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Definition

Some exotic plants spread in nature and increase their populations dramatically...

What is exactly an invasive plants ? Scientists have defined those species following precise criterias. Invasive plants are vegetal species which :

  • have been introduced by man (voluntarily or accidentaly) outside their natural distribution area (alien species)
  • have been introduced after the year 1500
  • are able to naturalize, meaning able to survive and reproduce in nature
  • present high dispersal capacities leading to an exponential increase of their populations.

Only species meeting all these definition criterias can be qualified as invasive

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       On the left : a canal invaded by the floating pennywort (Hydrocotyle ranunculoides). Photo : E. Delbart 
       On the right : alluvial forest invaded by the white meadowsweet (Spiraea alba). Photo : E. Branquart

Invasive plants is a serious issue (see impact), specially concerning environment. It is part of a recent scientific discipline called invasion biology. But the global process of introducing alien plants is not recent at all. Man used to transport species from all continents for a very long time. Today this process is amplified with intensification of organisms transports through globalisation and international trade exchanges. We all benefit from these introductions as alien plants are used in varied economic sector of our societies like agriculture, sylviculture, horticulture, medecine, etc. For example, potatoes, tomatoes or maize are alien plants very usefull for man. Most alien plants make no problem and are harmless for environment. Only a small proportion of introduced species are actually detrimental.

All alien plants do not become invasive, only a small proportion do.
For 1000 alien plants introduced, only one can become invasive.

Invasive plants grow in a wide array of habitats, both terrestrial or aquatic, in natural or urban areas. They are found in cities, forests, grasslands, wetlands and many others ecosystems. Maybe you have one in your own garden without knowing it ! They are called Japanese knotweed, giant hogweed, Himamayan balsam, for the most famous, but they are many others... (see list of invasive plants). Man is often responsible for their dissemination (see introduction pathways). Each year, those plants are moving forward more and more into nature.

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Invasive plants are moving forward !

The Himalayan balsam (Impatiens glandulifera) is an herbaceous plant highly invasive along rivers. In the United Kingdom, the expansion rate have been estimated at 38 km pea year. In the Czech Republic, I. glandulifera covers 56 % of the rivers length. Due to a constant rate of spread, the species is expected to cover all rivers of the country in 2025 !
Photo : S.Vanderhoeven