The spread of alien species to new geographical areas is intentionally or unintentionally facilitated by human activity. Globalisation, rapidly increasing international trade and transport and, in particular, new transport connections by sea, land and air have led to the increased spread of alien species.
Intentional import and distribution
Alien species have been and continue to be imported for commercial purposes. These intentional imports involve crops for cultivation and ornamentation, fish for aquaculture, and game for hunting and farming. Fish, reptiles, birds, mammals and invertebrates are imported as pets. Imports make it possible for alien species to spread further; an alien species may be intentionally or unintentionally released into the wild or, in the case of an animal, an individual may escape.
The trade in animals and garden plants is considered the most important individual factor in both intentional (about 70%) and unintentional (about 30%) introductions of alien species. Also, online commerce has made it easy to order products such as plant seeds or live food for aquarium fish from anywhere in the world. This too is conducive to the uncontrolled spread of alien species.
Unintentional migration of alien species caused by humans
The migration of alien species from one area to another on means of transport, or in raw materials and equipment has become more common in recent decades. Alien species may migrate with untreated timber or wood packages, for example. Alien species may also migrate on means of transport, for instance in the ballast water of ships or attached to their hulls. Unintentional imports mostly involve seeds, eggs (of invertebrates), diseases, parasites or species migrating with other species (epibionts). In recent years, the level of unintentional imports of alien species has risen compared with intentional releasing into the natural environment.
Canals, tunnels and other built routes as pathways
Species may also migrate through pathways built by humans: roads, railways, canals, bridges, tunnels or fish passes. Rapid and easy transport between and within continents also contributes to the spreading of alien species.
Several alien species have migrated from the Black Sea and Caspian Sea through rivers and canals to Finland’s coastal waters and to the almost-freshwater eastern end of the Gulf of Finland, including the mouth of the Neva River. The zebra mussel (Dreissena polymorpha) is both the best-known and the most harmful of the invasive alien species to have entered Europe via canals. It is thought to have entered Finland’s waters attached to floated logs. The significant growth of shipping on Russia’s inland waterways will probably contribute markedly to the spread of alien species. It is thought that climate change will increase the spread of animal species from the south towards the north through rivers and canals.