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Invasive plants is one of the major ecological problem of the 21th century. It is a very important matter for preserving our biodiversity

Ecological impact

Once established in nature, invasive plants form dense and monospecific populations which outcompete native vegetation and recude species diversity, altering the botanical composition of natural habitats. This affects the ecological network depending on vegetation structure and diversity.

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On the left : a forest invaded by the black cherry (Prunus serotina) in Belgium. Photo : S. Vanderhoeven
On the right : another woody invasive plant in forest : rhododendron (Rhododendron ponticum). Photo : Y. Barbier

Invasive plants deeply modify ecosystems functionning by changing soil properties, for example by altering litter composition or minerals cycles. Other species are able to produce allelopathic substances which inhibit the growth of other plants. By this way invasive species strongly disturb the balance between plants and soil. Some invasive aquatic plant like the water primrose (Ludwigia grandiflora) can cover the whole surface of water bodies, causing asphyxiation and the death of aquatic life.

On the left : a water body invaded by the water primrose (Ludwigia grandiflora) in France. Photo : A. Dutartre
On the right : an aerial photograph of another water body invaded by the same species. Photo : A. Dutartre

There are plenty of others ecological impact like the risk of crossing (hybridization) with native species (impact on populations genetic), effects on pollinators and other animal communities (soil fauna, insects, birds, etc.). Due to the amplitude and multiplicity of impacts, invasive plants are considered as one of the main cause for the loss of biodiversity throughout the world.

Economical impact

In Flanders, managing one single invasive aquatic plant, the floating pennywort (Hydrocotyle ranunculoides), costs annually 1.5 million euros.

Economic impacts due to plant invasions are considerable. Invasive plants are very difficult to control when widespread in nature. There are two distinc economic loss. First, there are direct costs for controlling the spread of established populations. It requires important workforce and specific materials during several years. Direct costs also include damage to infrastructures (degradation of human constructions).

On the left : Management of the parrotfeather (Myriophyllum aquaticum) in Flanders (Northern Belgium). Photo: Natuurpunt
On the right : Management of rhododendron (Rhododendron ponticum) in the Bristish Islands. Photo: RPSGroupPlc  

Second there are indirect costs resulting of the loss of ecosystem functions once invaded. For example, invaded areas in forests are less productive for sylviculture (wood production). Invaded riverbanks or water surfaces loose a part of their esthetic or functional value, preventing people from fishing or navigating.

In the United-States, controlling invasive plants amount to approximatelly 35 billion dollars annually !

Public health impacts

Some invasive plants also cause public health problems. The sap of the famous giant hogweed (Heracleum mantegazzianum) may cause severe burns. The pollen of the common ragweed (Ambrosia artemisiifolia) is highly allergenic, causing hay fever. In Francen 10 % of the population are sensitive. This species is not widespread in Belgium.

The giant hogweed (photo : E. Delbart)

The  common ragweed (photo : Daglial)