Asian hornet, raccoon, water hyacinth, red swamp crayfish, red-eared slider, monk parakeet, fountain grass, red palm weevil, wels catfish, Xyllela fastidiosa… They are all names of organisms that increasingly dominate the headlines of the major media in various countries. They are a real problem in many ways. The environment, the economy and the health system are directly and significantly impacted by these and many other species. They are invasive alien species and in this article we will reveal their secrets.
They come from the most unexpected places, travelling abysmal distances and using the most unthinkable means of transport. The entry of many of these species could be avoided with effective prevention plans, which on the other hand would save many headaches, but the population in general and administrations in particular still need one or two lessons about the importance of this phenomenon.
They are invasive alien species, or simply invasive species, and they are the protagonists of biological invasions, a natural phenomenon that, like climate change, is being accelerated by humans. The reader may not have noticed this, but many of the great epidemics and pandemics that humanity has suffered throughout history and that have eliminated entire populations at the stroke of a pen have been caused by exotic species that have found an appetizing niche to colonize. The Black Death, Ebola, AIDS, the Spanish flu, mad cow disease and many others began to wreak havoc in a very specific and fenced region, but human movements managed to serve as transport around the world for these microorganisms.
We hope that this article will serve as an in-depth analysis of this very worrying issue and, above all, as a tool for raising awareness of a problem to which we must devote all our efforts.
A brief presentation
The term “invasive alien species” consists of two epithets, so let’s start by distinguishing them. In a given region, we consider a species to be exotic, foreign or alien when it comes from another region. Its antonym would be the autochthonous or native, the one considered natural of a determined geographic region. On the other hand, an invasive alien species is a species that comes from elsewhere and is capable of establishing self-sufficient populations in the area in which it has been introduced, capable of surviving, reproducing successfully and spreading rapidly to other regions. In addition, these species often cause negative impacts on the environment, on native species and/or on the economy and/or health of human populations. This last criterion, the impact, is widely used to determine if an alien species is invasive or not, however, later we will discuss if this criterion is really useful to consider an alien species as invasive, since in many occasions it is very complex to determine the impact that an alien species has on ecosystems that, on the other hand, is not always negative.
Other criteria are also used to label an alien species as invasive. For example, the International Union for Conservation of Nature (IUCN), in addition to the above, adds that an invasive alien species is one that has also been accidentally or intentionally introduced by humans. Of course, an indigenous species can also become invasive when its populations suffer significant demographic explosions or when they “invade” undesirable territories from a human point of view (a crop for example), although in these situations it is not labelled as invasive, but as a pest, plague, weed, etc.
It should be made clear, therefore, that alien is not synonymous with invader. There are invasive alien species and others that are not. Think of many plant species that are an essential part of our diet or that have been used for ornamental purposes in parks and gardens. Their populations have failed to achieve reproductive stability and a worrying propagation capacity and, by themselves, they do not cause any harm, on the contrary, they bring benefits. In fact, if they are present outside their natural range, it is because we deliberately cultivate them and ensure their survival. Tomatoes, potatoes, sweet potatoes and peppers are native plants of South and/or Central America, from where they have spread to the rest of the world and are not recognized as invasive species.
There are more terms related to biological invasions although their use is rather anecdotal. For example, we speak of subspontaneous and adventitious species in reference to those exotic species capable of establishing ephemeral and not very viable populations. If they persist, it is because they are found near an anthropic zone or because new individuals are regularly introduced, renewing the population. The difference between the two terms is that adventitious species have been accidentally introduced and subspontaneous species have escaped from a delimited area, a crop or a farm for example. However, one term that is implicitly more serious and more widely used than the previous ones is that of naturalised alien species, because these species are capable of establishing long-lasting populations, an essential prerequisite for eventually becoming an invasive species.
However, a number of nuances need to be made. Firstly, because labelling a species as autochthonous or alien is not as easy as it might seem. For example, can a species introduced in the 1st century A.D. be considered alien after so long? Or let’s look at the case of the horse. Horses emerged 50 million years ago in North America and later spread to Asia and South America. 8000 years ago horses became extinct from America and did not trot through their rugged meadows again until the colonizers reintroduced them. The same goes for beavers in Spain. Is a species reintroduced after extinction considered as alien? If a species that was originally exotic has been integrated into the ecological community and, in fact, its eradication could cause an ecological imbalance, is it still alien or does it become native? We see, therefore, that this question has many edges, to such an extent that biologists have been forced to establish a series of criteria to give priority to certain species when establishing management programmes. Some of these criteria are time or certain historical events. At the beginning of the 20th century, the year 1500 was established as a turning point in Europe because of the great role played by the discovery and colonisation of the New World in the transfer of species between America and the Old World. Such is the case that species introduced and naturalized before 1500 are called “archaeophytes” and those introduced from that year onwards “neophytes”. It is the latter that are of most interest to environmental conservation programmes. Some authors have criticized this categorization for lack of objectivity, because, in the same way, could have been chosen as a turning point specifically for Great Britain the Roman occupation, not in vain the Romans introduced a large number of alien species, especially plants.
The alien-native dichotomy has also had its critics. A group of scientists maintains that many conservation plans base their actions on this erroneous and counterproductive criterion, as these labels face several epistemological problems, some of which we have already mentioned. For example, ecologist Mark Davis and 18 other contributors directed a commentary to the journal Nature on this issue. They point out that the definitions of native and alien, first sketched out by the British botanist John Henslow in the 19th century and still in force today, bring with them a xenophobic concept, because according to them, the objective of this classification was to separate “pure” and “authentic” British biodiversity from that which was not, that is, from the alien and “poor”, attributing a series of unscientific negative connotations. This is not uncommon, since in Victorian England there was a strong feeling of racial segregation that had a certain impregnation in the scientific field. Therefore, and according to these biologists, it is imperative that conservationist movements and science free themselves from these anachronistic and negative chains and from the criterion of the geographical origin of the species in order to obtain a more objective vision of biological invasions. In fact, we will see later that most alien species do not pose a problem. A very interesting debate, no doubt.
As if that were not enough, and to add to the complexity, all these terms must take account of the specific cases. For example, the form of dispersion of a plant is not the same as that of an animal, mainly because plants are sessile. Therefore, the following criteria are used to consider an invasive plant species: if that species reproduces by seed and in less than 50 years individuals have appeared within 100 metres of the focus of introduction, it can be considered invasive. If that species, on the other hand, reproduces by stolons or rhizomes and in less than 3 years an individual has appeared more than 6 metres from the focus of introduction, it is also considered invasive.
An intensified interest in invaders
When does the concern about this whole thing begin? Human beings are perceptive and for centuries have been aware of the presence of previously non-existent species in the regions in which they lived. The botanist Pehr Kalm travelled in 1747 to Philadelphia, USA, under the orders of the famous naturalist Carl von Linnaeus. There he contacted the ethnic groups of the region. From the conversations he had with them he extracted their bewilderment by the presence of what they called “English flies”, insects that until the arrival of the Europeans to their lands did not swarm there. They were not really flies, but the common bee (Apis mellifera). Likewise, they called the broad-leaved plantain (Plantago major) “white man’s foot”, because that herbaceous, before nonexistent, grew profusely in the European settlements. Other great scientists, such as Charles Darwin or Charles Lyell, were also aware of this phenomenon.
However, the real turning point was a small book published in 1958. It was entitled The Ecology of Invasions by Animals and Plants. Its author, the British ecologist Charles Sutherland Elton.
Elton (1900-1991) was an interesting person. Born in Liverpool, from an early age he showed a great interest in naturalism and the enigmas of nature, although his fascination ended up focusing on animal ecology. He studied at Liverpool College and graduated in zoology at Oxford. He was a member of the remarkable Royal Society and received the Darwin Medal in 1970. In addition to the work already mentioned, he wrote other books on ecology: Animal Ecology (1927), Voles, Mice and Lemmings (1942) and his magnus opus, The Pattern of Animal Communities (1966). His first book gave a notable boost to the discipline of animal ecology, to such an extent that some authors consider him its father. He was also the author of several scientific articles. From 1932 until his retirement in 1967 he worked in the Bureau of Animal Population, located in the Department of Zoology at Oxford University, with a small group of students and ecologists. He was an affable man and maintained a cordial relationship with his colleagues. He had an open mind and never tried to impose his working style on the team. He lived through both world wars, a factor which, according to some authors, influenced him in some way, to such an extent that some of his analyses of biological invasions would have a militaristic and sometimes even xenophobic character (the terms “invasion” or “invader” could have a warmongering influence, as well as other terms and expressions he uses throughout his work). In fact, during the Second World War Elton worked as a pest controller for the army’s food reserves, which surely reaffirmed his interest in the phenomenon of invasions.
In his work, which is actually an extension of three BBC radio broadcasts from March 1957 and a compilation of several articles he previously wrote, Elton draws attention to what he calls “ecological explosions,” i.e., exaggerated demographic growths of certain species. He begins his book by exemplifying this concept with various pandemics and epidemics of diseases that have decimated human populations and economies and changed history, such as the Black Death, the Spanish flu, malaria, rabbit myxomatosis, etc., and in whose ecological explosion is closely related to the spread of the microorganisms causing these pathologies all over the world from their place of origin with the help of human beings.
The main thread of his work is based on biogeography, that is to say, the branch of biology that investigates the geographical distribution of organisms, giving special relevance to Wallace’s Realms. The famous naturalist Alfred Russell Wallace (1823-1913; followers of this blog will remember that we already mentioned him when we talk about the flat Earth theory), co-discoverer of the theory of evolution by means of natural selection, delimited a series of regions that, like pieces of a puzzle, divided the Earth’s surface in relation to the distribution of flora and fauna. However, for Elton, the true importance of biogeography lay in the movement of fauna and flora through these realms, and in the significant success of adaptation in the new regions that some of these species manifested, increased when the hand of humans intervened.
