Introductions of new species should be viewed as natural within a large geologic timescale.

Common Lodge
Apr 3, 2019 · 22 min read
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Opening: For

Insight from History

F.O.1 We are in Utah. It’s the late Tertiary period, two million years ago. Looking across the landscape, we see rolling mountains and valleys. Volcanic eruptions spew debris in the distance. Streams snake through the valleys joining a giant lake that stretches through current day Nevada, Idaho, and Utah. After the extinction of the dinosaurs, mammals diversified. Now camels, mastodons, horses, and ground sloths graze, while saber tooth cats and giant wolves stalk nearby.

The Concept of Invasive Species is Erroneous

F.O.4 Invasive species often cause biodiversity loss within a habitat. This statement introduces the topic in countless scientific papers, which often end with a call for management or eradication of said species. This is the prevailing view and bias: human activity has introduced countless invasive species that can threaten the integrity of ecosystems by eliciting changes within native communities. Therefore, we need to mitigate their impacts by focusing on prevention, management, and eradication.

Environments Change, Species Adapt

F.O.8 The planet is an enormous living, breathing organism that has changed and evolved for the past 4.5 billion years. No two snapshots are the same. Scrolling through time, tropical landscapes become deserts; forests rise out of freshwater lakes; gargantuan glaciers melt to reveal waving prairies; continents move; species split, morph, and evolve, including humans; others disappear.

The Way Forward

F.O.16 Once science identifies a harmful invasive species, efforts get underway to control it. Trying to manage the infamous Purple Loosestrife has cost $45 million per year (Pimentel et al. 2000). In the United States, $100 million is spent annually on invasive species prevention, detection, management, research, and restoration (U.S. Fish and Wildlife Service).

Opening: Against

What is an alien species?

A.O.1 Species that have been moved by humans to a region where they have not previously existed are known as alien or non-indigenous. This movement can be intentional, such as taking a plant from its native region and moving it to a new region for ornamental purposes. It can also be unintentional, such as species that grow on submerged areas (e.g., hull, keel or water inlets) of boats or ships travelling among ports (Hulme et al. 2008). This unnatural movement of species is yet another direct result of human activity that causes unprecedented and often negative changes in the natural world. Introductions of new species through human means, particularly intentional introductions, are not the natural way species migrate and adapt to new climates. As such, these species are typically not dispersing as a survival strategy, but rather, are being taken from their native environments by humans, and being moved to new regions, where in some cases they are unable to survive and in others, they are able to thrive and often do significant harm.

How do introduced species become successful invaders?

A.O.2 Once a species is introduced, it has overcome the barrier of geography (which usually prevents dispersal unless humans intervene) and there remain several other barriers before being able to establish itself in the new environment (Blackburn et al. 2011). First, it needs to be able to withstand the new environmental conditions. If environmental conditions in the recipient region are substantially different from that of the species’ native region, it could either die off, or adapt to the new environment.

What impacts do they have?

A.O.5 Environmental impacts imposed by invasive species include changes in native community composition, competition for resources, hybridizing with native species, population loss, and changes in ecosystem processes. Often, environmental impacts can threaten the economy, social structure and health of humans. While the majority of impacts observed have been negative, impacts can also be positive, as is the case with the African Black Oystercatcher in South Africa (Branch and Steffani 2004): an invasive mussel, Mytilus galloprovincialis has acted as a food source for these birds, allowing them to increase their reproductive output and re-establish their declining population numbers. Invasive species are, therefore, not always bad. Why, then, should we prevent, manage or eradicate them? For fear of the unknown? Or do we have evidence that invasive species can cause real and dramatic impacts on our environment, our economy, and our society?

Examples of invasive species in a terrestrial, marine, and freshwater ecosystem

A.O.7 A group of terrestrial invasive species that is receiving increasing attention, due to the conflict it has created, is the Prosopis tree genus (Mesquite and related plants). Known as one of the world’s most damaging woody invasive taxa, it was introduced to many regions for its various uses such as fuelwood, fencing poles and fodder. Prosopis, therefore, starts off being quite beneficial but if left unmanaged, its negative impacts outweigh the benefits (Wise et al. 2012). These impacts result from the formation of dense, thorny thickets, and include reducing the water supply to humans, plants and animals, reducing native forest cover, adversely affecting livestock health, and restricting access to water sources, roads, and other areas in villages.

Management can reduce or prevent the impacts of invasions

A.O.13 Not all introduced species cause negative impacts from an environmental, social, or economic standpoint. However, those that do, have demonstrated clear and sizable negative consequences, with real-world challenges. Attempts to address the problem of invasive species, therefore, focus largely on the prevention of their introduction in the first place (Simberloff et al. 2013). While eradication is initiated in regions where it is affordable and where negative impacts have outweighed the costs that would be required to counteract those impacts, more and more regions are aiming to prevent introductions. When invasive species are left to thrive, the impacts can become overwhelming, costly, and often harmful to ecosystems. Do we stand aside and allow ecosystems to degrade and livelihoods to crash? Or should we do something about it? We can and should, by implementing management programs that are able to reduce or prevent negative impacts, before they reach catastrophic levels.

Rebuttal: For

Humanity’s Involvement: A Red Herring

F.R.1 In paragraph F.O.5, I introduced two problems underlying the traditional view toward invasive species. First is the assumption that human interference of species’ distributions is unnatural. But it isn’t. We’re just a species that’s wildly successful at colonizing and altering habitats (that makes us an invasive species). In her opening paragraphs (A.O.1 through A.O.4), my opponent makes this assumption while failing to explain what quantifies native environments, or natural species movement.

The Impact of Time

F.R.5 The second problem introduced in F.O.7 is the opposition’s adoption of a short timeframe. This perpetuates the illusion of static environments in which change is often seen as negative, especially when attributed to invasive species. The author’s arguments in A.O.5 and A.O.6 operate from this snapshot perspective.

