The arrival in a given territory, populated so far by so-called “indigenous”* species of another “outsider” species is a natural phenomenon linked living beings’ ability to disseminate and colonize. However, this type of event is even rarer when the distance between country of origin and country of arrival is great and the obstacles to getting there are also huge. But Man’s intervention is slowly going to change the rules: the number of introductions of “exotic” plants has grown at an exponential rate during the course of the history of Man.
Some significant stages deserve to be mentioned:
- Sedentarization linked to the development of agriculture, approximately 10,000 years ago, resulted in the geographical dispersion of cultivated plants and their accompanying weeds. In fact, it was no longer Man who moved to follow available resources, but the resource itself (cultivated plants) that was exchanged between one human group and another.
- The improvement in sea transport from the 15th Century onwards enabled Europeans to discover America, to make the first round-the-world voyages and therefore explore other continents in order to exploit the resources and farm the land. Many plants thus began to be imported from one continent to another.
-The method of acclimatization in the second half of the 19th century accelerated the introduction of plant species for utilitarian then ornamental purposes. France played a special role in this movement with the creation of the first Acclimatization Society by Geoffroy Saint-Hilaire during the Second Empire. While a certain number of current botanical gardens, particularly in the inter-tropical regions, were originally the acclimatization gardens of former colonies, all the botanical gardens have contributed more or less, to testing exotic plants’ ability to adapt to their climate.
- Finally, the globalization of trade that has developed over the last fifty years has more than doubled the pace of introduction.
How many exotic plants have been or are grown today for agricultural, horticultural or forestry purposes? Some “escape” from the field, garden or forest plot and gradually find their way into uncultivated areas.
Unauthorized dumping of garden waste or the emptying of aquariums in the open countryside may also be responsible for the introduction of foreign species into our natural environment.
The building of infrastructures almost always requires re-planting after work on large areas of land. Depending on the plants used, this re-planting may also be a significant way in which exogenous plants are introduced into nature.
As well as these deliberate introductions, there are many plant species imported in an inadvertent way through international goods transport: vehicles (planes, boats, lorries, trains), containers and goods themselves (minerals, earth, seeds, fodder, wool, etc.) are all excellent diaspora* vehicles (seeds, cuttings*, etc.). These “new arrivals” are spotted first of all by botanists in the immediate vicinity of ports and stations; some of them then end up by becoming naturalized.
Less well-known is the dissemination of plants by armed conflict: these almost always generate significant movement of troops, with animals and equipment, from one country or continent to another, leaving their selection of foreign plants in their wake, known as polemochorous* or obsidional* plants. The two World Wars in the 20th century made a strong contribution to this phenomenon.