The book is extremely interesting, not only because it has a rich scientific base supported by numerous studies of the time, but also because Charles Elton echoes many of the hypotheses with which biologists continue to work today to explain the adaptive success of invasive species, such as the pressure of propagules, phenotypic plasticity, the absence of enemies…, as well as the potential pathways and modes of transmission of these species. It also highlights the influence of globalization and global connectivity on the distribution of these species, phenomena that began to suffer a monumental impulse in their time. Elton also emphasizes the importance of the conservation of the biosphere through a very modern conservationist vision and the lack of dissemination of the problem of biological invasions (a problem that unfortunately is still present):
“For thirty years I have read publications about this spate of invasions; and many of them preserve the atmosphere of first-hand reporting by people who have actually seen them happening, and give a feeling of urgency and scale that is absent from the drier summaries of text-books.”
Elton predicted in his book a world in which geological and orographic borders would be completely annulled due to the development of transport and globalization, in which mountain ranges and oceans were mere reefs easily surmounted by species. A planet with an impoverished and homogenized biodiversity, made up of a few species scattered all over the world. A kind of futuristic Pangaea, but instead of being connected by land the continents will be connected by a hairless monkey. This was a matter of great concern to Elton, in fact he thought that greater biodiversity increased the stability of ecological communities, so an impoverishment of diversity triggered by biological invasions could not bring anything good.
In short, it is a work that synthesizes, in a way that everyone can understand, the keys of a problem that in its time was not given the importance it deserves and with which it provides a large number of examples of continental, island and maritime invasions. For this reason, Elton is considered the prophet of biological invasions. While it is true that he did not invent any new science, it is also true that he helped to spread the issue and put it in the spotlight of conservationist associations. For this reason, some claim that Elton’s book marked the true birth of the ecology of biological invasions or, at least, was a major turning point. Others, on the other hand, argue that his work was of relative importance, as it would not have had an imminent influence. It wasn’t until the 1980s that scientific publications on biological invasions were booming, and not precisely because of Elton’s work.
In the 1980s and 1990s, a large number of publications on biological invasions began to see the light of day. Many of them were covered by the Scientific Committee on Problems of the Environment (SCOPE). At one of its international conferences is when an interest in expanding knowledge on this issue appears and it is proposed that SCOPE take over the patronage of this research. Initially these studies focused on the context of Mediterranean ecosystems, but in 1982 scientists aspired to a more global vision. In this way, similar programmes were created in different nations: Great Britain, South Africa and the United States. From then on, the citations of the first publications and, therefore, the publication of new studies on biological invasions began to grow rapidly, generating in turn scientific journals dedicated exclusively to biological invasions, being the most representative Biological Invasions, which would give the definitive impulse to this young branch of ecology.
Where are they from?
Perhaps the easiest question to answer in this field. They can come from virtually anywhere and from the most unsuspected regions. The same goes for the means they use to move. It is difficult to predict how an exotic species will come unless we have previous examples. In this way, preventive measures and controls of these potential routes of introduction can be established.
Fristly we must distinguish two modes of introduction mediated by the hand of humans: the deliberate and the accidental. We could add a third, a mixture of the two previous ones, by which a species is deliberately introduced because it has an economic interest and that by accident that species accesses the natural environment. This is the case of the American mink (Neovison vison) in Spain, highly appreciated for its skin by the fur industry. Some fur farms, which had an infrastructure that left much to be desired, suffered damage during storms that offered an outlet to these animals. We would be speaking, therefore, of a case of negligence, although in the entry of the American mink in Spain it has had more influence the deliberate and negligent releases by animalism militants.
Most introductions are accidental. It should be noted that many organisms are not transported in their adult stage, but as tiny larvae or other germinal forms that can adhere to virtually any surface going totally unnoticed unless exhaustive control is carried out. And let’s not talk about organisms, mainly plants, that have the impressive capacity of vegetative reproduction, that is, the generation of an individual from any part of another. The transfer of any fragment of an organism with this capacity would also be sufficient. Moreover, many of the animals introduced are invertebrates, whose adult stages are small too. With all this it is clear why most introductions of alien species are accidental.
However, while moving from one region to another may seem easy, the truth is that it is not. It is in these circumstances that we realize the immense strength and will that life has to survive. An alien species has to overcome a large number of barriers to arrive finally to a new habitat, which is not easy at all:
Geographical barriers: orographic barriers, water masses, mountain ranges…
Local environmental barriers: abiotic conditions (temperature, soil type, humidity, ion concentration, etc.) and biotic conditions (competition with other species).
Reproductive barriers: the exotic species must be able to generate stable populations through reproduction.
Environmental barriers of disturbed hábitats: environments altered by humans delimit accessible niches.
Environmental barriers of undisturbed habitats: environments not disturbed by humans also offer resistance, not for nothing are already occupied by other species.
It is a hard road to success… From these barriers scientists tried to deduce a rule to estimate the number of alien species that may eventually become established in the recipient ecosystem and become invasive. This estimate, known as the “Ten Rule”, tells us that of the total number of exotic species that can reach a new region, only 10% can overcome the first barrier. Of those survivors, only 10% would be able to overcome the second barrier. Again, 10% of those who have overcome the second barrier would overcome the third, etc. That is to say, finally a tiny percentage of all the alien species that manage to introduce themselves in a new place would end up reaching the desired end. We can also extract more positive information, since most of the alien species that manage to settle do not become invasive and, therefore, would be harmless. It’s a good thing, because we have enough with the few that become invasive. Unfortunately, this rule is not always followed. Actually, invasion rates depend on the species, the characteristics of the recipient ecosystem, the frequency with which the species is introduced, etc. In this way, they do not have the same adaptive success, for example, exotic plants that are introduced for ornamental purposes and exotic mammals that manage to escape from their captivity. The latter tend to have better success rates.
Below we will describe several examples to illustrate the unique transmission routes that these species are able to use to reach their random destinations. Let’s start with some cases of alien animals that have been accidentally introduced.
The Asian tiger mosquito (Aedes albopictus) displays its name by the pattern of whitish stripes that alternate with the black background of the tiny body of adults. It is native to a wide geographic region that includes tropical Southeast Asia, several Pacific and Indian islands, southwest Asia to Madagascar and part of China and Japan. From here it has spread to all continents except Antarctica. Today we find stable populations in the Americas, Australia, much of Europe, Africa and several Asian countries where it was not before. We might think that this insect has reached some of these destinations by itself thanks to its ability to fly and easily overcome geographical barriers. However, the distances they can fly are short. Their cosmopolitanism is due to shipping and the incorrect overcrowding of wet tyres. As with other mosquitoes, the first three vital stages (egg, larva and pupa) are aquatic, while adults are terrestrial. When it rains, small puddles form inside the tires, thus creating the perfect habitat for these animals to develop without problems. Any container containing some stagnant fresh water is really a good place for this insect. In fact, another important means of transmission have been containers storing lucky bamboo plants (Dracaena spp.) sent from China, especially when they began to be preserved in stagnant water. This is therefore a case of dispersion by passive transport.
Another common means of dispersion is the ballast water of boats. It is the ideal medium for many aquatic species, mainly invertebrates. This ballast, whose function is to maintain the stability of ships, is mainly water, whether brackish, fresh or salty that the ship collects from the environment in which it is, although before the end of the nineteenth century, when ballast water began to be used more widely, it was more common to use “dry” ballast, consisting of sediments and land materials, such as cobbles and stones. In the latter case, the animals most likely to travel by this means were small invertebrates that inhabited the coasts, mainly crustaceans and tiny insects, but also algae. Finally, the ballast water is discharged at the port of destination.
Ballast water has been a real headache keeping in mind that the maritime transport and trade have been constantly in crescendo since the end of the 19th century. Such is the case that it became necessary to implement a series of laws requiring inspections of ballast water and containers to at least reduce the rate of introduction of exotic organisms by this means. Because ballast water has proven to be a very effective means of transmission for many invasive species. This has been the case for the Asian clam (Corbicula fluminea), native to Southeast and East Asia, the quagga mussel (Dreissena rostriformis bugensis), native to some Ukrainian rivers, the New Zealand mudsnail (Potamopyrgus antipodarum), the ctenophore Mneiopsis leidyi, native to the Atlantic coasts of the Americas, the European shore crab (Carcinus maenas), native to some coasts of Europe and Africa.
However, the most paradigmatic case is that of the zebra mussel or Dreissena polymorpha. This bivalve was restricted to the basins of the Caspian, Aral and Black Seas and its arrival to the rest of Europe has been a piece of cake, because those seas are connected through various canals with Western Europe and Russia. From Europe it would later reach the American continent, probably submerged in the ballast waters of merchant ships. This species is on the IUCN list of the 100 of the world’s worst invasive alien species.
Another form of passive dispersion in which ships also intervene is the adherence to the surface of the same, which in biology is known as biofouling, i.e., the adherence of a community of organisms to wet surfaces. In this way, not only animals (mainly molluscs), but also algae, plants and microorganisms can be dispersed.
The much-feared Asian hornet (Vespa velutina) arrived in southwestern France in 2004 in a rather unsuspected way. This hymenopteran is native to a large region of Asia spanning northeast India, south and central China, part of Indonesia and Taiwan. It didn’t not arrive on its own to the Gallic country. The resaponsible could be a shipment of bonsai pots from China. It seems that these pots served as refuge to a group of queens that were hibernating. When the administrations realized their presence it was too late, wasps populations were numerous and stable, making their eradication impossible.
A similar example of accidental introduction was that of the brown tree snake (Boiga irregularis), also included on the 100 of the world’s worst invasive alien species. This species is insular, native to several Pacific islands, but not to the island of Guam. The populations of this island are the only invaders, at least so far, because they could also have established a stable population in Saipan. Guam Island is dotted with U.S. military bases. Precisely, this ophidium infiltrated military cargoes destined for the island on several occasions and subsequently escaped into the natural environment in the late 1940s or early 1950s. We see, therefore, that another frequent means of transmission are cargoes and goods, prone to serve as a refuge for stowaways. This same route served the black rat (Rattus rattus), carrier of various parasites and pathogenic microorganisms, to spread throughout the world from the Indian subcontinent for centuries.
