What are alien species and why are they dangerous?

The economic damage caused by alien species in the European Union is estimated at 12 billion euros a year


An alien species is a species that humans have intentionally (for ornamental purposes, for fur) or accidentally (with the bilge water of ships) removed from its normal range. The species would not have reached the new range on its own (due to changes of the climate or by currents).


The muskrat. By: Karl Adami
ALIEN SPECIES. The muskrat was brought to Estonia sometime between 1940 and 1950. By: Karl Adami


While in the case of plants, it has been agreed that species that were brought to Estonia by humans after the mid-eighteenth century are considered alien species in Estonia, for other species, that point in time is the end of the nineteenth century. The species brought to Estonia before that have adapted here (become naturalised) and the local species and communities have accepted their presence to a certain extent. In the case of certain species, it is difficult to say whether they are alien species, because the manner in which they arrived in Estonia is unknown.


Not all, but some alien species can become a great nuisance. In that case, the so-called rule of three tens applies, which illustrates the probability of the introduced species becoming invasive: about 10% of the introduced species survive in the new area, 10% of those become natural and might even begin to reproduce, and 10% of those can become problematic or invasive.


Invasiveness is the ability to endanger people, other species, or entire ecosystems by causing economic, health, or environmental damage. Native species can also be invasive, but invasive alien species for which the native species have not developed protective adaptive traits are particularly dangerous. Often, the time between the arrival of an alien species and the manifestation of the invasiveness is very long, sometimes even a hundred years. For example, the copper tops was first found in the Estonian wild in 1939, but signs of invasiveness began to appear only about a decade ago.


The danger of alien species lies in their entry into the local food chain and in competing with native species for food and habitat. Invasive species are usually very aggressive and crowd out the naturally occurring species. They may also be toxic or pathogenic (disease-causing) to local species (including humans) or may transmit pathogens and parasites.


Alien species threaten local biodiversity. For example, an alien species similar to the native species may hybridise with the latter, thereby altering and weakening the population of the native species. In addition to habitat loss and fragmentation, invasive alien species are one of the most important factors reducing biodiversity. It has been found that in almost 40% of cases of bird extinctions, which began in the seventeenth century, the cause has been alien species [1]. Cats, rats, and goats have caused the most extinctions of birds. The trend is similar in other species groups as well, and alien species are one of the biggest threats to the preservation of wildlife, with small carnivores, reptiles, cattle, and various plants also causing problems.


The International Union for Conservation of Nature and Natural Resources (IUCN) has gathered information on the 100 worst invasive alien species of the world – the list includes representatives from all biota groups. The alien species have the greatest impact on isolated communities, such as islands, where local biota is narrowly specialised and not accustomed to alien enemies. Common human synanthrope species, such as pigs, rats, and cats have dramatically reduced local biodiversity in, for example, New Zealand, Madagascar, and Hawaii [2]. There are no cases that dramatic in Estonia, but our native European mink is also extinct in our wilderness due to its alien relative, the American mink. Today, with great effort, a new population has been created in Hiiu County.


The most well-known invasive plants in Estonia are the large hogweed plants – Sosnowsky’s hogweed and giant hogweed. National control of these species began as early as in 2005 and their spread is now under control. The total area of the hogweed colonies has stabilised, and many of the colonies are weakened and on the verge of extinction due to constant control.


Among invertebrates, the most well-known invasive species is, for example, the aggressive Portuguese slug and among aquatic animals, the alien crustaceans (signal crayfish, marbled crayfish, spinycheek crayfish). Gobies (Amur sleeper and round goby) are undemanding, rapidly reproducing, and resilient fish that are already incredibly abundant in some places in our waters and who are probably impossible to get rid of [3]. Alien crustaceans have brought the crayfish plague to Europe, which is devastating local crustacean populations.


In addition to ecological threats, alien species can also cause significant economic damage. It is estimated that alien species cause economic damage in the European Union of 12 billion euros a year, which is more than, for example, Estonia’s 2020 state budget [4]. It is a sum of reduced production in, for example, forestry, agriculture, and aquaculture, direct control costs, but also medical costs caused by alien species that are dangerous to humans. In Canada, the economic damage from alien species in forestry alone is 20 billion dollars (the lion’s share of that is spent on controlling the beetle Agrilus planipennis), with an addition of 7 billion dollars in damage due to aquatic alien species, and 2.2 billion dollars in damage due to agricultural alien species [5].


A total of 1001 alien species have been registered in Estonia, but due to the lack of systematic monitoring and inventories of alien species, the actual number may even exceed two thousand. Alien species are divided into four hazard classes: invasive, potentially invasive, non-invasive, and undetermined. Among the alien species known in Estonia, 65 species are considered invasive and 89 potentially invasive; the invasiveness of most species is undetermined.


In Estonia, a list of alien species endangering our natural balance has been established by a regulation of the Minister of the Environment. The introduction of live specimens of the species included in that list to Estonia is prohibited.


In the European Union, the regulation on the prevention and management of the introduction and spread of invasive alien species has been in force since 2015. The regulation provides a confirmed list of alien species that can not be intentionally:


  • brought into the territory of the Union, including transit under customs supervision;
  • kept, including in contained holding;
  • bred, including in contained holding;
  • transported to, from, or within the Union, except for the transportation of species to facilities in the context of eradication;
  • placed on the market;
  • used or exchanged;
  • permitted to reproduce, grown, or cultivated, including in contained holding;
  • released into the environment

Alien species should be reported to the Ministry of the Environment at alien@envir.ee or added to the nature observation database.



Last modified: 23.03.2022



[1] M. Linnamägi. Võõrliik: kas edukuse musternäidis? Eesti Loodus. Märts (2019), lk 16–23.

[2] Bioloogilise mitmekesisuse konventsiooni veebileht: https://www.cbd.int/island/invasive.shtml

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

[4] M. Linnamägi. Võõrliik: kas edukuse musternäidis? Eesti Loodus. 3/2019, lk 16–23.

[5] https://www.invasivespeciescentre.ca/invasive-species/