The media often focuses on animals when discussing invasive species. Plants hardly occupy one of every ten news on the subject, despite the fact that they represent more than 35 % of species regulated by the Ministry of Ecological Transition.
Invasive plants are the great forgotten ones that, although they can cause serious impacts and of greater magnitude than many animal species, by acting slowly and in an inconspicuous way, they are ignored by the bulk of society.
There are different ways to approach the issue of most invasive plants and various ways to assess it. We can deal with how widespread the species is , the number of impacts it causes , or a quantification of the capabilities that make it invasive .
If we take this last scale as a reference, some species can show a very invasive behavior, but they are still less widespread than other less invasive species, but they have been around for much longer.
Another even more complex way of evaluating would be the ‘ Priority Index ‘, developed in 2019, which includes not only the invasive potential of the species, but also the level of impact they can cause and the interest it arouses in the population — a direct indicator of the social demand of the species—. If we take these parameters into account, the list of species would be different, however, this ‘Priority Index’ has only been applied to ornamental plant species, and never to species that are already regulated as invasive.
In order to include any invasive species —both those that are regulated by the Ministry and those that are not, and both those that are for ornamental use and those that are not—, the invasion risk assessment method developed in 1999 will be used in this list. by an Australian research team led by Dr. Pheloung, and adapted to the Iberian flora through research led by Dr. Núria Gassó in 2010 .
5. Water hyacinth
It is an aquatic plant from the Amazon basin. The water hyacinth ( Eichhornia crassipes ) has colonized the lower reaches of the Guadiana, causing a true ecological disaster .
It presents a very aggressive invasive character from the beginning, quickly colonizing all the environment that is favorable to it. It covers the entire surface of the water, preventing light from entering the bottom of the wetlands and eliminating bottom vegetation. As it breaks down, the oxygen levels of the water drop dramatically.
Water hyacinth populations are an ideal environment for the proliferation of mosquitoes , favoring the appearance of diseases that affect both fauna and human health. Among other socioeconomic impacts, the impediment to the passage of boats, the reduction of fishing, the clogging of canals, and even the blockage of hydroelectric power plant turbines stand out.
Physical, chemical and biological means have been used to try to eliminate the water hyacinth invasion. However, when the species is already widespread and established, eradicating it becomes impossible , the most that is achieved is to control its expansion and mitigate its impacts.
4. Acacia mimosa
It is important not to confuse the so-called mimosa wattle ( Acacia dealbata ) with the true mimosa ( Mimosa pudica ). Both have similarities, but one unmistakable feature differentiates them: the true mimosa closes its leaves when hit, and the acacia does not.
This tree, native to Australia, was introduced as an ornamental, to fix slopes and for the extraction of dyes, essential oils and gum arabic. It is a very dangerous invasive plant in temperate and subtropical areas. It is very easy to germinate and resprout after a fire , and its rapid growth makes it an excellent competitor, especially in areas disturbed by fires or deforestation, or in scrubland areas.
Being a species that cannot invade mature forests, prevention through the maintenance and conservation of plant cover is one of the best ways to avoid its invasion . Eliminating its presence in parks and gardens is a priority.
3. Spanish flag
The common name of this shrub from the entire tropical area of America, “Spanish flag” ( Lantana camara ) , is given by the characteristic color of its flowers. In its most common variety, those that occupy the center of the inflorescence have an intense yellow color, while those that form the outer ring are red. However, garden varieties with other colors are found today.
This plant is highly appreciated in gardening due to the ease with which it is grown and its striking inflorescences. That ease of cultivation is what makes it a powerful invader. In addition to colonizing disturbed areas and open spaces, it manages to penetrate the margins of the forests and displace the native species that develop there.
Through the roots it emits allelopathic substances , which are toxins that spread through the soil and inhibit the growth of other plants. It is poisonous to livestock , especially horses and donkeys, and cases of fatal poisoning in children have been reported by accidentally consuming its fruits.
Although this species is not included in the Spanish catalog of invasive alien species, it is considered by the IUCN as one of the 100 most dangerous invasive species in the world.
2. Rabogato
Originally from northeast Africa, the rabogato ( Pennisetum setaceum ) has found its optimum place of invasion in the Canary archipelago .
It is currently present on all the islands, although it has been almost completely eradicated on some.
It is easily dispersed along the margins of the roads, taking advantage of the air currents generated by the passage of vehicles. In the Canary Islands, this species strongly competes with native vegetation, to the point of displacing it .
This can be really problematic in island ecosystems, where there are many endemisms and a high sensitivity to disturbances.
Prevention is once again a priority against this species. Manual elimination must be carried out when the plant is not in bloom, and repeated several times a year to be effective, and eliminate the juveniles that germinate after the first passes.
1. Pampas Duster
The Pampas duster ( Cortaderia selloana ) is an herbaceous plant from Argentina that began to be planted as an ornamental species, due to the attractiveness of its bright white duster-shaped inflorescences. On its leaves it has tiny silica needles that protect it from attack by herbivores. If handled with bare hands, it can cause skin cuts.
It is a lover of soils with a large amount of nutrients , such as the edges of roads and highways and the banks of rivers. In this type of environment it also finds the great advantage of the movement of people, animals, vehicles or water, which generate air currents that drag the seeds miles away. Once established, it forms dense, evergreen clumps.
It can invade slopes, banks, dunes and estuary tails.
It is a difficult plant to eradicate. Once the aerial part has been cut with a chainsaw, it is necessary to uproot the roots to prevent regrowth. Manual removal of tillers must be carried out with personal protective equipment that protects hands, face and eyes.
This operation must be carried out before flowering, otherwise, shaking the plant could cause the seeds to break off and be released to the wind.
REFERENCES:
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
BOE. 2013. Royal Decree 630/2013, of August 2, which regulates the Spanish Catalog of invasive alien species. BOE, 185(Sec. I.), 56764-56786.
Gassó, N. et al. 2010. Predicting plant invaders in the Mediterranean through a weed risk assessment system. Biological Invasions, 12(3), 463-476. DOI: 10.1007/s10530-009-9451-2
Pheloung, P. C. et al. 1999. A weed risk assessment model for use as a biosecurity tool evaluating plant introductions. Journal of Environmental Management, 57(4), 239-251. DOI: 10.1006/jema.1999.0297
SanzElorza, Mario. et al. (Eds.). 2004. Atlas of Invasive Alien Plants in Spain. General Directorate for Biodiversity.
