As is well known, invasive species are a global problem of exclusively anthropogenic cause. Each and every one of the invasive species that exist in the world is because they have been introduced by the human hand , in one way or another. However, if the human being is just another animal and forms part of nature, wouldn’t it be logical to think that all transport mediated by anthropic activity is also natural transport?
In addition, after all, species have the ability to move , to move from one region to another, and sometimes to colonize new territories , without the need for human influence. Taking this into account, the process called “biological invasion” would only be the consequence of colonization, a natural event in which we have no reason to interfere. That is the basis on which some authors hold a position that, as a result, denies the occurrence of biological invasions . And it is that the ecologists who study biological invasions do not include in the process of naturalization and invasion the species that access new ecosystems through natural colonization processes.
Where are camels from?
For critics, it is a very anthropocentric way of seeing the process, as if only human beings were the cause, as if only we could take a species to a new place where it never was, when in fact the species, naturally, they move and colonize new territories.
Books and articles have been written on the subject, and it has even been suggested that “invasive alien species will be the salvation of the planet”. To support their hypothesis, they take camels as an example, and their true origin. These animals, which today we consider native to North Africa and some regions of Asia, have their origin in the American continent . In the past, populations of camelids crossed the Bering Strait and gradually colonized large areas of Asia, first, and Africa, later.
In reality, this natural colonization process has been going on since life has existed. On different scales, it has been going on for billions of years. What is so special about the cases of the American mink, the eucalyptus or the blueberry, that we consider them “invaders”?
The climate change analogy
A very good analogy can be drawn between the colonization process of species and changes in climate. The climate , due to its inherent conditions, is a changing entity and, in fact, has undergone great modifications throughout the history of the earth; Similarly, the distribution of living beings is also changing, and has been modified throughout the history of life.
Why then do we speak of an anthropogenic climate change , if the climate is already a changing entity in itself? What is special about the climate change that we are currently experiencing? The answer lies in the speed at which it happens, and in the magnitude . No climatic change in the history of the earth has been of such magnitude in such a short time. And that has direct consequences: it happens so quickly and so abruptly that species and ecosystems don’t have time to adapt to the changes.
What differentiates biological invasion
The transport of species by humans also has a different importance than natural colonization events. Returning to the camel example, molecular studies and the fossil record indicate that this colonization event began about 11 million years ago. It is clear that these animals did not arrive from North America to Morocco in days or hours – as happens today if they are transported by ship or plane – but in thousands, tens or even hundreds of thousands of years. So long, in fact, that they had enough margin so that, from the same common ancestor, there are now seven species belonging to three different genera: Vicugna , Lama and Camelus .
For one thing, the speed at which we move has not been matched by any living thing on the planet in all of Earth’s history. Taking just one train and two-stop flight, you can get from Valladolid, Spain, to Wellington, New Zealand in less than 36 hours—two cities that are almost perfect antipodes. This trip for most living beings is simply impossible to do, because biogeographical barriers would prevent it, and the few that can do it would take weeks, or months.
Another factor to take into account is the number of individuals being transported, the so-called propagule pressure . This represents the magnitude . The greater the quantity, the greater the chances that a species will reproduce successfully, spread uncontrollably and generate impacts on the environment.
Greater volumes of cargo are being transported in less time, and when that cargo is living beings, it translates into an increase in the number of invasions.
The natural processes of colonization are slow, gradual, gradual and give the ecosystems the opportunity to adapt to the new tenants, as well as the latter to adapt to the former. But when humans transport species outside their native distribution ranges, they do so quickly, almost suddenly, and very massively. As is the case with anthropogenic climate change.
The new Pangea in the ‘anthropocene’
With the breakdown of biogeographic barriers we are turning the biosphere into something similar to what it was during the last great supercontinent Pangea. The active transport of species from one place to another acts as a bridge between the continents. But again there is a difference of magnitudes: the formation of the supercontinent Pangea was a process that lasted tens of millions of years, with continents colliding and merging into increasingly extensive land masses slowly and gradually, while this ‘ new Pangea ‘ that we’re creating through mass transport of species is much faster: it’s barely happening in any significant way since the end of the 15th century, and it’s really worrying in the last century.
Again, we have the difference between a slow, gradual, incremental change and a fast, abrupt, massive change . And if the study of life on earth teaches us anything, it is that rapid, abrupt and massive changes rarely end well.
References:
Blackburn, T. M. et al. 2019. Alien versus native species as drivers of recent extinctions. Frontiers in Ecology and the Environment, 17(4), 203-207. DOI: 10.1002/fee.2020
Lean, C. H. 2021. Invasive species increase biodiversity and, therefore, services: An argument of equivocations. Conservation Science and Practice, 3(12), e553. DOI: 10.1111/csp2.553
Mckinney, M. L. 2005. New Pangea: Homogenizing the Future Biosphere. Proceedings of the California Academy of Sciences, 4th series, 56, Supplement I(11), 119-129.
Russell, J. C. et al. 2017. The Rise of Invasive Species Denialism. Trends in Ecology & Evolution, 32(1), 3-6. DOI: 10.1016/j.tree.2016.10.012
van Kleunen, M. et al. 2015. Global exchange and accumulation of non-native plants. Nature, 525(7567), 100-103. DOI: 10.1038/nature14910
