overcome. Invasion further requires that introduced plants produce reproductive offspring in areas distant from sites of introduction (approximate scales: > 100 m over < 50 years for taxa spreading by seeds and other propagules; > 6 m/3 years for taxa spreading by roots, rhizomes, stolons or creeping stems). Taxa that can cope with the abiotic environment and biota in the general area may invade disturbed, seminatural communities. Invasion of successionally mature, undisturbed communities usually requires that the alien taxon overcomes a different category of barriers.

We propose that the term ‘invasive’ should be used without any inference to environmental or economic impact. Terms like ‘pests’ and ‘weeds’ are suitable labels for the 50–80% of invaders that have harmful effects. About 10% of invasive plants that change the character, condition, form, or nature of ecosystems over substantial areas may be termed ‘transformers’.
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Keywords: aliens; barriers; biological invasions; casual alien plants; invaders; naturalized species; pests; plant introduction; transformers; weeds

Document Type: Research Article

Affiliations: 1: Institute for Plant Conservation, Botany Department, University of Cape Town, Rondebosch 7701, South Africa. Email:, Email: [email protected] 2: Institute of Botany, Academy of Sciences of the Czech Republic, Cz-252 43 Průhonice, Czech Republic. Email:, Email: [email protected] 3: Section of Evolution and Ecology, University of California, Davis, CA 95616, USA. Email:, Email: [email protected] 4: Department of Environmental Horticulture, University of California, Davis, CA 95616, USA, Email:, Email: [email protected] 5: Alan Fletcher Research Station, Department of Natural Resources, PO Box 36, Sherwood, Queensland 4075, Australia, Email:, Email: [email protected] 6: Southland Conservancy, Department of Conservation, PO Box 743, Invercargill, New Zealand, Email:, Email: [email protected]

Publication date: 01 March 2000

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