Many have moved into natural landscapes, some with significant ecological or economic impacts. Increased trade in unusual pets introduces an increasing variety of species which may escape or be released by owners. The aquarium trade is the source of a large number of aquatic species accidentally or intentionally released into waterways. Aquaculture has also increased the spread of fish and other aquatic species and associated diseases. Huge metal containers used in shipping goods via boat, train, and truck provide entry for seeds and small animals to new locales.
Flooding can also transport nonnative aquatic and marsh species to new regions.
Human activities have improved the odds for many nonindigenous species. A rapidly increasing human population has led to greater land disturbance which favors the spread of invasive species. Increased demand for food and fiber and overuse of public lands for recreation and commercial purposes have also contributed to nonnative invasions. When ships unload their cargo, they often fill their ballast tanks with water to provide balance for their return journey. In addition to water, many aquatic organisms are sucked into these tanks and given transport.