And since we are talking about disease-carrying animals, it is worth mentioning some of the organisms that cause these pathologies and that can also be considered as invaders due to their rapid spread and their harmful impacts. It is well known that the black rat facilitated the etiological agent of the black plague and bubonic plague, the bacterium Yersinia pestis, to spread and wreak havoc (although the role of the black rat in the spread of this pathology has recently been called into question). Less well known are the exotic pathogens that are massively annihilating different taxa and have spread along with their hosts. Therefore, another effective way to disperse exotic species are the living beings themselves.
The fungi Batrachochytrium dendobatidis and Batrachochytrium salamandrivorans are responsible for chytridiomycosis, a pathology that fatally affects Anura (frogs and toads) and Urodela (salamanders and the Iberian ribbed newt) amphibians respectively. These pathogens are estimated to have decimated 501 amphibian species worldwide in half a century, according to a recent study published in Science. These fungi would be native to Asia, although it is known that the oldest genetic lineage, from which the four lineages currently described come from, is native to the Korean Peninsula. How did they get so far? Thanks to the international trade of amphibians for different uses (food, pets, experimentation, etc.). Amphibians from the regions of origin of these fungi have a greater immune resistance against them for having coevolved with them over a long period of time together. On the contrary, the amphibians of the receptor ecosystems have never come into contact with these fungi, so they have not developed the necessary defences and, consequently, succumb to infection. It is the same thing that happened when the Old and New World came into contact from 1492. Human populations on both continents infected each other and spread exotic pathogens against which they didn’t have immune defences.
Exactly the same thing happens with the aphanomycosis of crayfish. This deadly disease is again caused by a fungus, Aphanomyces astaci, and its dispersal is again explained by human negligence. Crayfish (some of them also invaders) carrying this disease are the cause of its widespread and the alarming decline of numerous populations of freshwater crayfish.
To end with the examples of accidental transmission routes of alien animals, we will mention two major engineering works that have greatly facilitated numerous biological invasions. We refer to the Panama Canal and the Suez Canal. These constructions have destroyed two geographical barriers that separated different oceans and seas respectively and prevented, or at least made it unlikely, that biodiversity from one side would pass to the other. Since 1869, the Suez Canal has allowed the exchange of more than 500 exotic species between the Mediterranean Sea and the Red Sea. Such is the case that the Mediterranean is considered the most invaded sea in the world. The situation of the Panama Canal is similar and, if that were not enough, the recent expansion to allow a more fluid transfer of larger vessels is not going to help reduce the transfer of species. It should be noted that species not only cross these canals autonomously, but also receive the help of boats, either stuck in the hull of the same or through ballast water. We have also mentioned before how the aquatic species of the Ponto-Caspian bassin have been able to access the rest of Europe through the endless canals that run through its entire geography. Water transfers, of course, also facilitate the transmission of exotic species between different bodies of water.
Let’s see now some ways in which alien animals are deliberately introduced. Obviously, there is always a human interest behind this, be it economic, aesthetic or otherwise. From an economic point of view, the two most prominent routes of deliberate introduction would be fishing and hunting, both from a sporting and gastronomic point of view. Taking Spain as an example, several species have been introduced for these purposes. Some of the most representative would be the titanic wels catfish (Silurus glanis; native to Eastern Europe and Western Asia), the black bass (Micropterus salmoides; native to some areas of North America), the pike-perch (Sander lucioperca; from Eastern Europe), the red swamp crayfish (Procambarus clarkii; native to the northeast of Mexico and the south of the USA), the aoudad (Ammotragus lervia; native to Africa), the European mouflon (Ovis aries musimon; native to Corsica and Sicily). Such has been the intensity of alien fish introductions into Spain for fishing, especially during the twentieth century, that now make up about 30% of the freshwater fish fauna.
On the other hand, other alien species were introduced, among others, for ornamental interest. These are the common carp (Cyprinus carpio) and the goldfish (Carassius auratus), both native to Asia and introduced to Spain in the 18th century.
The introduction of species as biological controls precisely for the management of invasive alien populations is also a potential route of introductions, although it is true that this methodology is presented as one of the most promising options to curb the expansion of alien organisms, provided that these projects have previously gone through risk analysis to avoid introducing a new invasive species. However, there have been some accidents. It has happened with attempts to control and deplete populations of the giant African land snail (Achatina fulica), considered one of the 100 world’s most damaging invasive species by the IUCN. Another gastropod, the rosy predator snail (Euglandina rosea), native to North America and voracious predator of other snails, was deliberately introduced to the sites invaded by the former. It was thought that it could end the populations of Achatina fulica, but it not only failed in its objective in many regions, but also managed to establish populations with a fruitful future and, today, in many regions coexist both gastropods. In fact, it is now on the same list as his alleged victim.
In Spain, an alien fish from the Atlantic slope of North America was introduced in 1921 known as Eastern mosquitofish (Gambusia holbrooki) to eradicate malaria-carrying mosquitoes, as the larvae of these insects are a delicacy for this fish. It was introduced in different bodies of water: lagoons, ponds, rafts, etc. In this case the introduction was somewhat successful, as the Eastern mosquitofish played an important role in the eradication of malaria (in 1964 it was officially eradicated). The benefits were relative because Gambusia holbrooki, like the snails already mentioned, succeeded in implanting stable populations. This species is also part of the IUCN list. On the other hand, the cat (Felis silvestris catus) has also been used as a biological control, mainly on islands, to eradicate other invasive species such as rabbits, mice or rats (although the presence of cats on numerous islands around the world is mainly due to their domestication). The point is that the cat has established permanent populations on hundreds of islands, triggering very detrimental effects on the biodiversity of these ecosystems.
Not long ago there were some institutions, although one could rather describe them as real nonsense, known as acclimatisation societies. These societies sought the biological enrichment of different regions through the introduction of alien species. Normally these societies belonged to the colonizing powers and settled in the new territories. Their crazy pretensions were to recreate the flora and fauna of the colonists’ native countries in order to satisfy a strange feeling of nostalgia and to complement the biodiversity of those places, considered poor and simple by the colonists, with the most “sophisticated” fauna and flora of the countries of origin. Possibly the best known case is that of the American Acclimatization Society, or rather that of its president, the New York pharmacist Eugene Schieffelin who, in an access of eccentricity and madness, did not think of anything other than to introduce into America all the species of birds mentioned in the Shakespearean plays (about 50 species). To him we owe the population of 200 million starlings (Sturnus vulgaris) that America currently has (the starling is also on the IUCN list), generated from the 100 specimens he introduced into New York’s Central Park between 1890 and 1891.
Finally, it is interesting to talk about those cases of introductions that we have called “mixed”, that is, species that have been introduced by human interests that have accidentally and/or negligently escaped to the environment from the enclosures that contained them. In this category we can include some pets, possibly one of the most serious causes of biological invasions, because it denotes the lack of environmental and ethical sensitivity of the population, general ignorance and the lack of involvement of the competent political institutions when it comes to disseminating and promulgating preventive education and ecological awareness. The abandonment and release of pets for lack of interest is already serious, but so are the consequences of these acts. To think that an alien animal will have better living conditions if it is released into nature is a fallacy. Not only are they condemned to death and suffering (after all they won’t have anymore the usual care and comforts they had in their outbuildings), biodiversity is also condemned to suffer the penalties of a new biological invasion if that alien animal manages to establish breeding populations.
A large and worrying number of invasive alien animals have been introduced through this pathway, some of which have been listed by IUCN. The raccoon (Procyon lotor; native to southern Canada, part of the USA, Mexico and some Central American countries), the monk and rose-ringed parakeets (Myiopsitta monachus and Psittacula krameri; from South America and Asia respectively), the black-headed weaver (Ploceus melanocephalus; native to sub-Saharan Africa), the Nasua (Nasua spp.; from Central and South America), the raccoon dog (Nyctereutes procyonoides; native to Southeast Asia), various species of turtles (freshwater turtles of the genus Trachemys sp, Pseudemys sp., Chrysemys sp., Graptemys sp., among others), the giant African land snail and many other animals are a clear example of pets that have become invasive species.
The problem of exotic pets is permanent and is closely related to absurd society fashions. Just as it was fashionable a few years ago to have raccoons at home, now it is fashionable to buy suricates or lovebirds. To prove a point: a few weeks ago, the Spanish media echoed the expansion of the Iberian catalogue of invasive alien species to include four new species of alien animals that have become the ultimate pets: Sus escrofa domestica, the savannah monitor (Varanus exanthematicus), the royal python (Python regius) and the peninsula cooter (Pseudemys peninsularis).
Exotic pet market, therefore, is an important pathway for the introduction of invasive alien species. Unfortunately, it is not the only “mixed” pathway. Many species reach the environment because they find a means of escape from their enclosures. Consequently, zoos, aquariums, fish farms and farms of alien animals are potential pathways of introduction. In this respect, see the ruddy duck (Oxyura jamaicensis), the aforementioned American mink (Neovison vison), the muskrat (Ondatra zibethicus), the American bullfrog (Lithobates catesbeiana) or the golden apple snail (Pomacea canaliculata).
It’s the same with plants. Plantae kingdom has invasive alien species that have been introduced accidentally, deliberately and “mixed”. As the pathways are similar to those of animals, we will focus on listing several examples so as not to extend this section much further.
