Naturalisation (or naturalization) is the ecological phenomenon through which a species, taxon, or population of exotic (as opposed to native) origin integrates into a given ecosystem, becoming capable of reproducing and growing in it, and proceeds to disseminate spontaneously.^[1] In some instances, the presence of a species in a given ecosystem is so ancient that it cannot be presupposed whether it is native or introduced.^[2]

Some populations do not sustain themselves reproductively, but exist because of continued influx from elsewhere. Such a non-sustaining population, or the individuals within it, are said to be adventive.^[3] Cultivated plants, sometimes called nativars, are a major source of adventive populations.

Botany

In botany, naturalisation is the situation in which an exogenous plant reproduces and disperses on its own in a new environment. For example, northern white cedar is naturalised in the United Kingdom, where it reproduces on its own, while it is not in France, where human intervention via cuttings or seeds are essential for its dissemination.^{[citation needed]}

Two categories of naturalisation are defined from two distinct parameters: one, archaeonaturalised, refers to introduction before a given time (introduced over a hundred years ago), while the second, amphinaturalised or eurynaturalised, implies a notion of spatial extension (taxon assimilated indigenous and present over a vast space, opposed to stenonaturalised).^{[clarification needed][citation needed]}

Degrees of naturalisation

The degrees of naturalisation are defined in relation to the status of nativity or introduction of taxons or species:^[2]

• Accidental taxon: non-native taxon growing spontaneously, which appears sporadically as a result of accidental introduction due to human activities (as opposed to intentional introductions)
• Subspontaneous taxon: taxon naturalised following an introduction of accidental origin (fortuitous introduction linked to human activities) or unknown, and which, after acclimatization, can reproduce like native plants but is still poorly established
• Spontaneous taxon: native or non-native taxon growing and reproducing naturally, without intentional human intervention in the territory considered, and is well established (mixes with local flora or fauna)

Zoology

Animal naturalisation is mainly carried out through breeding and by commensalism following human migrations.^[4][5]

The concerned species are thus:

• either introduced voluntarily into an ecosystem where they are not native;
• either accidentally introduced or become feral;
• or by naturally following human migratory flows by commensalism (eg: arrival of house sparrow in Western Europe following Huns, and previously in Eastern Europe from Asia Minor in Antiquity).

It sometimes happens that a naturalised species hybridizes with a native.^[5]

Introduction and origin areas

The introduction site or introduction area is the place or, in a broadlier way, the new environment where the candidate species for naturalisation takes root. It is generally opposed to the origin area, where this same species is native.

There is also a more ambiguous notion that is the "natural distribution area" or "natural distribution range", particularly when it comes to anthropophilic species or some species benefiting from anthropogenic land settlement (canals, bridges, deforestation, etc.) that have connected two previously isolated areas (e.g. the Suez canal, which causes Lessepsian migration).

Impact on the ecosystem

Naturalisation is sometimes done with human help in order to replace another species having suffered directly or indirectly from anthropogenic activities, or deemed less profitable for human use.^[6]

Some naturalised species eventually become invasive. For example, the European rabbit, native to Europe and which abounds in Australia; or the Japanese knotweed which is invading Europe and America where it is considered to be amongst the one hundred most invasive species in the 21st century.^[7] Apart from direct competition between native and introduced populations, genetic pollution by hybridization can add up cumulatively to environmental effects that compromise the conservation of native populations.^[8]

Some naturalised species, such as palms, can act as ecosystem engineers, by changing the habitat and creating new niches that can sometimes have positive effects on an ecosystem. Potential and/or perceived positive impacts of naturalised species are less studied than potential and/or perceived negative impacts.^[9]

However, the impact on local species is not easy to assess in a short period. For instance, the African sacred ibis (Threskiornis aethiopicus) escaped in 1990 from an animal park in Morbihan (France), gave rise to an eradication campaign in 2008. In 2013, however, the CNRS stated that this bird species is not a threat in France, and may even promote Eurasian spoonbill and limit the development of the invasive Louisiana crayfish.^[10]

Naturalised species may become invasive species if they become sufficiently abundant to have an adverse effect on native species (e.g. microbes affected by invasive plants^[11]) or on biotope.^[12]