Invasive species have been introduced through many avenues. Intentional introductions have been made for agricultural crops, landscaping, ornamental plants, and for the pet trade. Once introduced, a few species have escaped cultivation or domestication and have established viable populations in the wild. Some of these populations not only survive, but thrive and increase.
Accidental introductions include contaminated freight such as plants, wood, or food that contain propagules of invasive species, such as seeds, eggs, spores, or other biological material capable of propagation. Invasive species can come in as contaminants or hitchhikers in shipping crates or packaging made of contaminated wood products, including shipping pallets, bracing, and other dunnage. The packaging material used to cushion products being shipped can be contaminated with invasive species.
A few species make it to new regions on their own without help from people.