Non-native plants that cause negative environmental and economic impacts.
Many of the plants that surround us have been transported from their natural habitat to other locations, being these plants called exotic (from Greek exotikós, “from outside”). Some of these species coexist with the native species in a balanced way, but others develop very quickly and become beyond the control of Man, becoming harmful – these species are called invasive. They are able to overcome geographical barriers, and are also species that can overcome biotic and abiotic barriers while maintaining stable populations.
An exotic plant shall be considered invasive when it produces numerous breeding populations and that are separated from the initial plant, both in space (over 100 m) as in time (less than 50 years for species dispersed by seed; over 6m, every three years, for species with vegetative reproduction), regardless of the degree of environment disturbance and without the direct intervention of man. The proliferation of these species often promotes environmental change and/or economic loss.
Although we refer to invasive plants, there are invading organisms in all groups of living things: since the rust fungus to cholera, and including the red swamp crawfish (Procambarus clarkii), the argentine ant (Iridomyrmex humilis), numerous wood insects, the pine wood nematode (Bursaphelenchus xylophilus) or the red palm weevil (Rhynchophorus ferrugineus) and many others.
Invasive species are compared to a form of pollution that, unlike the others, does not stop when it eliminates the emission source.
What characterizes an invasive plant?
The great diversity of invasive plants implies that their characteristics are diverse. However, there are some characteristics that are common to many of these plants, such as:
– they have rapid growth and/or high dispersibility
– they compete more effectively for available resources than native species
– they produce many seeds, which may be viable for long periods of time and can be stimulated by fire, which is particularly serious in a Mediterranean area like the one in which we live in
– where they are invasive, the plants have no natural enemies once they are displaced from their place of origin
– they reproduce vegetatively without need for production of seeds to disperse.
These features are not necessarily all present in invasive species. Additionally, other features may contribute to the invasive behaviour of the species.
Why do they represent a problem?
Despite the positive aspects that have sometimes justified the introduction of invasive species, these are responsible for many negative impacts, often of difficult and expensive resolution and, in some cases, irreversible. Among the negative impacts, the following are noted:
1) high economic impact, both in terms of production, in particular when they are species that invade agricultural, forestry or fisheries areas, either in the application of control measures and restoration of invaded systems – at an European level, an estimate made recently referred losses close to 10 billion €/year associated to invasive species (Hulme et al. 2009);
2) impacts on public health, when they are species that cause diseases, allergies, or act as pest vectors;
3) decrease in the availability of water in aquifers, in the case of very demanding species, implying substantial losses in this resource that is scarce in many parts of the world;
4) impacts on the ecosystems’ balance, achieved over thousands of years of evolution, being nowadays one of the main threats to biodiversity. This includes, for example, the change of biogeochemical cycles (carbon and nitrogen cycle), the standardization of ecosystems, changing fire regimes and food chains, and competition with native species, at times replacing them completely.
In this context, invasive species are one of the greatest threats to the planet’s economic and environmental well-being.
All are exotic plants invasive?
No. Many exotic plants do not have invasive behaviour. Considering all the exotic plants that are introduced (the full circle in the Figure), the majority remain with a restricted distribution to the sites where they were placed. Others may bloom and even reproduce occasionally, but cannot accomplish self-sustaining populations, relying on repeated introductions for their persistence – casual plants.
Of these, a fraction is established beyond the place of initial introduction, it breeds persistently and forms populations that are maintained without the direct intervention of man, staying in balance in semi-natural habitats, for a variable amount of time. When this happens, it is said that these plants are naturalized.
In a fraction of the naturalized species, the balance can be interrupted by any phenomenon that encourages the rapid increase of its distribution (the stimulus can be a natural disturbance, such as the adaptation of an agent that may disperse the seeds or a pollinator, the occurrence of a storm or climate change; or a disturbance caused by human activities such as land-use changes, the occurrence of a fire or even the control of another invasive species.), triggering the process of biological invasion.
Main steps of a process of biological invasion.
Some species adapt very easily and reveal invasive behaviour attacker shortly after its introduction in a new territory, apparently without the need for any stimulus.
Many of the aforementioned disorders result in open clearings, which is an excellent opportunity for an invasive species to secure itself. Taking into account global changes, it is likely that some of these disorders become more frequent, which may aggravate and accelerate processes of biological invasion.
The subsequent increase in the distribution of an invasive species (upward curve in the Figure) depends on its rate of growth and reproduction, the efficiency of its dispersal mechanisms and characteristics of the invaded habitat. Generally speaking, well-kept habitats are more resistant to invasion, but may not always be the case.
