Invasive species means an alien species which becomes established outside its natural ecosystems or habitats, is an agent of change, and threatens native biological diversity (IUCN). Once an invasive species becomes firmly established, its control can often be difficult and eradication is usually impossible (Primental et al., 2000). It can have some serious consequences on ecological, economic and social systems (Primental et al., 2000). Though the answer of the question ‘what makes a species invasive?’ is still unclear (Baker and Stebbins 1965, Kornberg and Williamson 1991, Lodge 1993), short juvenile period, short interval between large seed crops and larger number of seeds produce- these three variables seems to discriminate invasive species from others (Rejmanek M.
, Richardson M. D., 1996).
Also, resource competition has always been considered as a major mechanism for invasive plant success (Tilman, 1997; Levine et al., 2003). Phenological differences resulting in competition avoidance, niche and fitness differences, phylogenetic relatedness, recruitment limitation, indirect competition, or allelopathy have been key factors for resource competition (Gioria M., Osbome A. B.,2014). Many invasive species possess higher values of competitively advantageous traits than native and non-invasive species.
These include a superior capacity to acquire and retain resources and/or to advantageously exploit resources better than co-occurring native species (Huenneke et al., 1990; Burke and Grime, 1996; Rejmánek, 1996; Callaway and Aschehoug, 2000; Daehler, 2003). This increased competitive ability often results from rapid hybridization or genetic drift from founder populations, in the absence of herbivores or pathogens that would control them in their native environments (Richardson M. D. et al.,2006).
A naturally aggressive plant can be especially invasive when it is introduced to a new habitat. It can also have a huge effect on other native species and biodiversity.