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Introduction pathways

Man plays a central role in introductions pathways. Plants are introduced voluntarily or accidentaly. Pathways are mutliple for introducing alien plants : agriculture, sylviculture, horticulture, beekeeping, etc. All invasive plants imported as ornamentals are considered as deliberate introductions. So are plants deliberately planted for sylviculture.

In Europe, 70 % of alien plants have been introduced deliberately.

In several European countries (Germany, the United-Kingdom, The Czech Republic), studies have demonstrated the major part of introduced alien plants are ornamentals. Horticulture is therefore considered as one of the main pathways for plant invasions (see invasive plants and horticulture).

Voluntary introductions and accidental introductions

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Voluntary : the American black cherry (Prunus serotina), native from North America, is a forest tree first cultivated in Europe near Paris in 1623. The tree has been massively planted to improve soils and sylviculture productivity in forest located on poor and acidic soils. Millions of specimens were planted all over Europe. It has now become an aggressive inveder, costing millions of euros for control measures. Flanders (Northern Belgium) is heavily invaded by this species.

(Photo : Rasbak, GFDL)

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Accidental : the South african ragwort (Senecio inaequidens) has been introduced in Belgium (region of Verviers) at the end of the 19th century through wool industry. Wool containing seeds of S. inaequidens have been accidentaly imported to different wool industry centers in Europe, like Mazamet (France) or Verviers. After a slow naturalization in the surrounding of these areas, the species has spread progressively. It is now a widespread invader along railways, roadways and waste lands.

(Photo : P. Busselen)

Besides initial introduction, other factors are responsible for spreading invasive plants.These factors are direct use for plantations or sowings, soil transports containing fragments of invasive plants (seeds, stems or roots fragments, etc.), deposition of garden waste. They are sometimes called as secondary releases vectors. Secondary releases continue to disseminate species during decades or even centuries after the initial introduction, whereas established populations continue to spread naturally (see how do they spread). Today, established populations of some invasive plants mainly come from secondary releases. To reduce invasion risks in nature, attemps at prevention should focus on secondary releases as well as on initial introductions (see what can we do).

The Japanese knotweed and soil transport as a secondary release vector

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Soil transports containing rhizomes fragments of Japanese knotweed (Fallopia japonica) is considered as an important dispersal vector. Rhizomes (underground stems) of Japanese knotweed easily create new individuals. They form an underground network which spread several meters away around the plant. New populations frequently appear when soil is excavated near a knotweed population and deposited in another place (for embankment works for example) Rhizomes fragments contained in the soil will regrow and form a new population. This dispersal pathway often occur in urban areas or along roadways (Photo : G. Frisson).