Invasive alien species

By Thomas Evans

The aliens have landed. They’ve taken up residence in our waterways, our cities, our farmland and forests. Their negative impacts to both the environment and the economy are enormous, and they aren’t going home any time soon. So what are ‘alien species’? In short, they are animals, plants or other organisms that we have introduced to areas outside of their natural range. They are considered to be ‘invasive’ when they become established, disperse and have negative impacts on people and / or wildlife.

Free oyster cards

Pretty much every habitat across the globe has been invaded by alien species – and the inalienable fact is, we put them there. The world is getting smaller – with improvements to transport by air, land and sea, international travel and trade have increased dramatically. As a result, species that wouldn’t otherwise have the opportunity to travel (mussels don’t have feet – well actually they have a foot, but it’s not made for walking long distances) are hitching a ride across the globe.

Alien species have been introduced to new surroundings both intentionally and unwittingly. The unrelenting frogmarch of the cane toad along the eastern seaboard of Australia, much to the detriment of large predator populations (think quolls, snakes, lizards and goannas) is an example of a deliberate introduction with significant environmental consequences. The Shine Lab at Sydney University has been working to contain the toad invasion for many years.

A death adder killed by a cane toad. Source:

A death adder killed by a cane toad. Source:

The zebra mussel’s grip on the Great Lakes, where it clogs water supply infrastructure and outcompetes native species, reducing the food supply for important commercial fish species, is an example of an accidental introduction with disastrous environmental, recreational and economic impacts. In this case, the tenacious mollusc was transported to North America in the ballast water of ships embarking from ports in Western Europe. The Centre for Invasive Species Research (CISR) at the University of California, Riverside (UCR), recently estimated that costs to control zebra and quagga mussels in the Great Lakes amount to $500 million/year.

Ponto-Caspian catastrophe?

Zebra mussels smothering a current meter (Lake Michigan). Source: National Oceanic and Atmospheric Administration (NOAA).

Zebra mussels smothering a current meter (Lake Michigan). Source: National Oceanic and Atmospheric Administration (NOAA).

The UK has not escaped the impacts of invasive alien species. Earlier this month, the quagga mussel was found in the UK for the first time. Prior to this unwelcome discovery, the quagga mussel was identified by scientists at the Centre for Ecology and Hydrology (CEH) as the single greatest threat to British wildlife of all alien species. Equally alarming is this month’s revelation that the threat of invasion by the quagga mussel, along with several other species from the Ponto-Caspian region (that’s Southeast Europe to you and me) is now so real that parts of Great Britain face the grim prospect of invasional meltdown.

Evicting toad from Toad Hall

Last month, in recognition of the ever growing threat posed by potential invasions such as the quagga mussel, along with the lack of a coherent EU wide strategy to address invasive alien species, and the mounting costs associated with their containment and management (recently estimated to be €12 billion/year in Europe), the EU adopted a new regulation on invasive alien species. This legislation requires Member States to develop a list of species of Union concern, informed by risk assessments for individual species. It therefore encourages a cross-border approach to a cross-border problem – something that has been lacking to date. The regulation will be published in the Official Journal of the European Union in the coming weeks.

Instructions to boaters at Lake Mead, Nevada to prevent quagga mussel transportation. Source: JN Stuart. (CC BY-NC-ND 2.0).

Instructions to boaters at Lake Mead, Nevada to prevent quagga mussel transportation. Source: JN Stuart. (CC BY-NC-ND 2.0).

A number of recent research initiatives within the field of invasion biology aim to compliment and inform the above process. A protocol for scoring the impacts of invasive species has recently been developed and refined, enabling the severity of a suite of environmental, economic and social impacts associated with a species to be calculated, informing risk assessments for potential invasions. Studies have shown that this methodology can be effectively applied to different taxonomic groups (see also Kumschick et al. In press), and can also be used to inform studies that aim to predict the impacts of a species prior to invasion

This scoring methodology is also integral to a proposal to develop a global list of alien species classified by their environmental impacts that aligns with the existing IUCN Red List of Threatened Species (which is celebrating its 50th birthday in 2014, having become a global reference tool for biodiversity). The development of a standardised approach to quantify and compare the impacts of alien species will help us to predict whether potential invaders are likely to have negative impacts in their new range. This is important, as it allows for early intervention to prevent the establishment of alien species, and enables us to efficiently allocate money and resources where they are most needed to prevent or contain an emerging invasion.


  • Gallardo B, Aldridge DC (2014) Is Great Britain heading for a Ponto–Caspian invasional meltdown? Journal of Applied Ecology. doi: 10.1111/1365-2664.12348.
  • Nentwig W, Kühnel E, Bacher S (2010) A generic impact-scoring system applied to alien mammals in Europe. Conservation Biology 24: 302–311. doi: 10.1111/j.1523-1739.2009.01289.x
  • Kumschick S, Nentwig W (2010) Some alien birds have as severe an impact as the most effectual alien mammals in Europe. Biological Conservation 143 (2010) 2757–2762.
  • Kumschick S, Bacher S, Blackburn TM (2013) What determines the impact of alien birds and mammals in Europe? Biological Invasions 15: 785–797.
  • Kumschick S et al. (In press) Comparing impacts of invasive plants and animals using a standard scoring system.
  • Evans T et al. (2014) Comparing determinants of alien bird impacts across two continents: implications for risk assessment and management. Ecology and Evolution 2014; 4(14): 2957–2967.
  • Blackburn T et al. (2014). A unified classification of alien species based on the magnitude of their environmental impacts. PLoS Biology 12(5): e1001850. doi:10.1371/journal.pbio.1001850.
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