Biological invasions are events of colonization and massive proliferation of populations in an environment, by species that are not native to that place, and that have been transported by human action, either deliberately or accidentally. These types of events, together with changes in land use, overexploitation of resources, pollution and climate change, make up the main drivers of anthropogenic global change . In fact, the impact produced by invasive species is currently the greatest factor in biodiversity loss on the planet.
The impacts of biological invasions not only have consequences on the species; They also alter natural cycles, modify the structure of ecosystems, cause disruption of ecological succession processes and generate negative effects in the socioeconomic sphere. Agriculture, livestock, aquaculture, many infrastructures, the well-being of our pets and even human health can be affected by some invasive species.
However, when we talk about exotic species with an invasive character , the saying is applied that “they are all that are, but not all that are”. Every invasive species is exotic, since it has been transported to a new ecosystem —deliberately or accidentally—; but many exotic species do not become invasive. For example, of the more than 700 exotic species currently grown in nurseries, only 30 are considered invasive and another 73 are considered potentially invasive. And it is that in the process of a biological invasion, a species faces a series of barriers and only those species capable of overcoming them all become invaders.
“It should not surprise us that the species of any region
be defeated and supplanted by naturalized productions from another
Charles Darwin, The Origin of Species.
1. The geographical barrier
The first barrier that alien species face in a given environment is the geographic barrier . Of course, only a species transported to another location can become invasive. And that transport must be mediated by the human hand. Therefore, colonization events produced by the natural movements of species are not counted as transport, from the point of biological invasion.
Any species that is moved from one place to another is called non-native, or exotic . It may happen that this species remains in the same place; that this living being never goes out into the natural environment. This is the case of an animal intended as a domestic pet, which is sterilized and remains at home; we would then speak of an exotic or non-native animal. But for an invasion to occur, other barriers still need to be overcome.
2. The environmental barrier
Some transported organisms never reach the natural environment. But it is more likely that the propagules of these organisms get out of human control and end up settling in the environment. Either because people deliberately release them, because they escape, or because their carriers are not aware that they are being introduced. They are crossing the environmental barrier .
When a ship releases the ballast water that it loaded elsewhere, a multitude of spores, algae and plankton can be released into the natural environment after being transported. When someone releases their parrots because they didn’t know they were going to be so noisy, and leaves them in the nearest urban park; when a group of animalists, with the best of intentions, releases thousands of mink from a farm; or when someone walks through the field with their boots dirty with mud adhering to another place, the introduction of the species is taking place.
In all these cases, the species is jumping the environmental barrier, and is now considered an introduced species.
3. The Survival Barrier
An exotic living being, at the end of the journey, can reach an ecosystem for which it is not biologically adapted. Seeds of a plant that needs abundant humidity, introduced in a desert. Or plants that need an arid environment, introduced on the banks of a river. A marine animal in a freshwater lake, or one that needs warmth, in the snows on top of a great mountain.
Many of the living beings introduced into the natural environment are not able to adapt, they are unable to survive for different reasons: environmental conditions, the presence of relentless predators, or even the existence of a lethal disease for them in the area.
However, other living things do survive. They overcome the survival barrier and integrate into the new ecosystem. They properly acclimatize to new conditions. Organisms that overcome the survival barrier are called subspontaneous .
4. The reproductive barrier
It is possible for an animal to successfully survive in the new environment. But as long as it is unable to reproduce, its population will depend directly on successive introductions. If a subspontaneous population stops receiving individuals, it will probably end up disappearing.
But, as always, life makes its way, and it often happens that these organisms find a way to reproduce. At that moment, when they overcome the reproduction barrier , the population becomes capable of maintaining itself generation after generation and perpetuating itself in time.
When a population exceeds the fourth or reproductive barrier, we say that it is naturalized .
5. The dispersion barrier
A population can remain stable over time, reproduce and occupy an available ecological niche —or displace another species to occupy its own—, within the area where it was originally introduced, and stay that way.
These naturalized species need not pose a significant problem beyond the area of original introduction. However, sometimes a naturalized species is able to cross the dispersion barrier , and colonize new environments —either by the movement of its individuals, or because they spread their propagules with great efficiency—. When naturalized species use stable populations as a source of expansion, and settle in new ecosystems different from those in which they were introduced, we say that they are invasive species .
Thus, an invasive species is not just any exotic species. An invasive species is one that has been transported by human action, introduced into a new environment, where it has successfully survived, naturalized, and eventually spread to new environments.
Is there a sixth barrier?
There is the proposal of a sixth barrier , which is not so much biological, as the previous ones, but legislative. In many countries, in order to act with total guarantees in the control and management of certain invasive species, it is necessary that they be included in a catalog that is regulated by law. It is much easier for an entity, such as a city council, to propose measures to eradicate ailantos of his municipality, than to eradicate the eucalyptus trees, despite the fact that both are very invasive .
Unlike the previous barriers, in which it was the species that was overcoming them —thanks to human action in some cases, or on its own in others—, in this case the barrier is ours. If we, as a society, do not provide the means to fight against invasive species, either by eradicating them, controlling their expansion, mitigating their pernicious effects or, in the best of cases, preventing their invasion, this problem will continue to be one of the great drivers of anthropogenic global change, and the world’s leading cause of biodiversity loss .
Bayón, Á. et al. 2019. Horizon scanning to identify invasion risk of ornamental plants marketed in Spain. NeoBiota, 52, 47-86. DOI: 10.3897/neobiota.52.38113
Bayon, A. 2021. Deliberate introduction patterns, invasion prediction and impact analysis of ornamental plant species in Spain. Sevilla University.
Blackburn, T. M. et al. 2011. A proposed unified framework for biological invasions. Trends in ecology & evolution, 26(7), 333-339. DOI: 10.1016/j.tree.2011.03.023
Blackburn, T. M. et al. 2019. Alien versus native species as drivers of recent extinctions. Frontiers in Ecology and the Environment, 17(4), 203-207. DOI: 10.1002/fee.2020