Let’s look at some plant species that were accidentally introduced and propagated by different routes:
Hidden or contaminating goods: would be the case of the anual meadowgrass (Poa annua; native to temperate regions of Eurasia and famous for being the only exotic plant with flowers that has reached the Antarctic), a species that usually appears as a contaminant of grass seeds, the South African ragwot (Senecio inaequidens; native to South Africa, its seeds have spread widely throughout Eurasia and other regions of Africa adhering to shipments of sheep wool) or tamarisk (Tamarix parviflora; native to Mediterranean regions, it can be propagated through shipments of wood. We could safely say that it is one of the most hated species in some regions of North America).
Adhered to clothes or transported by animals: this dispersal mechanism not only plays an important role in the dispersal of the exotic plant in question from its native region to new ecosystems, but also in the propagation within the recipient region. A series of adaptations that have greatly helped plants of all types to take advantage of this mechanism are the development of seeds with appendages (whether barbed, with adhesive substances, hooks, etc.) that facilitate adherence to the body surface and the development of fleshy and appetizing fruits that surround the seed, which favour the ingestion of the seeds and their transport in the digestive tract of animals. The process of propagation by means of animals or people is called zoochory. Some examples of alien species that use this mechanism are the Russian olive (Elaeagnus angustifolia; native to Eastern Europe, it has been introduced, among other regions, in North America, where the also exotic common starling helps its propagation by ingesting its fruits) and the mesquite (Prosopis juliflora; coming from Mexico, Central America and Northern South America, it is widely distributed in almost all continents and in its local dispersion ungulates that ingest its seeds intervene).
By wind or water: other seeds, on the other hand, have developed adaptations that favour their transport by wind (anemochory) or water (hydrochory) by means of aerodynamic appendages or appendages that allow their buoyancy. Exotic plants with this type of seed that have been planted in places with frequent gusts of wind (such as a road due to the passage of vehicles) or near bodies of water benefit greatly from these mechanisms. We could mention the case of the tree of heaven (Ailanthus altissima; originally from China, it has been introduced in several countries deliberately as an ornamental plant in road ditches) or the water hyacinth (Eichhornia crassipes, originally from the tropical regions of South America, has spread through various freshwater habitats in Africa, Australia, Europe and Asia. Water currents have had a great influence). Of course, algae also benefit greatly from hydrocory, as is the case with the marine Codium fragile.
Through engineering works that have broken down geographical barriers (such as the Panama and Suez canals): : see the Asian kelp (Undaria pinnatifida; native to northeast Asia, it has settled steadily on the Californian coasts thanks to the ships that cross the Panama canal).
On the other hand, deliberate and “mixed” pathways of introduction have been worryingly frequent in the genesis of new plant invasions:
Introduced alien species with ornamental motifs: pampas grass (Cortaderia selloana; native to several South American countries, has been introduced to all continents except Antarctica for its colorful silver panicles), butterfly bush (Buddleja davidii; this plant of beautiful purple flowers is native to southwest China), Russian tree (Elaeagnus angustifolia), multitude of species of Acacia sp.., (mostly from Australia), fountain grass (Pennisetum setaceum; originally from North Africa), black locust (Robinia pseudoacacia; native to central and eastern USA), tamarisk (Tamarix sp.), etc.
Alien species introduced for gardening: Japanese knotweed (Fallopia japonica), various species of prickly pears (Opuntia sp.; native to Central and North America), Japanese honeysuckle (Lonicera japonica; often used to cover fences and walls), Russian vine (Fallopia baldschuanica; native to Tibet and western China, has had a similar use to Japanese honeysuckle)…
Alien species introduced for fish keeping: the killer algae (Caulerpa taxifolia; green algae native to tropical regions of the Atlantic, Indian and Pacific oceans, it is known with precision that it arrived in the Mediterranean Sea through the emptying of the content contaminated with this algae of an aquarium of Monaco), water hyacinth (Eichhornia crassipes), floating pennywort (Hydrocotyle ranunculoides; from North and South America and some regions of Africa), yellow waterlily (Nymphaea mexicana, from the Southern USA and Mexico), kariba weed (Salvinia molesta; native to South America), water fern (Azolla filiculoides; native to the Americas)…
Alien species introduced for industrial purposes: prickly pears (the best known, Opuntia ficus-indica, is appreciated by prickly pears and was also used to raise cochineal dyes), eucalypts (Eucalyptus sp.; most of them come from Australia and have been planted extensively throughout the world for wood and paper), giant hogweed (Heracleum mantegazzianum; it comes from Central Asia and the Caucasus and is used as a fodder plant and for beekeeping), Japanese knotweed (Fallopia japonica; used as a fodder plant and in beekeeping)…
Other species have been introduced, for example, to control erosion or stabilise soils, such as the hottentot fig (Carpobrotus edulis), native to South Africa, the European alder (Alnus glutinosa), originating in Europe and North Africa respectively, and the tamarisk (Tamarix sp.) in the USA.
Because of the reasons why all these species have been introduced, we can suspect the main potential sources of introduction and spread: nurseries, florists, horticulture and gardening establishments, etc. Also, many of these alien species have not been introduced directly into the environment, but, as with many alien animals, have managed to escape and spread from relatively restricted locations (shops, private properties, etc.).
So far we have investigated the mechanisms of dispersion and introduction mediated by humans. However, life is autonomous and capable enough to scatter on its own, which has been a constant since it exists, thus taking advantage of plate tectonics, flight capacity, swimming or buoyancy, droughts and floods to reach various destinations. It goes without saying that organisms are capable of overcoming those imaginary barriers established by Homo sapiens of which we have not spoken so far and which constitute another probable entry of alien organisms: borders. The best case illustrating this circumstance is that of the Asian hornet, which naturally crossed the Spanish-French border to reach northern Spain in 2011 from France. This forces us to establish more rigorous and robust cross-border controls.
Consequences of biological invasions
We come to the primary key of biological invasions: the consequences or impacts of the establishment of these controversial species in the ecosystems that host them. If invasive alien species are famous and feared, it is precisely because of their harmful effects in different areas. It should be remembered that this criterion is often used to characterise these species and differentiate them from alien species that are not invasive. However, as we have also mentioned, impact is an anthropocentric criterion and one that is not always easy to determine. Furthermore, an invasive alien species does not always have harmful effects and these do not always have the same magnitude. Let’s take the New Zealand mudsnail or Potamopyrgus antipodarum as an example. While some authors have reported harmful effects in the populations of native species by this mollusk, others on the contrary have found positive effects and others, even, have not discovered any type of impact, neither positive nor negative. The same occurs with many other alien species, depending on where they have been introduced, they have some effects or others to a lesser or greater extent.
Even so, biological invasions are a matter of real concern because of the actual or potential damage they can cause. Several authors reiterate that biological invasions are the second cause of biodiversity loss on a global scale. The first cause would be habitat destruction and landscape fragmentation. Bearing in mind the two fundamental features that characterise biodiversity, i.e. that it is unique and unrepeatable, we are therefore talking about a problem that is becoming irreversible. To be more concise, biological invasions take first place if we are talking about the causes of extinctions of amphibians, reptiles or mammals.
This assertion, however, has been criticized by a group of biologists who consider it to be an unfounded exaggeration. In general, they often cite an article published in the journal BioScience in 1998 by David S. Wilcove and collaborators, who establish a ranking of the main causes that threaten endangered species in the United States. These authors come to the conclusion we have already described: the first threat to endangered U.S. biodiversity is habitat destruction in a variety of ways, and the second is invasive species. The point is that this study only focuses on U.S. territory, not globally. When this criterion is established at a global level is in 2004 in the Red List prepared by the IUCN, a very extensive and complete inventory that collects the state of conservation of animals and plants worldwide. Since then, invasive species have been carrying this nefarious fame. Be that as it may, regardless of whether they are the second or the fifth cause, experts largely agree that biological invasions are one of the main causes of biodiversity loss.
Let’s start by detailing the negative impacts. The most worrying are those that affect the balance of ecosystems and biodiversity. Virtually every organism has a way of altering recipient ecosystems, so it is best to illustrate this with examples.
Zebra mussel (Dreissena polymorpha), native to the Ponto-Caspian bassin, has expanded widely by a great diversity of freshwater ecosystems of great part of the planet. Its problem lies in two factors: the enormous population densities it is able to reach in the invaded ecosystems (in 1992 more than 500 billion individuals were reported in the Hudson River, New York) and its way of feeding. Like most bivalve molluscs, these animals feed by filtration. Thanks to a system of cilia located in their gills they are able to generate a small stream of water directed towards these organs, which are pierced by a series of pores that act as a net to trap the nutritive particles of the medium. In addition, these pores are usually impregnated by a mucilaginous and adherent substance secreted by adjacent cells that facilitates the uptake of food. Zebra mussel is a real filtration machine. If we add to this the high number of individuals that can compose a population, the effects on the environment are very significant. By filtering solid particles suspended in water, including also phytoplankton, they reduce the turbidity of the medium. From an anthropocentric point of view, it could even be appreciated, since in some way they beautify the environment, but the truth is that in this way they significantly alter the chemical and biotic profile of the ecosystem. By making the water more transparent, this favors a greater penetration of sunlight at greater depths, thus facilitating the growth of photosynthetic macrophytes (macroscopic plants). To an increase in the photosynthetic rate of the medium must be added the large quantities of waste products that these animals excrete and defecate as a result of their feeding in the form of faeces and nitrogen, causing an increase in the concentration of nutrients and eutrophication (accumulation of organic waste). In feedback, this triggers a proliferation of algae also with photosynthetic capacity. Through filtration, Dreissena polymorpha also consumes high levels of oxygen. Consequently, zebra mussel not only competes for resources (food and oxygen) with other species of bivalves (often winning) but favors the growth of some organisms to the detriment of others by altering the biomass of the habitat in particular.
