Some possible explanations are that the alien species may be:

• Vigorously growing and compete with the natural plants for minerals, water etc. The less vigorous local species may be eliminated.
• Natural pests and predators of the alien species may not be present in the introduced area-leading to proliferation in their number.
• The introduced species may harm the local species by production of chemicals (Amensalism).
• The alien species by proliferation may make conditions unfavourable for the growth of local native plants. (e.g., Eichhomia).