Introduced species, those dispersed outside their natural ranges by humans, now cause almost all biological invasions, i.e., entry of organisms into habitats with negative effects on organisms already there. Knowing whether introduction tends to give organisms specific ecological advantages or disadvantages in their new habitats could help understand and control invasions. Even if no specific species traits are associated with introduction, introduced species might out-compete native ones just because the pool of introduced species is very large (“global competition hypothesis”). Especially in the case of intentional introduction, high initial propagule pressure might further increase the chance of establishment, and repeated introductions from different source populations might increase the fitness of introduced species through hybridization. Intentional introduction screens species for usefulness to humans and so might select for rapid growth and reproduction or carry species to suitable habitats, all which could promote invasiveness. However, trade offs between growth and tolerance might make introduced species vulnerable to extreme climatic events and cause some invasions to be transient (“reckless invader hypothesis”). Unintentional introduction may screen for species associated with human-disturbed habitats, and human disturbance of their new habitats may make these species more invasive. Introduction and natural long-distance dispersal both imply that species have neither undergone adaptation in their new habitats nor been adapted to by other species there. These two characteristics are the basis for many well-known hypotheses about invasion, including the “biotic resistance”, “enemy release”, “evolution of increased competitive ability” and “novel weapon” hypotheses, each of which has been shown to help explain some invasions. To the extent that biotic resistance depends upon local adaption by native species, altering selection pressures could reduce resistance and promote invasion (“local adaptation hypothesis”), and restoring natural regimes could reverse this effect.