Something similar happens with the New Zealand mudsnail (Potamopyrgus antipodarum), since it is also capable of reaching unimaginably high population densities (more than 700000 individuals per square meter in the Polecat Creek River, Wyoming, for example). This animal doesn’t feed by filtration, however, it is also capable of altering the concentration of nutrients and the chemical profile of the aquatic habitats it colonises. For example, teams led by researcher Robert O. Hall Jr. determined that in some regions this species can consume 75% of the primary productivity of some regions (i.e., the amount of biomass produced by autotrophic organisms, those that are capable of generating their own food) and represent no less than 80% of the secondary productivity (amount of mass produced by heterotrophic organisms).
Other species, on the other hand, are capable of altering the disturbance regime of an ecosystem, that is to say, they modify the rates at which certain phenomena that alter the environment occur, such as fires. Examples include the eucalypt (Eucalyptus sp.), native to Australia, and the cogongrass (Imperata cilindrica), a grass from Eurasia and the tropics and a member of the list of the 100 of the world’s worst invasive alien species. Both species are pyrophytes, that is, they are highly combustible plants and prone to burn, either because of the type of wood or because of the combustible substances they produce. On top of that, the second one sprouts easily in areas that have burned. The establishment of populations of these species, therefore, increases the rate of fires in host ecosystems, annoying native species that were accustomed to less common fire regimes. In Spain, the competent institutions are constantly denounced for not working hard enough to limit eucalypts populations, who are blamed for the spread and intensification of fires mainly in the north of the Peninsula. On the other hand, the leaf litter of Eucalyptus globulus interferes with the nutrient cycle, since its chemical composition slows its decomposition and, consequently, limits decomposers’ access to this resource. When this species replaces others whose litter is more accessible to decomposer organisms, the entire saprophytic community is affected.
Another plant that has produced harmful impacts on the ecosystems where it has been introduced is the firetree or faya (Myrica faya), an island shrub native to the Canary Islands, Azores and Madeira Islands. This species was introduced to Hawaii in the 19th century by the Portuguese, where it has successfully adapted to its lava soils. A positive aspect of this type of soil is that it is poor in some nutrients, such as nitrogen, making it difficult to establish exotic species that require high levels of this component. The establishment of the firetree would have been unlikely if it had not had an unexpected ally: the nitrogen-fixing bacteria of the genus Frankia sp. The symbiotic mutualist relationship they have established with these bacteria has been essential for their adaptation in Hawaii. Together they form what are known as actinorrhizal nodules, places where soil bacteria and plant roots come into contact. Both species work together to ensure survival: while the firetree yields part of the nutrients resulting from photosynthesis to the bacteria, Frankia sp. captures atmospheric nitrogen and transfers it to the soil through a series of chemical reactions, generating a nitrogen reserve very useful for the plant. A collateral effect of this relationship is the increase in nitrogen concentration and the consequent fertilization of Hawaiian soils, which has facilitated the entry of other alien nitrogen-demanding plants.
The water hyacinth (Eichhornia crassipes) is considered to be the most dangerous aquatic invasive plant in the world. Not in vain, its effects are devastating. It reproduces very quickly, either by seeds or asexually (a mere fragment of the plant is enough to generate a new individual), being able to create a thick and dense green carpet in a very short time. If the reader is still skeptical, we invite him to look for photographs of the Guadiana River (Spain), intensely invaded by this species. As we say, this plant creates leafy carpets in the places where it is introduced. As a result, the light falling on the water mass is drastically reduced, affecting various photosynthetic organisms. It also consumes large amounts of oxygen, generating environments close to anaerobiosis, a harmful effect for plants, fish and other living beings. If this is not enough, their populations are so dense that, sometimes, it can slow down the circulation of water.
These examples are enough to observe how invasive alien species can, in many cases, tragically alter ecosystems. Evidently, these ecosystem alterations have a direct effect on the survival of native species, leading to the reduction of their populations and to their potential extinction. This ultimately results in the loss of biodiversity. In the previous examples, we can see some mechanisms by which invasive species negatively affect native species. Competition for resources (e.g. food, ecological niche, space, refuges, forage areas, pollinating insects, etc.) is a clear example. Less adapted populations end up reduced and displaced to other places in search of resources, while the victorious can occupy the niches that have left these species empty and, consequently, expand. In general, competition usually occurs between species belonging to the same taxonomic categories, such as, for example, between the American mink and the European mink, the zebra mussel and the bivalves of the family Unionidae, the wels catfish and other fish, the red-eared slider and the Spanish pond turtle (Mauremys leprosa) or the European pond terrapin (Emys orbicularis), and so on.
A phenomenon that we have not yet mentioned and which is extremely useful for plants is allelopathy, consisting of the secretion into the environment of a series of chemical substances that influence in some way the reproduction, growth and survival of other organisms. Allelopathy does not always cause harmful effects on native species, in fact positive effects have sometimes been found (by stimulating growth for example). The fact is that these allelochemicals may be able to inhibit the growth of native species, interrupt the flow of nutrients or attack mutualistic interactions of native plants with soil fungi or bacteria. Of course, these substances can also modify the characteristics of the soil or surrounding water, altering its pH, the concentration of certain components, affecting populations of microorganisms, etc. Some invasive species that have taken advantage of allelopathy are several species of the genus Centaurea sp., Typha angustifolia, Artemisia vulgaris, etc.
Another circumstance that favours the decrease of native species is the hybridization with alien species. In many cases, both species are related enough to generate fertile offspring. The best known examples are usually the ruddy duck (Oxyura jamaicensis), a water bird native to Central and North America that is capable of mixing its genes with those of the white-headed duck (Oxyura leucocephala), a species that already maintained a critical state of conservation, that of the domestic pig (Sus scrofa domesticus) with the wild boar, resulting in a new species that in Spain has occupied much of the northern half, or the hybridization between the plant Spartina alterniflora with species of the same genus native to U.S. and Great Britain. On the other hand, however, gametes may be wasted when the result of hybridization is an abortion. This happens when American mink males try to fertilize European mink females.
Of course, predation also affects native species. We have several examples, such as the Asian hornet (Vespa velutina) which skilfully hunts other social insects; the pike (Esox lucius), the wels catfish, the pumpkinseed (Lepomis gibbosus), the black bass (Micropterus salmoides), the perch (Perca fluviatilis), the pike-perch (Sander lucioperca) and many other fish that devour native fish, their larvae or eggs or other organisms (the wels catfish, in addition, and due to its colossal size can devour pigeons and other birds); the aoudad (Ammotragus lervia) and the European mouflon (Ovis orientalis musimon) prey on various herbaceous plants and native shrubs, many of which are threatened; the American mink hunts intensively on European crayfish, amphibians, Iberian desmans… One of the most significant cases is that of feral cats that have managed to establish themselves in insular ecosystems. According to an extensive study signed by 10 researchers and published in the journal Global Change Biology in 2011, these generalist predators are responsible for 14% of the extinctions of birds, mammals and reptiles in these ecosystems and the main threat of 8% of birds, mammals and reptiles that are critically endangered.
However, if we had to choose the most threatening cause for biodiversity, we would certainly opt for the transmission of diseases, because, unfortunately, in many cases an exotic species does not come alone. And we chose this impact because it is the one that has bequeathed us the most worrying and too high figures for losses. For example, as we have already pointed out, chytridiomycosis, a pathology caused by the Asian fungi Batrachochytrium dendrobatidis and B. salamandrivorans is decimating amphibian populations worldwide. These fungi reproduce on the skin of these animals, a fundamental organ involved in gas exchange and respiration, causing asphyxiation of the animal. According to the latest analyses, these fungi are behind the impoverishment of the populations of 501 species of amphibians and, presumably, 90 extinctions in half a century. Thanks to the trade in amphibians such as the American bullfrog (Lithobates catesbeiana) and the African clawed frog (Xenopus laevis), which, on the other hand, compete with and displace native amphibian populations, these fungi are widely scattered.
We have also talked about the aphanomycosis of crayfish, whose etiological agent is also a fungus, Aphanomyces astaci, endemic to North America, and which has reached other continents along with invasive species of crayfish, such as Pascifasciatus leniusculus and Procambarus clarkii. This fungus begins reproducing in the cuticle-free zones of the abdomen. From there it progressively emits its hyphae, which are prolonged by the soft regions of the cuticle until colonizing great part of the organism. It causes 100% mortality in European crayfish and causes the death of the animal in 6 or 10 days. American crayfish are immune to this pathology.
Many exotic fish can transmit parasitic diseases by nematodes or flatworms, such as the bleak (Alburnus alburnus), the black bullhead (Ameiurus melas), the goldfish (Carassius auratus auratus)… Some of these parasites can be accidentally transmitted to humans through the food chain. It is also the case with the Florida red-eared and yellow-eared sliders, which can transmit a trematode flatworm to the European turtles, something that has recently been discovered. They can also be carriers of salmonellosis like many other reptiles.
In this respect, plants are no exception. Possibly, one of the most remembered pandemics and one of the most prevalent echoes is that of the graphiosis of the elm, a pathology caused by the fungi Ophiostoma ulmi s. l. and Ophiostoma novo-ulmi and which, since it began in the twentieth century, has devastated extensive hectares of elms of various species in Europe and North America. These fungi are transmitted by several beetles of the genus Scolytus sp., whose females build curious galleries inside the bark to deposit their eggs. It is during their stay in the elms when they deposit the spores. The trees die irremediably because the fungi introduce their mycelium in the xylem of the tree, destroying these conductive vessels and interrupting vital functions such as the conduction of the xylem sap along the whole plant. There are, however, elm species from western Asia, such as Ulmus pumila or Siberian elm, which are immune to this pathology. It is possible that the disease started in that region and was transported with these elms to the rest of the world. The beetles would do the rest…
Going to more modern times, we will highlight an epidemic caused by a set of bacteria that are wreaking havoc both environmentally and economically. We are referring to the bacterium Xyllela fastidiosa (so called because of the complexity of isolating in laboratory), which has three subspecies. It is distributed mainly in America. In Europe it was detected for the first time in 2013, specifically in olive crops in southern Italy. The problem with this bacterium is that it is incredibly generalist, that is to say, it can use a great diversity of plants as hosts. Specifically, it has been determined that Xyllela fastidiosa is able to colonize about 523 different species of woody plants (trees, shrubs) and is the etiological agent of several critical plant diseases. It is an authentic plague, because the mortality it causes is high, which is not strange, since this microorganism accumulates in the xylem of the plants and clogs it, preventing the flow of sap. Of course, its wide distribution is due to the global transfer of plants and arthropods, especially of the Cicadidae family (cicadas and company), which serve as a vector for this microorganism. To get an idea of how infectious it is, when a focus of infection is detected, it is necessary to isolate and destroy plants that can potentially become infected within 100 meters around, as well as to establish a buffer zone (i.e., a delimited region in which a phytosanitary treatment is applied) of one or more kilometers around. When stably installed, eradication becomes impossible. In the case of Spain, the bacteria has reached Mallorca, Menorca and Ibiza and its eradication is considered impossible. Outbreaks have also been found in Valencia, Andalusia and recently, in April 2018, there was a detection in an olive tree in Madrid that fortunately has not gone any further.