Management is Unrealistic and Unpredictable

F.R.9 In A.O.13, the author poses a final question: “[should] we stand aside and allow ecosystems to degrade and livelihoods to crash?” She answers saying no, we should manage invasive species via prevention and eradication “before [impacts] reach catastrophic levels.” First, change doesn’t equal degradation, as mentioned above.

Rebuttal: Against

The key arguments in the authors opening statement has been identified and addressed below.

1. Humans are part of the natural world and our role in species movement should be left as is.

A.R.1 While I appreciate the opponent’s view regarding the definition of invasive species in F.O.5, current invasions cannot be equated to those in the fossil record. The present scale of invasions, their subsequent impacts and evolutionary importance is unique. We have created pathways for species to spread faster, further away, and at much higher densities than ever before (Richardson and Ricciardi, 2013). Human-created pathways are also profoundly distinct from species’ natural dispersal mechanisms as mentioned in A.O.1.

2. Invasion scientists view the natural world as a static environment and aim to preserve a world that does not change by focusing on a short time-span.

A.R.4 The author suggests in F.O.7 and F.O.15 that all forms of species movements, including human-mediated introductions change ecosystems and by managing introductions and looking at a single snapshot, we advocate for a pristine and unchanged environment. I disagree. We acknowledge natural dispersal mechanisms and natural geographic range shifts of species that ultimately result in constantly changing ecosystems. We further acknowledge naturalized species, which are introduced species that have self-replacing populations outside of captivity or culture but have not spread from their point of introduction. As such, invasion scientists are not aiming to create a static environment but rather trying to prevent the evidence-based impacts that can occur as a result of human-mediated introductions. Furthermore, invasion ecologists are assessing new concepts such as novel ecosystems (Richardson and Gaertner, 2013), mentioned in F.O.17, among others, in order to gain a better understanding of global change effects.

3. Scientists only seek out the negative impacts of introduced species, with the assumption that native species do not cause harm.

A.R.5 In F.O.12, the author states that not all “newcomers will damage the landscape”, a point that has already been addressed in A.O.5. The author then continues, declaring that “scientists try to identify how an invasive species is having a negative impact.” However, scientists try to determine whether there is an impact at all, in order to inform management decisions and whether action is indeed warranted.

4. Spending money on controlling invasive species is ill-placed and should rather be directed towards other, imminent threats.

A.R.7 While I agree that money could often be better spent on more pressing issues (see F.O.18), I don’t agree that we should neglect oversight of our own movements of species that could cause and have caused harm. By suggesting that we let introduced species run their course and harm human livelihoods, and doing nothing about it when we have the ability to do so, debunks the opponent’s portrayal of species (including humans) adapting to change. Implementing management programs is one of our ways of responding to a changing ecosystem as a result of our own actions. This is exactly why prevention is important, so that we are not forced to eradicate species that we introduced in the first place.

Closing: For

Species on the Move, Naturally

F.C.1 The global movement of species into new environments has been a phenomenon throughout time. Species move in response to changing climates and ecosystems within broad evolutionary timescales. Static, or pristine, environments in which certain species have always been present don’t exist. Impermanence rules.

Misguided and Distracted

F.C.4 If the opposition’s desire is to prevent species introductions (A.O.13 and A.R.7), the solution is clear: We must cease altering environments in ways that encourage species’ movement. Practically, that’s not possible. Our species will grow alongside novel environments, technology will advance, and global trade and travel will increase. In response, species will adapt to their changing surroundings and move to new environments (F.O.10).

Closing: Against

Human activities are the ultimate cause of invasive species threats

A.C.1 Humans are a part of the natural world, but our conscious, increasing and accelerated movement of species cannot be viewed as a natural way for species to disperse. The core arguments to support this are as follows:

  1. A.C.1.2 Negative impacts of invasive species currently outweigh positive impacts (A.O.6). This is prevalent in numerous ecosystems and is supported by evidence-based research (for examples, see A.O.7 to A.O.12). “This is further supported by cases when invaders have demonstrated stronger impacts in their introduced ranges compared to those in their native ranges” (A.R.6).
  2. A.C.1.3 Humans are cognizant of the ecological and socio-economic impacts and have the ability to address them, as with every other human-induced threat. Therefore, implementing management programs is our duty, and the best approach currently is to prevent or reduce harmful introductions (see A.O.11 and A.R.3).



Haley R. Pope has a Master of Science in Zoology from Stellenbosch University in South Africa where she studied the spread, life-history strategies, and impacts of an invasive marine barnacle under the context of climate change. Her research, related to the topic of this debate, can be found here. Haley is the President of TerraLens Photograph, providing conservation photography, archiving, and writing services.


Koebraa Peters has a PhD in Zoology from Stellenbosch University in South Africa. For the past 8 years, she has worked extensively on invasive species. Her work on marine invasive species has included developing prioritization tools for monitoring alien species in regions where resources are lacking. Koebraa is published in the Journal of Environmental Management, BioInvasions Records, Management of Biological Invasions and African Journal of Marine Science. Her research, related to the topic of this debate, can be found here.

Common Lodge

Common Lodge is a place for written debates on cultural and…

Common Lodge

Written by

A place for written debates on cultural topics between opposing academics, practitioners, and public intellectuals.

Common Lodge

Common Lodge is a place for written debates on cultural and political topics between opposing academics, practitioners, and public intellectuals.

Common Lodge

Written by

A place for written debates on cultural topics between opposing academics, practitioners, and public intellectuals.

Common Lodge

Common Lodge is a place for written debates on cultural and political topics between opposing academics, practitioners, and public intellectuals.

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