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The definition of invasive species or plants varies according to author. Some of the common definitions are:

An invader plant is any indigenous or exotic plant species having a detrimental effect on the growth of commercial tree species, giving rise to particular management problems or growing where it is not wanted (Le Roux, 1981).

Invasive plants are naturalized plants that produce reproductive offspring, often in very large numbers, at considerable distances from parent plants (approximate scales being more than 100 m in less than 50 years for taxa spreading by seeds and other propagules; more than 6 m/3 years for taxa spreading by roots, rhizomes, stolons or creeping stems) and thus have the potential to spread over considerable areas (Richardson et al., 2000a).

Invasive tree species are species that are able to survive, reproduce and spread, unaided, and sometimes at alarming rates, across the landscape (van Wilgen et al., 2001).

Invasive species means an alien species which becomes established in natural or semi-natural ecosystems or habitats, is an agent of change, and threatens native biological diversity (IUCN, 1999).

Invasive alien species are species introduced deliberately or unintentionally outside their natural habitats where they have the ability to establish themselves, invade, out-compete natives and take over the new environments (CBD News, 2001).

‘Transformer species’ are a subset of invasive plants which are species that change the character, condition, form or nature of ecosystems over substantial areas relative to the extent of that ecosystem (Richardson et al., 2000a).

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