We see that biological invasions are not a trivial issue. So far, we have described the effects of invasive species on ecosystems and native biodiversity. But this does not end here, from an anthropocentric point of view they also generate diverse economic, health and social impacts.
From the economic point of view, biological invasions accumulate enormous investments all over the world. We are talking about several billion that are invested annually in the prevention, early detection, control and management of invasive species. Let us give a few examples to illustrate this better. In 2005, a review signed by David Pimentel of Cornell University, New York, and two collaborators estimated an expenditure of more than $120 billion per year only in the U.S. for the management of the approximately 50000 alien species present in the country. Without leaving the U.S., it is estimated that the destruction of grain reserves by the brown rat and the black rat would be equivalent to $19 billion; the farming sector loses $13 billion annually due to invasive insects; the Massachusetts Department of Conservation and Recreation spends more than $300000 annually only on zebra mussels. If we travel to other continents, the figures don’t change much. In Europe, the cost of biological invasions is estimated between 12000 and 20000 million euros per year. In South Africa, only the control of invasive plants is estimated at $40 million per year. If these exorbitant amounts are harsh for developed countries, imagine how costly it will be for developing countries…
These estimates should not seem strange to us actually considering the large number of biological invasions that have already been recorded and the impacts that they have in various areas. Many of these species become real pests for crops, destroying months of hard work and essential food sources. Others, such as zebra mussels, due to their ability to adhere to solid surfaces, clog pipes and tubes, causing damage and breakdowns in systems and machinery. The Asian hornet can cause losses to the beekeeping sector by reducing bee populations. Tamarisk, an arch-enemy shrub of North Americans living in the driest regions of the Southwestern United States, has often been accused of consuming large amounts of water, a problem that is exacerbated in regions with scarce water reserves. Invasive pyrophytes plants can increase the risk of fire and the destruction of large hectares of forest and pasture, areas that will subsequently have to be reforested. Invasive fish capable of displacing native species affect the recreational fishing sector. Livestock can be decimated by exotic diseases that can even affect human health, such as mad cow disease or avian influenza. Tourism can also be affected in this respect, although perhaps to a lesser extent: the exaggerated proliferation of the water hyacinth makes it difficult for boats to circulate in the rivers it invades; invasion by exotic jellyfish on various beaches around the world prevents bathers from having carefree access to them, and so on. Of course, conservation institutions also need to invest more effectively in the protection of endangered species threatened by invasive species.
Health sector is also affected by biological invasions. We have already mentioned on several occasions that many exotic species have been the cause of major pandemics that have sometimes led to the annihilation of entire human populations, either by direct infection or by the systematic destruction of the livelihoods of those populations. We can speak in this sense of the causes of the Great Irish Famine (1845-1849). This was possibly the most tragic episode in the history of Ireland, an event that has left an indelible mark on the Irish people, because as a result of it nearly 1 million people died of hunger and another million were forced to migrate to England and the United States. The cause was the destruction of potato crops, the exclusive food of the poorest, who were in fact the main victims. The cause was a fungus, Phytophtora infestans, which affects not only the potato, but also the tomato and eggplant. He arrived on the emerald island possibly on board shipments of infected tubers from America, and there left an imprint impossible to forget even if several generations go by.
In this regard, we have already described the harmful impacts of the bacterium Xyllela fastidiosa on hectares of woody plants, obviously including crops of various species, such as the olive tree. Well, there is a fungus, Fusarium oxysporum, which acts in a similar way, blocking the xylem of the plants it infects and causing their death if no remedy is applied in time. It is known to be the etiological agent of Panama disease, also known as Fusarium wilt, in banana trees. It has caused serious damage to banana crops in South America and Asia, leading to substantial economic losses. It is feared in several regions of Africa. In fact, in some African banana farms, such as Matanuska, the visitor is obliged to receive a disinfectant bath to eliminate all potential traces of this fungus. Even so, this fungus does not only affect banana growers. Some subspecies can also infect oil palms (from which the controversial palm oil is obtained).
As for the direct impacts on human health, we can mention the paradigmatic case of the Asian tiger mosquito (Aedes albopictus). If its bite is already annoying (both in humans and in cattle), even worse are the hosts it can potentially transport. Because this mosquito is a means of transmission suitable for more than 10 different pathologies, some very worrying, and that in recent years have acquired great media coverage for their spread to continents and regions in which they were previously absent. We could mention the West Nile virus, Yellow Fever, Zika virus, Chikungunya, Rift Valley Fever, Dengue, etc. The raccoon is another carrier of diseases, in particular a very problematic one. This mammal carries a nematode worm known as Baylisascaris procyonis. When the worm reaches a person and reproduces, a phase known as larva migrans is generated, a larval stage that is able to move and cysts in different regions of the body, such as the head or under the skin. The problem is when it settles in the central nervous system, where it produces a pathology very similar to meningitis, which can lead to a misdiagnosis and a critical complication of the disease. It is also a carrier of rabies, West Nile virus and other parasites that can infect livestock or domestic animals. In addition, an aggressive raccoon is extremely dangerous, it is an animal with great strength in its limbs and jaws and, of course, very sharp claws and teeth.
Exotic turtles acquired as pets can carry salmonellosis. On the other hand, many exotic fish may be carriers of nematodes or flatworms that may accidentally reach humans through the food chain, causing eating disorders.
On the other hand, it is dangerous that certain invasive alien species swarm uncontrollably because of their toxicity. For example, the toxin of the cane toad (Bufo marinus) is especially dangerous, even deadly, for children, the elderly and immunosuppressed people. Asian hornets are also a problem in the places they have invaded, especially for people allergic to wasp venom. South African ragwot (Senecio inaequidens) is also toxic to humans (and livestock). Other species, on the other hand, can favour the appearance of allergies, such as the pampas grass, while others can trigger annoying reactions when pricked or in contact with them, such as prickly pears, chollas or giant hogweed.
So far we have only exposed the negative impacts of invasive alien species. But this issue is more complex than it seems, and just as an invasive species can be harmful in a particular locality, it has no effect in another, or can have a positive impact. Even positive and negative effects can occur in the same place at the same time. We saw it with the zebra mussel that, at the same time that it competes and displaces other species of bivalves, favors macrophytes and algae by increasing the transparency of the water and the arrival at greater depths of more sunlight. It goes without saying that a more transparent lake attracts more attention to tourists, another benefit, in this case economic. Similar effects have been registered with the New Zealand mudsnail. Its introduction in certain regions has served as food for endangered fish species, such as the Northern tidewater goby Eucyclogobius newberryi, for whom, moreover, it is a much more nutritious food than other species of native molluscs with which it coexisted and which it could not digest completely because of their shells. This fish is capable of completely digesting Potamopyrgus antipodarum. In addition, the introduction of another prey into the environment can reduce predatory pressure on native species.
Several authors point out that most exotic species do not reduce the biodiversity of the recipient ecosystem, quite the opposite: if they do not cause any damage or threat to that ecosystem but have beneficial effects, this implies that their establishment entails an increase in biodiversity, not for nothing that new species are added to the biological staff. In addition, it should also be noted that the perception of certain invasive alien species may change over time. A paradigmatic case is that of the tamarisk, the famous shrub accused of stealing large quantities of water in the southwestern United States and of salting the soil. At least that was the conclusion of the first published analyses on the subject. It has even been labeled as the second worst invasive plant in the U.S. However, in more recent times reviews that highlight the exaggeration and inaccuracy of those early studies have been published, subtracting magnitude to the effects of tamarisk. This shrub was also accused of displacing and replacing the native flora, affecting in turn some birds that nested in that flora, such as the willow flycatcher (Empidonax traillii). On the contrary, more recently it has been observed that in some areas, a high percentage of willow flycatchers nest in tamarisks. The tamarisk has been the target of millionaire investments for its control, perhaps now it will be necessary to invest to use it in the restoration of ecosystems…
With all this in mind, how should we label these species? Such as invasive in the places where they cause damage and as alien in the regions where they pose no threat? What happens to species with such double impacts? A fascinating subject, no doubt.
Several authors claim that these beneficial effects of alien species are often neglected by some scientists due to an opinion bias that is reflected in the language sometimes used with warlike or even xenophobic tints. However, they should be taken into account for future conservation programmes, since alienc species could become allies for the maintenance and protection of ecosystems and biotic networks. For example, it has been recorded that some introductions of alien flora in eroded regions that require restoration favour the recovery of these ecosystems and create microclimates for the regrowth of native plant species, since, on the other hand, they attract pollinators. In addition, there is a number of evidences involving certain alien species as suitable substitutes for native species that for some reason have become extinct or their populations are severely depleted, taking over as seed dispersers, population controllers, niches, etc. And of course we do not forget the economic and health benefits that several alien species provide us, mainly plants useful for food or with a therapeutic function, although we could also highlight the bees. It is important to consider this alternative facet of biological invasions when making decisions on whether or not to eradicate a particular alien species, because its absence may lead to an environmental imbalance.
