Around invasive alien species there are numerous controversies, including those who deny their mere existence. Part of his argument is based on the very definition of an invasive species: one that, having been transported by the human hand, becomes naturalized in a new natural environment and manages to disperse.
Biological invasions or colonization?
The defense of these ‘deniers’ is that species already move naturally, colonize new territories, and that a group of animals, over millennia, can come from the American continent to colonize North Africa. In fact, it is the case of the origin of the camels, without the human hand intervening in the process. But following the same argument, one could speak of anthropogenic climate change that we are currently experiencing.
As well as the objective fact that the climate changes naturally over time, the seriousness of anthropogenic climate change lies in its great magnitude and the speed at which it is happening. Similarly, colonization processes differ from biological invasions to the same extent: the human hand transports species much more massively, faster, and with greater magnitude.
And as with anthropogenic climate change, many populations and ecosystems are unable to adapt to sudden change, nor are native species able to adapt to the presence of new ones, which have massively colonized the new ecosystem in a few centuries or even decades, and gradually, in tens or hundreds of millennia or in millions of years.
For a biological invasion to occur, therefore, the first step is the transport of species from one place to another , crossing biogeographical barriers. That transport is mediated by human activity. Although sometimes living beings are transported deliberately and sometimes not.
Deliberate transport of species
Human beings have deliberately transported alien species for millennia, for various purposes, although early in our history this transport was slow and of low magnitude.
We can identify three moments in the history of humanity in which radical changes occur in the rate and magnitude of such transport.
The first coincides with the end of the Middle Ages , with the creation of new maritime routes for the transport of spices, and the arrival in America of crops typical of the Old World, and vice versa. At this time, plants such as potatoes, tomatoes, corn, tobacco or cocoa began to arrive in Europe.
The second significant historical moment is the Industrial Revolution , and the consequent improvement of transport systems, which allows the transfer of a greater quantity of merchandise in less time.
We are currently experiencing the third moment and it coincides with the phenomenon of globalization . Since the 1950s, technical and logistical improvements have accelerated the ease with which all kinds of goods, including invasive species, are transported. At this time, the deliberate transport of species, the starting point for biological invasions, is directly related to economic growth. In general, higher economic levels are associated with more exotic flora and fauna. And it is that many of these plants and animals enter through trade .
Today, the deliberate entry of invasive species into new, more common ecosystems is almost exclusively for commercial purposes; either the entrance of ornamental plants , such as the Spanish flag; or timber harvesting , such as eucalyptus; or of animals considered as pets —despite being wild fauna, such as the coati, the raccoon, the tanuki or the Kramer’s parrot—; or for use in hunting or fishing , as happened with the black bass or the arruí
In most cases of deliberately introduced invasive species, the source of the introduction can be found by following the money trail.
Accidental transportation
But not all invasive organisms enter with human intent. Throughout history, many animals, plants, and microorganisms have infiltrated transportation systems as stowaways .
The black rat is closely associated with human populations, practically since the human being became sedentary and the first relatively permanent settlements were formed. Its small size, high reproductive rate, and ability to feed on almost anything made this animal a perfect candidate for world invasion. From the Middle East , their region of origin, they colonized Eurasia and Africa. They arrived in the British Isles with the Romans, in the 1st century, and in the American continent with the Spanish settlers, at the end of the 15th century. And, finally, they landed on the shores of Botanic Bay in Australia, with the arrival of the eleven ships of the First Fleet, under the command of Captain James Cook, in 1770.
With the arrival of the Niña, the Pinta and the Santa María, not only settlers and rats arrived in America. They also landed pathogens still unknown in the New World, such as smallpox. And it is that diseases are also invasive species, which arrive through animals or plants that act as vectors. Furthermore, these vectors can also be invasive species. This was the case with certain species of parrots in Hawaii, which in addition to invading the islands, transmitted diseases to local birds such as avian malaria or avian pox, extinguishing entire lineages in the process.
Living things can accidentally arrive through many different means. Insect eggs or tiny seeds can travel in dried mud on shoes or clothing; insects and other animals can arrive with the ornamental plants; algae, crustacean larvae or fish fry can be easily transported from one coast to another thanks to the ballast water of large freighters…
Better safe than sorry
But all this phenomenon has a positive point. Knowing and understanding these routes of introduction of invasive species can help us avoid repeating the mistakes of the past . It is possible to develop strategies that allow predicting which species are candidates to become invasive in a given place and prevent their introduction, either by regulating trade and prohibiting their possession, or by implementing effective measures that prevent the accidental transport of these species. .
Applying the proverbial principle that “ better safe than sorry ”, in a problem such as invasive species, should be considered of paramount importance in some regions. Not without reason, invasive species are now the second largest driver of global change – after climate change – and the leading cause of biodiversity loss in the world.
References:
Bayon, A. et al. 2019. Horizon scanning to identify invasion risk of ornamental plants marketed in Spain. NeoBiota, 52, 47-86. DOI: 10.3897/neobiota.52.38113
Blackburn, TM et al. 2011. A proposed unified framework for biological invasions. Trends in ecology & evolution, 26(7), 333-339. DOI: 10.1016/j.tree.2011.03.023
Hulme, PE 2009. Trade, transport and trouble: managing invasive species pathways in an era of globalization. Journal of Applied Ecology, 46. DOI: 10.1111/j.1365-2664.2008.01600.x
