Naturalisation: How Non-Native Species Integrate into New Ecosystems
When an organism travels from its original home to a new part of the world, it faces a critical turning point. It may simply fail to survive, or it may undergo naturalisation. This ecological process occurs when a species, taxon, or population of exotic origin becomes fully integrated into a new ecosystem, establishing self-sustaining populations that can reproduce, grow, and spread without any further help from humans.
In some instances, a species has been present in an area for so long that scientists cannot definitively say whether it is truly native or the result of an ancient introduction. In these cases, the taxa are often regarded as naturalised due to the impossibility of confirming their original status.
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Key Facts
- Naturalisation occurs when exotic species establish self-sustaining, reproducing populations.
- Adventive populations are non-sustaining and rely on a continuous influx of new individuals from elsewhere.
- Naturalised species can become invasive if they harm native species or the local biotope.
- Some naturalised species act as ecosystem engineers, creating new niches and habitats.
- The distinction between native and naturalised can sometimes be blurred by historical introductions.
The Botany of Naturalisation
In the field of botany, naturalisation refers specifically to exogenous (outside) plants that manage to reproduce and disperse on their own in a new environment. A clear example is Thuja plicata; it is naturalised in the United Kingdom because it reproduces independently, whereas in France, it remains dependent on human intervention through seeds or cuttings to spread.
Classifying Plant Naturalisation
Botanists use specific terms to categorize how and when plants have integrated into new lands:
- Archaeonaturalised: Refers to species introduced more than a hundred years ago.
- Amphinaturalised (or Eurynaturalised): Describes species that have spread across a vast spatial area, becoming assimilated into the indigenous landscape.
- Stenonaturalised: The opposite of amphinaturalised, referring to species with limited spatial extension.
Degrees of Naturalisation
Not all non-native plants reach the same level of integration. Scientists categorize them based on their status and how they arrived:
- Accidental taxon: A non-native plant that grows spontaneously and appears sporadically due to accidental human activity.
- Subspontaneous taxon: A plant that arrived accidentally or via unknown means and has undergone acclimatisation to reproduce like native plants, though it is not yet well established.
- Spontaneous taxon: A well-established plant (native or non-native) that grows and reproduces naturally without intentional human intervention, often mixing with local flora.
Zoology and Animal Integration
Animal naturalisation often follows different patterns, frequently driven by breeding or commensalism—a relationship where one species benefits from living near humans (such as following human migration flows). For example, the house sparrow arrived in Western Europe following the movement of the Huns.
Animals may enter an ecosystem in three primary ways:
- Voluntary introduction by humans.
- Accidental introduction or becoming feral.
- Natural movement alongside human migratory patterns.
In some cases, a naturalised animal species may even hybridise with a native species, altering the genetic makeup of the local population.
Impact on the Ecosystem: From Helpers to Invaders
The impact of naturalised species is complex and not always negative. While many are introduced to replace species lost to human activity, others can become invasive species. An invasive species is one that becomes so abundant that it adversely affects native species or the surrounding biotope.
Common examples of invasive naturalised species include:
- European rabbit: Native to Europe but highly abundant in Australia.
- Japanese knotweed: A major invader in Europe and America, ranked among the top 100 most invasive species of the 21st century.
Beyond direct competition, naturalised species can cause genetic pollution through hybridisation. However, some species, like certain palms, act as ecosystem engineers by changing habitats and creating new niches, which can have positive ecological effects.
The long-term impact is often difficult to predict. For instance, the African sacred ibis (*Threskiornis aethiopicus*) escaped an animal park in France in 1990. While an eradication campaign was launched in 2008, research in 2013 suggested the bird might not be a threat and could actually help promote the Eurasian spoonbill while limiting the invasive Louisiana crayfish.
Summary of Ecological Terms
| Term | Definition | Key Characteristic |
|---|---|---|
| Adventive | Non-sustaining population | Requires continuous human or external influx |
| Naturalised | Integrated exotic species | Self-sustaining and reproduces spontaneously |
| Invasive | Harmful naturalised species | Causes adverse effects on native biota |
| Ecosystem Engineer | Habitat-altering species | Creates new niches or changes the environment |
Frequently Asked Questions
What is the difference between an adventive and a naturalised species?
An adventive species is a population that does not sustain itself through reproduction and relies on a constant influx of new individuals from elsewhere. A naturalised species, however, has established a self-sustaining population that can reproduce and spread on its own.
When does a naturalised species become invasive?
A naturalised species is classified as invasive when its population becomes sufficiently abundant to cause negative impacts on native species, local microbes, or the overall biotope.
Can naturalised species have positive effects?
Yes. Some species act as ecosystem engineers by creating new habitats or niches. Additionally, some studies suggest certain non-native species may help control other invasive species or support native ones.
How do animals become naturalised?
Animals typically naturalise through intentional human introduction, accidental introduction, becoming feral, or through commensalism, where they follow human migratory patterns.
What is genetic pollution in this context?
Genetic pollution occurs when a naturalised species hybridises with a native species, which can compromise the conservation and genetic integrity of the original native populations.