It is not uncommon for alien species to have neutral or even positive effects on ecosystems and biodiversity. We must remember that alien species that become invasive or harmful are fortunately a minority.
The causes of success
We’ve already looked intensely at how invasive species spread and the effects they have. However, one of the facets that still raises many doubts is why an alien species is successful in becoming invasive, that is, capable of establishing solid, self-sufficient populations with reproductive capacity and the ability to compete with the species with which it must coincide. To shed some light, experts are investigating several hypotheses that focus on invasive species traits, the original and the recipient ecosystems. So far, several possibilities have been postulated, and everything indicates that the particular combination of some of them determines to a greater or lesser extent the invasive success of each species. Let’s see what they are.
Let’s start with those hypotheses that point to the traits of the species. One of the unknowns that experts are trying to uncover is why invaders appear to be more successful in the regions where they have been introduced than in the regions where they originate. Thus, the identity of what is known as “the ideal invader” remains controversial. The following traits that would explain invasive success are often emphasized in the scientific literature:
High fertility and growth rates and early sexual maturity. Species that produce a large number of seeds or individuals ensure their continuity, since the larger the number of offspring, the more likely it is that some offspring will reach sexual maturity and may produce more offspring. Rapid growth implies rapid sexual maturity and earlier access to certain resources, which in a competitive relationship with other species is a great advantage. The invasive success of the tree of heaven (Ailanthus altissima) or of the New Zealand mudsnail could be framed by these hypotheses, as they produce a high number of offspring, grow rapidly and possess early sexual maturity.
The type of reproduction. A slower and more energetically expensive reproduction does not bring the same success as a shorter one, such as asexual reproduction. The second allows to generate a greater number of individuals in less time. We could highlight vegetative reproduction, a type of asexual reproduction consisting of the generation of a complete individual from a fragment of the parental individual. This happens with the water hyacinth. Also interesting is parthenogenesis, a type of sexual reproduction (since one of the gametes is involved) that does not require sperm, the egg is capable of starting cell division on its own and generating a complete individual. Invasive populations of the New Zealand mudsnail reproduce in this way essentially. However, although sexual reproduction is usually more expensive and slower, it also depends on the reproductive strategy followed by the organism. Two types of strategies are distinguished: strategy r and strategy K. The first consists in the massive production of descendants who are left to their own devices. This strategy plays with statistics, since producing a high number of descendants ensures that, statistically, some survive. This is the case of the zebra mussel, capable of producing up to 1 million larvae annually. On the contrary, the organisms that follow the strategy K produce a reduced number of descendants although with a more assured survival, because the parental care appears, etc. This would be the case of birds or mammals, for example.
Phenotypic flexibility or plasticity. This hypothesis refers to the ability of many invasive species to tolerate or acclimatize to a wide range of biotic and abiotic conditions, usually greater than the range of native species. This feature allows for a more effective acclimatization or adaptation to new or changing conditions. To illustrate this hypothesis, we can turn again to the case of the New Zealand mudsnail, which is capable of tolerating wide temperature ranges (0-29ºC) or wide conductivity ranges (66-7390 μS/cm).
Easiness to hybridize with similar species. This process provides greater genetic variability to the invasive species, which could lead to the acquisition of new traits that favor its invasive potential. We remind the reader of the example of the exotic ruddy duck, capable of hybridizing with the white-headed duck.
Evolution of increased competitive ability. This hypothesis was proposed to explain the invasive success of certain alien plants. Broadly speaking, what it predicts is closely related to the enemy release hypothesis. Basically, when an exotic species leaves its original habitat, it leaves behind its enemies (predators, parasites, etc.). By freeing itself from this selective pressure, mortality and physiological damage decrease, reproductive and expansion success would increase, it could reach more abundant populations, etc. In other words, its adaptive and invasive success would increase. This leads us to the hypothesis of the evolution of increased competitive ability. If that species has freed itself from its enemies, it can redirect its resources to make its competition against the species of the recipient ecosystem more effective. This is very well seen in plants, which are able to produce a number of toxic substances (actually secondary metabolites generated during the metabolic process) to defend themselves from their predators and parasites. These resources will continue to be produced in the invaded habitat, but if they do not have enemies they can reallocate these resources, for example, to improve their fecundity, grow faster, etc., and thus increase their competitive capacity.
New weapons. According to this hypothesis, invasive species possess a series of “weapons” or biochemical strategies that are obviously novel for native species that have come into contact with these species. On the contrary, the species with which these invasive species previously coexisted in their habitat of origin would be more resistant to these weapons, as they would have coevolved for longer together. In this way, competition in the original habitat would be greater, since the surrounding species are more immune and have adaptations against these weapons that prevent them from retreating. On the other hand, the native species of the recipient ecosystem, since they have never faced these strategies, are unprotected against them, which represents an important competitive advantage for the invasive species. A classic example is that of allelopathic substances.
However, two other main aspects are fundamental to a successful invasion: the traits of the recipient ecosystem, which is where the adaptive abilities of alien species are tested, and the intensity and frequency with which individuals or propagules of the species are introduced. Thus, scientists have long wondered whether some ecosystems are more likely to be invaded than others. Again, a set of hypotheses try to shed light on this:
Propagule pressure. Many invasive species have established because their populations were replenished by the repeated introduction of members of their species or propagules. Possibly, a single introduction would not have been sufficient for these species to form a self-sufficient population. The success of an invasion in this regard depends not only on the frequency of propagule introduction but also on their number.
Empty niche. It could be said that sometimes there are ecosystem functions that are not exploited by anyone, either because the species in the vicinity do not have the necessary adaptations or because some biogeographic barrier prevents their access. In this way, invasive species can take advantage of this void: if there is no one to compete against and, furthermore, these “unowned” ecosystem functions are entirely available, species with appropriate adaptations are guaranteed to succeed.
Enemy release hypothesis. We refer the reader to the description we have given earlier.
Mutualist facilitation or invasional meltdown. Many times, invasive species achieves success by collaborating with other species in the invaded ecosystem. Unity is strength. It must be borne in mind that, just as an exotic species frees itself from its enemies when it leaves its region of origin, it also gets rid of the organisms with which it had mutual interactions or which it took advantage of. The most illustrative cases are those of plants that established mycorrhizae with fungi or actinorrhizae with bacteria. If these plants manage to re-establish these relationships with microorganisms in the receiving ecosystem, they will obtain an important advantage. Many times plants do not necessarily need these interactions, but it is true that by establishing them they can increase their fitness. We refer the reader to the example of the firetree, a plant that managed to invade and adapt in Hawaii thanks to the establishment of a mutualist relationship with nitrogen-fixing soil bacteria, although we could also talk about how the harmful bacterium Xyllela fastidiosa managed to establish itself in North America: it did not succeed until an insect (Homalodisca coagulata) that would serve it as a vector was introduced.
Disturbed environments. An ecosystem disturbed, for example, by humans is more likely to be invaded than one that is well preserved for various reasons that have to do with hypotheses that we have already explained. In the first place, because the disturbance of ecosystems often leads to the displacement of the species established there, thereby reducing competitiveness and increasing the empty niches. Secondly, and in relation to the above, the disturbance of an ecosystem means a greater release of resources. For example, if a region is contaminated and disturbed by industrial waste, the concentration of nitrogen in the soil can be increased, thus favouring alien species that tolerate high concentrations of this element. If this resource pulse occurs from time to time, populations of the invasive species may grow. Even so, we have also seen that invasive species themselves can disturb the ecosystem and thus favour the introduction of new species, as happened with the firetree in Hawaii which, after establishing a mutualist interaction with nitrogen-fixing bacteria, the concentration of nitrogen in the soil increased, thus displacing plants that did not tolerate well higher concentrations of this component and leaving room for more tolerant invasive species.
Pre-adaptations. If an exotic species already carries a series of adaptations to certain conditions, such as disturbance, it will have less difficulty acclimatizing to the recipient ecosystem with properties similar to that of origin.
From all these hypotheses, it can be outlined that ecosystems are more likely to receive more invaders. Very susceptible are those that are poorly conserved or disturbed. So are islands because, compared to continents, they have fewer species, so invasive species would find themselves with less competition and more empty niches. They would also have milder and more stable climatic conditions as islands are surrounded by the sea. Perhaps these hypotheses could explain why 50% of the flora in Hawaii or New Zealand is alien and in continental areas percentages are lower. On the other hand, if we take a particular continent, in general alien species prefer to settle in temperate climates rather than in subtropical or tropical climates. Propagule pressure has much to say about this, although it is a factor that varies over time. For example, the degree of invasion was greater in newly discovered continents than in Europe, Asia or Africa. With today’s globalization, however, this pattern is much more blurred.
To end this section and to add a little more muddle, we will mention a curious phenomenon of population dynamics that occurs with some species, although it is outstanding especially in ants. Several studies have reported significant declines and collapses in the populations of the most important invasive ant species, which could lead to a process of extinction, since low densities in ant populations tend to lead to poorer competitive capacity. It is an extremely curious process, because after a period of population boom and after a period of time, these ants suddenly suffer a collapse and a drastic decrease in their numbers. It is not known exactly what it is due to, if to a modification in the availability of resources, to the appearance of some parasite or predator or to some question related to the reproduction. How can be integrated this boom-bust dynamic in this field? Science will give us the answers over time.
And what do we do?
At this point, the reader will be wondering whether there are solutions to such a complex issue. Fortunately yes, the problem is that many times they are not executed on time and we are too late. It is then when invasive species establish themselves with nothing to do but constantly monitor their populations.
