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
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