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The Threat Posed by Alien Invasives

What Is An Alien?

Aliens aren't the stuff of science fiction, but science fact.  An "alien" is any species that is found out of its historically normal range.  In the vast majority of cases, the species will not survive because it is not adapted to its new locale (it is estimated that only 10% of introductions survive).  For example, its new habitat may have slightly different temperatures, seasonal variation, humidity, wind, predators, pests or diseases.  However, in a minority of cases, a plant will be able to survive in its new location.

Even more rarely (perhaps in 10% of cases), an alien will thrive in its new location better than in its original location.  This is often because of the absence of a predator or pest that would normally keep the species in check, and because of a biological trait that predisposes it to fast colonisation, such as a fast growth rate and the production of many seeds.  When this happens the species can spread quickly to become very common and dominant in its new habitat, and is known as "invasive".

For example, Rhododendron ponticum is an ornamental flowering shrub that has invaded the UK acid woods and heathland from the Iberian peninsula.  It outshades native plants and is causing problems in many important oak and hazel woodlands on the West coast of Scotland.

The Ecological and Economic Problems Caused

Invasive species will quickly increase the range and density of their populations.  This can have several adverse for native species.


Clidemia hirta, an invasive of many
tropical countries, where it
outcompetes other native plants

They will outcompete native species, causing these species range and population's to decline.  This can be a significant problem for species who are in direct competition with this species, already rare or confined to a small range.  For example, Parrot's Feather Myriophyllum aquaticum is a native of Central America that has now colonised over 100 sites in Southern England, mostly in shallow ponds.  It currently threatens the native Brown Gallingale Cyperus fuscus.

Aliens can also be a problem for species attacked or consumed by the invader.  Some invaders can alter the abiotic (physical environment) - for example, increasing the frequency of fires, whilst others may hybridise with native species, eroding the native gene pool and producing plants with novel genetic composition.  For example, one of the threats to the globally threatened bluebell Hyacinthoides non-scripta is hybridisation with the non-native Spanish bluebell Hyacinthoides hispanica, to form Hyacinthoides hispanica x non-scripta.

The effects of invasives can be fast, widespread and unpredictable, so it is important that the introduction of alien species is minimised as far as possible. 

The careless introduction of alien invasives is causing problems for us all.  For example, it has been estimated that in the US alone, the problems caused by its 50,000 alien invader species cost it over $120 billion dollars per year (US Department of the Interior, 1999).

Humans Encourage Invasion 

The actions of humans have moved many plants across the world - many more, and much further, than would otherwise occur.  This, combined with the interaction with other human-caused threats, means that the problems caused by invasives are much more serious and widespread than previously.

Some of these introductions have been deliberate, as many plants have been imported to be grown as ornamentals (such as the Rhododendron).  Other introducations have not been planned.  For example, the Zebra Mussel Dreissena polymorpha have been introduced to North American waterways by clinging to the hulls of ships.  They now clog up the water inlets of power stations and factories, and their removal costs $2 billion per year.

Interaction with Other Threats

The problems caused by invasives can interact with other threats to cause catastrophic declines in biodiversity and ecosystem problems.  Climate change, species' over-exploitation, habitat loss and pollution can all contribute to the decline and possible extinction of species.  For example, disturbed and degraded habitats are less resilient to invasive species, whilst climate change can mean local conditions change to become more suitable for the alien species.

Invasives are an especial problem for island ecosystems.  Islands tend to be especially vulnerable to invasion not only because their populations are small and isolated but because they may have evolved in the absence of normal predation rates and diseases, whilst the introduced species have competitive abilities honed in a larger species pool.  Once introduced invasives are extremely difficult to remove, so it is especially important that humans minimise the risk of invasives entering islands.