The key measure is prevention. It is imperative to establish a series of stringent measures to prevent as far as possible the intentional or unintentional introduction of alien species. Because one of the things on which scientists agree is to consider any alien species as potentially invasive until scientific evidence proves the contrary through risk analysis. This is known as the precautionary principle. How can an effective prevention system be established? Through three basic pillars: education, legislation and scientific research. The education of the population and of the political and administrative sector is essential in order not to fatten up the problem any further. Sensitizing the population to the risks of biological invasions is essential so that they do not participate. This can prevent people from buying exotic animals, releasing them into the wild (on the contrary, people can approach to certain organisations and institutions to place the exotic pet in responsible care) and they are encouraged to report illegal activities in this regard. On the other hand, administrations need to have quality information to be aware of biological invasions and to allocate funds and budgets for the treatment of this phenomenon.
Education must also be expanded to specific groups, such as fishermen, hunters, conservationists, forest guards, etc., bearing in mind that some cases of biological invasions have been promoted by some of them. These groups can also be trained to prevent them from continuing to commit accidental or deliberate acts that favour biological invasions and, on the other hand, to help detect and capture invasive species. In this way, a detection network can be created to facilitate the early detection of these species. Inspections in certain shops (animal and horticultural shops, florists, nurseries, etc.) also help prevent, as well as check and verify that species that are prohibited by law are not being sold.
There are other methods of prevention in addition to education and awareness. In science, the use of computer models of species distribution is gaining ground in an attempt to prophesy whether a particular species can move to other regions and become invasive. Broadly speaking, these models refer to the distribution of a particular species and a series of biotic and abiotic variables that predominate in that range and act as predictors. By extrapolating these measures it is possible to know with what probability species can be established in a given recipient ecosystem (the more the recipient habitat resembles the original, the greater the probability of settlement) and therefore measures can be established before that species arrives there.
Another way is through the establishment of cross-border controls consisting of experts and specialists to review and inspect goods and cargoes likely to carry propagules of alien species. This is taken very seriously in Antarctica for example, where those trying to enter the icy continent are subjected to intensive decontamination in order to prevent the introduction of new species that could endanger the pristine Antarctic ecological balance.
In addition to prevention, it is essential to create an early detection network system for biological invasions. When an invasion is at its earliest stage, it is much less expensive to control and manage it. This is a critical moment, since by acting conveniently, the small nucleus that has just been established can be effectively eradicated, avoiding greater evils. When it is too late and neither prevention nor early detection has had any effect, the only thing left to do is to control and delimit the populations so that they do not expand further or cross biogeographic and/or political borders. The problem with the latter lies in the fact that a constant and prolonged reserve of money and people will be needed until a solution is found to eradicate the invasive species, an issue that too often becomes impossible.
In order to have a certain success, it is essential, as we have already outlined above, to collaborate on various fronts: science, Public Administrations (Urbanism, hunting and fishing, commerce, Port and cross-border systems, Customs, Agriculture and Livestock, etc.), the general population, even international cooperation, as invasive species do not understand borders. When the links begin to weaken, measures and management cease to be effective. In addition, as a result of the increase in invasive species, it has become necessary to create various programs. For example, the European Union’s LIFE programmes, the Global Invasive Species Programme (GISP), the United Nations Environment Programme (UNEP), etc., stand out. All these factors and actions must be backed up by strong and seamless legislation.
Since biological invasions began to be considered as a serious problem, both national and international bodies have implemented numerous laws or reformed pre-existing laws to deal with this phenomenon. For example, due to the increase of maritime traffic and the transfer of stowaways in ballast waters, the International Convention for the Control and Management of Ships’ Ballast Water and Sediments was adopted on 13 February 2004. In this way, vessels of Member States are obliged to undergo inspections and examinations of ballast water and the relevant documentation in order to minimize the transfer of alien species.
Focusing on the European Union, one of the most outstanding legislations is Regulation 1143/2014, which entered into force on 22 October 2014. The purpose of this Regulation is as follows:
“This Regulation sets out rules to prevent, minimise and mitigate the adverse impacts on biodiversity of the introduction and spread within the Union, both intentional and unintentional, of invasive alien species.”
It applies to all invasive alien species with the exception of a few cases (Article 2). One of the most important contributions of this Regulation is the adoption of a “list of invasive alien species of Union concern” (Article 4) which is reviewed from time to time and which is open to the inclusion or exclusion of taxa. What does it mean if an alien species is included in this list? From then on, a number of restrictions apply which all Member States have to abide by, namely: these species are prohibited from being intentionally introduced, kept, bred or cultivated, transported, placed on the market, used or exchanged or released into the environment within the E.U. Indeed, the adoption of these lists is commonplace. They are a useful tool for setting priorities on which to focus management efforts. These lists can be divided into three types: black lists, those that include fully recognized invasive species, white lists, which include exotic species with low probability of becoming invasive, and grey lists, which include questionable taxa that are not known to be invasive or harmless. Of course, the inclusion of species on these lists must be supported by scientifically proven risk analyses.
Spain also has its own legislation to deal with biological invasions. Since 1995 it has been a crime against the environment to release or liberate harmful alien species. However, the most important laws on the subject are Ley 42/2007 and Real Decreto 630/2013. In the first, the Spanish Catalogue of Invasive Alien Species was created for the first time, which includes nearly 200 invasive species of animals, plants, algae and fungi and has since undergone several modifications. The Real Decreto served to regulate this Catalogue and to update it. Thus, the inclusion of a species to this list
“[…] entails a generic prohibition on the possession, transport, trafficking and trade in live or dead specimens, their remains or propagules, including foreign trade.”
And what about the owners who owned any of the species in the catalogue before it came into force? They have the opportunity to continue maintaining them, although with the obligation to inform the corresponding Autonomous Community of their existence and register it. Of course, their breeding, marketing, etc. are totally forbidden. It is also forbidden to introduce these species into the nature, to return them to the nature if they are extracted from it (i.e. a fish included in the Catalogue cannot be returned to its habitat if it has already been fished) and to breed or cultivate them. These prohibitions do not apply to species that, although they form part of the catalogue, are going to be used in research, health or as biocontrols.
This Real Decreto also establishes various preventive measures, such as the requirement by the competent authorities for promoters of works on watercourses to inform of the presence of species in the catalogue before carrying out a transfer or diversion of the body of water or the possibility of using hunting and fishing as control and eradication measures. Again, any citizen or organisation may suggest the inclusion of new species in the catalogue, always on the basis of scientific-technical evidence that the species may be a danger to the environment and biodiversity. Approval of the inclusion or exclusion of taxa ultimately depends on the Ministerio de Agricultura, Alimentación y Medio Ambiente. For example, in the last year the domestic pig, the savannah monitor, the royal python and, in the case of the Canary Islands, the tree tobacco and the pampas grass have been included in the catalogue.
A bleak future
There’s still a lot to do. Although there are laws that prohibit the trade or possession of invasive species, a brief search on the Internet serves to realize that the sensitivity has not yet permeated enough, not for nothing advertisements for the sale of raccoons, parakeets and so many other species can still be found. The same applies to the illegal trade in animals, a scourge with perfectly organized mafias that operate around the world favoring biological invasions.
Legislation, as we have said, is fundamental. Unfortunately, economic interests sometimes take precedence over science and environmental sensitivity and succeed in reversing these legislations. In Spain there has been a debate about whether to consider species of broad economic interest such as the red swamp crayfish or the rainbow trout (Oncorhynchus mykiss) as invasive and include them in the Spanish Catalogue of Invasive Alien Species, which would lead to a ban on using them for economic purposes. Although the red swamp crayfish was included in the catalogue, in Real Decreto 630/2013 it was partially pardoned in order to be marketed in the food industry. In the case of rainbow trout, it does not even appear in the catalogue, in fact it is allowed to release it in a controlled way from monosex crops and after having gone through a sterilisation process (it would be necessary to check if this is strictly complied with or if there can be human errors). However, in 2016 the Supreme Court supported the appeal filed by Ecologistas en Acción, Sociedad Española de Ornitología and Asociación para el Estudio Mejora de los Salmónidos. Since then, the additional provision allowing the commercialization of the red swamp crayfish was eliminated and rainbow trout was included in the catalogue along with other species. Unfortunately this did not last long. In 2017 the Partido Popular presented a proposal for a law that would allow to modify the Ley de Patrimonio Natural to exculpate the red swamp crayfish, rainbow trout and other species of economic interest. It finally was approved thanks to the support of other parties, something that doesn’t work in the management and treatment of biological invasions’ favor.
There is one facet of biological invasions to which we have not paid much attention but which is extremely important. Biological invasions have become a fundamental element of anthropogenic global change, i.e. they already play in the same league as climate change, habitat fragmentation, increased concentration of atmospheric CO2 and alteration of its composition, alteration of biogeochemical cycles of nitrogen and other compounds, increased eutrophication and pollution, etc. And what does this imply? Well, something essential to better understand biological invasions: this element is interconnected or is directly influenced by the other elements of global change. In fact, this does not help to make the predictions flattering. A famous review published by Jeffrey Dukes and Harold Mooney in the journal TREE, both of Stanford University’s Department of Biological Sciences, determined that the rest of the elements of global change will favor the prevalence of many invaders. If global temperature increases and brings warmer temperatures to cold areas, we will surely see that those regions will be colonized by invasive species that previously had access to those areas vetoed by low temperatures. The same is predicted for Antarctica by a research published in Nature: the melting of the continent and the increase of temperatures will favor the expansion of invasive species to the detriment of endemic species that are closely linked to the current environmental conditions of Antarctica. If climate change threatens the rainfall regime of certain areas and makes them more arid, the species able to tolerate water stress better will prevail (many of them will be invasive), and so on with the rest of the elements.
From all this we can conclude that biological invasions do not present a promising future. Experts agree that invasions will grow, along with the rest of the elements of global change. The challenges they pose are undoubtedly formidable and we all have the key to reducing or diminishing their effects. It is time to get serious and stop giving the invaders a truce.
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