Factors that Promote Invasiveness

As mentioned above, every alien specie does not become invasive. There may be some inherent features of species combined with some external factors, which promote invasiveness. Some of the factors that promote invasiveness are explained below:

Human-induced disturbance of habitat:
Removal of native vegetation for farming, forestry, mining, etc. provides open niche for alien invasive due to lack of competition and altered soil structure, availability of moisture (by irrigation), and nutrients (Randall & Marinelli 1996). Many species do best in urban-fringe environments (McNeely 1998). An alien specie may be present in some area since quite long, but becomes invasive only after human-induced habitat disturbance.

Frequency and magnitude of introduction:
Repeated introduction of some species on large scale increases its chances for becoming invasive.

The inherent ability of species for profuse proliferation and rapid growth:
Some plant species have the ability to vegetatively propagate and produce large number of seeds. These species when fed to animals increases the chances of its invasiveness covering wider geographic areas.

Lack of natural competitors and enemies in the new habitat:
The native species in general do not have means to compete with the aliens because they had not co-evolved with them. For example, many plant species produce such substances, which are toxic to other plant species (allelopathy) or other organisms. In their own natural range, the invasive species must have survived with those plants and animals which had developed means to compete with them keeping their population in check. But in a new habitat, the native plant species do not have means to compete with them, and at the same time, the native grazing animals or insect predators do not touch them due to their toxic metabolites. As a result these alien invasives generally form pure, continuous and thick populations.

Despite the above mentioned factors, it is never possible to predict with certainty which of the alien species would become invasive. Generally the alien species need time to acclimatize with local conditions, and in some cases it may take decades to centuries for an alien specie to become invasive.