What are the main ways an alien species spreads?

Most of the alien terrestrial species have been brought to Estonia on purpose

Sosnowsky’s hogweed. By: Karl Adami
A TROUBLE OF OUR OWN MAKING. People used to spread Sosnowsky’s hogweed themselves. Now they are having trouble getting rid of the alien species. By: Karl Adami


Each species has its own potential entry routes, depending on the characteristics of the species and the habitat, but some modes of spreading are suitable for many species.


Alien water species spread mainly in the ballast water tanks of various watercraft (ships) or on the outer surface of the hull. About 80% of the world’s goods are transported by ships. Ballast water is taken into tanks in the port if the ship is not carrying cargo, and it is discharged in another port before the goods are loaded aboard. In the ballast water tanks, organisms can survive voyages lasting several months with ease.


Ballast water biota is abundant. At any given time, more than 4,500 species of organisms travel in the ballast water: there are benthic animals, microorganisms, as well as large fish that survive the voyage nicely and start spreading in the new port. More than 50,000 zooplankton specimens and 10 million phytoplankton cells can be found in a cubic metre of ballast water of one ship entering a port – one ship can carry billions of planktonic organisms to its destination [1].


The second most important migratory route for alien water species is man-made canals, but many alien species also enter the wildlife through aquaculture and the trade in aquarium and live fish (especially in Southern Europe and Asia).


Most terrestrial alien species have been introduced intentionally. Many species important for agriculture, forestry, or horticulture, which have since entered the wild, have been deliberately introduced. And about three quarters of all alien species found in Estonian nature are flowering plants. Many plant species have also entered the wild from botanical and other gardens. However, with difficult-to-control Internet trade, we may receive seeds of potentially very dangerous species – while the customs authorities can detect the seeds in larger quantities, very different species may enter Estonia in parcels and passengers’ pockets. As flowering plants are the most common alien species, any garden owner or farmer can pay attention to them – it is important to not grow invasive species and to safely dispose of garden waste.


Animals (for example, raccoon dogs, muskrats, American minks) have been deliberately released into the Estonian wild on several occasions. While minks escaped from fur farms, raccoon dogs were deliberately released into the wild to increase the diversity of local fauna, especially game. The deliberate release of alien species into the wild is now prohibited in Estonia, but they still accidentally end up there, such as exotic pets.


However, many species are also introduced unintentionally, for example with goods, uncleaned means of transport, or with humans themselves. Unwanted passengers may be found in special feed, cotton and wool, soil mixtures, and poorly cleaned seeds. Therefore, there are a lot of random alien species in railway stations, ports, and around wool factories. However, the proportion of unintentional introduction is much lower than intentional introduction. One of the important routes for the unintentional introduction of insects is the timber trade, where bark beetles and other timber-related species living in the bark and in the wood cross the border with the timber. Various insects and spiders have also arrived in Estonia with fruit [2].


The EU Invasive Alien Species regulation provides for an analysis of the risk of introduction, survival, and spread of alien species. An analysis of forty invasive alien species of concern to the European Union can be found here.




Last modified: 27.11.2021




[1] H. Ojaveer, L. Eek, J. Kotta. Vee võõrliikide käsiraamat. Tallinn, 2018.

[2] L. Eek, T. Kukk, T (koost.). Maismaa võõrliikide käsiraamat. Keskkonnaministeerium ja autorid. Tallinn, 2013.