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The invasive species issue


To better understand the concern about invasive species, it is first necessary to know what makes an invasive species successful, whether they are terrestrial plants, birds, marine species, or microorganisms.

From the moment a species is moved out of its native range by human action, this species is called an alien species. If this species establishes itself in a new ecosystem and is able to create a viable population capable of expanding to other locations or causing profound changes in the ecosystem it colonised, then it becomes an invasive species. For more details on these definitions see the glossary.

The shrimp "Palaemon macrodactylus", an invasive species present in the Guadiana estuary.

An invasive species generally has the following characteristics (e.g. Holle and Simberloff 2005; Colautti et al. 2006; Weis 2010; Havel et al. 2015):

1. High capacity to adapt and resist fluctuating environmental conditions (e.g. temperature, salinity, humidity, sun exposure).

Many species are constantly being transported by human actions, inadvertently or on purpose, to places far from their native range. However, the environmental conditions they encounter in non-native areas may be different from those where they evolved, so they simply do not survive. However, if the environmental conditions are favourable and if the species is simultaneously adapted to environmental fluctuations then the likelihood of it adapting to a new ecosystem and thriving increases.

2. High fertility, low age at first maturity, rapid growth.

The problem with invasive species is mostly associated with the abundance they reach. Assuming that the species has successfully adapted to the environmental conditions encountered, its ability to reproduce and grow dictates its invasive capacity. A high fertility means that each female has the capacity to produce a large amount of offspring in each reproductive event. Additionally, if both sexes reach the age of first maturity in a short period of time, for example at the end of the first year of life, the reproductive and expansion capacity of the species is very high. Thus, these three factors (fertility, maturation, growth) are often the key to the establishment and expansion of many invasive species.

3. High capacity for dispersal and mobility.

The expanding capacity of an invasive fish will be different from invasive plants. However, dispersal phenomena are usually very effective during the early stage of development, for example with the dispersal of seeds by wind or animals, or the transport of larvae of marine organisms through ocean currents.

4. Absence of predators and/or parasites.

When an exotic species is introduced, in most cases it will not encounter its native enemies, such as predators or parasites. This fact confers a competitive advantage over native species with identical ecological characteristics, which can allow the colonisation and expansion of exotic species.

5. Opportunistic and general feeding habits.

Opportunistic in the sense that they thrive in environments already impacted by human activities, being able to better exploit the available resources (e.g., food, space) when compared to native species.


1. Ecological impacts.

Invasive species have a great capacity to adapt and can directly affect native species. If native species are already under any kind of pressure, then the introduction of a competing invasive species can dictate their decline due to competition for food resources and space.

For example, the introduction of the crayfish (Procambarus clarkii) into several freshwater ecosystems in Europe has altered the bottom of rivers and lakes through the construction of holes in the sediment and the consumption of algae on the bottom. These changes have caused, for example, the disappearance of invertebrates and amphibians that lived in association with these algae, thus causing a cascading effect along the trophic web (Souty-Grosset et al. 2016).

Crayfish (Procambarus clarkii) (photo:

2. Economic impacts.

These impacts are mainly related to invasive species that have a strong impact on native species of high commercial value, that alter the functioning of an entire ecosystem, or that cause damage to high-value infrastructure, thus requiring significant financial investment to eliminate them and restore such infrastructure to operation.

For example, the uncontrolled growth of the zebra mussel (Dreissena polymorpha) has caused major economic damage. These impacts derive from the fact that this species colonises and clogs water pipes, including nuclear and power plants, refineries, fish farms, water supply facilities and ships (Petsch et al. 2021). Cleaning and/or replacing this type of equipment involves high costs. On a global scale, the impacts of biological invasions have been estimated at $13.5 billion for the 10 worst invasive species (Cuthbert et al. 2021).

Zebra mussel (Dreissena polymorpha) (photo:

3. Impacts on public health.

The appearance of invasive species that may cause health problems has been increasing in Europe. We highlight the Asian wasp (Vespa velutina), highly stinging marine jellyfish species (e.g. Rhopilema nomadica), or mosquito species of the genus Aedes spp. capable of carrying tropical diseases (Schindler et al. 2015).

The jellyfish "Odessia maeotica", an invasive species present in the Algarve.


Colautti, R. I., Grigorovich, I. A., & MacIsaac, H. J. (2006). Propagule pressure: a null model for biological invasions. Biological invasions 8(5): 1023-1037.

Cuthbert, R. N., Diagne, C., Haubrock, P. J., Turbelin, A. J., & Courchamp, F. (2021). Are the “100 of the world’s worst” invasive species also the costliest? Biological Invasions, 1-10.

Havel, J. E., Kovalenko, K. E., Thomaz, S. M., Amalfitano, S., & Kats, L. B. (2015). Aquatic invasive species: challenges for the future. Hydrobiologia 750(1): 147-170.

Holle, B. V., & Simberloff, D. (2005). Ecological resistance to biological invasion overwhelmed by propagule pressure. Ecology 86(12): 3212-3218.

Petsch, D. K., Ribas, L. G. D. S., Mantovano, T., Pulzatto, M. M., Alves, A. T., Pinha, G. D., & Thomaz, S. M. (2021). Invasive potential of golden and zebra mussels in present and future climatic scenarios in the new world. Hydrobiologia, 848(9), 2319-2330.

Schindler, S., Staska, B., Adam, M., Rabitsch, W., & Essl, F. (2015). Alien species and public health impacts in Europe: a literature review. NeoBiota 27: 1-3.

Souty-Grosset, C., Anastacio, P. M., Aquiloni, L., Banha, F., Choquer, J., Chucholl, C., & Tricarico, E. (2016). The red swamp crayfish Procambarus clarkii in Europe: impacts on aquatic ecosystems and human well-being. Limnologica, 58, 78-93.

Weis, J. S. (2010). The role of behavior in the success of invasive crustaceans. Marine and Freshwater Behaviour and Physiology 43(2): 83-98.


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