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Author: Patricia Ash

Alien invasion

Updated Thursday, 1st April 2010

Even plants and animals are not safe from alien invaders, so how can native species compete for survival?

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Since the 1960s, the threats from non-native or ‘alien’ species to native species have been increasing as more aliens reach the Americas, Asia, Australasia and Europe. Alien species compete with native species for resources.

Japanese knotweed spreads rapidly by vegetative reproduction and can quickly cover an area with dense shrubby growth, shading out other plants and depleting food resources for native species.

Japanese Knotweed

Japanese knotweed

Himalayan balsam (Impatiens glandulifera) and small balsam (Impatiens parviflora) are both annual herbaceous plants but they too can blanket out native herbaceous species. Himalayan balsam plants can grow up to two or more metres in height and shoot their seeds out of the exposed pods over a distance of several metres. The seeds also spread via running stream and river water.

Himalayan Balsam

Himalayan balsam

The plants shade out the native species and the rapid spread of this species along the banks of rivers and streams has resulted in a widespread abundance of the plant. Although the flowers are visited by bees and bumblebees, the alien plant reduces the diversity of herbaceous cover, which reduces the food supplies of insect species. This has impacts on the food available to amphibians, small mammals and birds. Himalayan balsam is now subject to eradication schemes.

As well as alien plants, there are also alien animal species. A recent invader is the harlequin lady bird (Harmonia axyridis). This species was introduced to the USA from Asia as an effective predator of aphids. It spread rapidly throughout the USA and then through China and Europe, and in 2004 it reached the UK.

A harlequin ladybird

A harlequin ladybird

In addition to eating aphids, harlequins also feed on other insects, including native ladybird species, their eggs and juvenile stages. Harlequins also out-compete the smaller native ladybirds by eating their food sources – aphids. This causes a concern that some native species in mainland Europe and the UK may become extinct.

How you can help

Volunteers are involved in reporting and eliminating invasive alien plants.


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