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

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.

F.O.2 Fast-forward 400,000 years. It’s the Pleistocene epoch. The Ice Age is in full swing. Mammoths invade North America from Asia and encounter Utah’s native flora and fauna. Over the next 1.5 million years, species compete and adapt as cycles of glaciation and deglaciation change landscapes and, with that, the requirements for survival.

F.O.3 Then, about 10,000 years ago, after Homo sapiens had arrived, the game changed ( Utah Geological Survey). As the Ice Age came to a close, humans proliferated and dominated. By the end of the Pleistocene, there was a net loss of biodiversity as extinction claimed all the species mentioned above (Utah Geologic Survey). The invasion of humans and subsequent loss of species has occurred throughout the world (Boivin et al. 2016).

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.

F.O.5 The problem with this assessment is twofold. First, the definition of ‘invasive’ hinges on human involvement. An invasive species is not only non-native with the potential to cause harm, but also humanity must have played a role in its introduction (U.S. Department of Agriculture). Conversely, native species haven’t been introduced by humanity and don’t cause harm.

F.O.6 However, no landscape, seascape, or species has escaped the influence of humankind. Through agriculture, animal and plant domestication, and the destruction and creation of ecosystems, we have affected Earth’s climates and species distributions and extinctions (Boivin et al. 2016). According to those definitions either no species is native, and all are invasive — an absurd idea — or we need to rethink the invasive species concept. The definitions also assume humans are separate from the natural world and therefore, our interference with species’ distributions is unnatural.

F.O.7 Second, this view employs a short time span, creating the illusion that landscapes are, and should be, static. Any change to a habitat would then be obvious and considered detrimental, since we are looking at a single snapshot in history instead of watching the entire movie. But our planet is in a constant state of physical and evolutionary flux. It’s naive to view the world as if frozen in time. The movement of species into new geographic areas is not only an expected consequence of changing climates and ecosystems within a broad evolutionary timescale, but those species are not inherently detrimental to native communities. Ultimately, it’s not realistic to control species’ movements. We need to reassess our view of invasive species.

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.

F.O.9 With such constant motion, how does one determine which species are native, if that is even an important question? In Utah, were dinosaurs native and mammoths invasive? What about humans? They certainly changed native communities. I’d argue that geographic shifts are part of the normal ebb and flow of species responding to changing environments.

F.O.10 As habitats change, the requirements needed to live there change and interspecies competition for resources like food and space is affected. In such a scenario, each species must adapt to survive — natural selection demands this. Those who can’t adapt don’t survive. Adaptations come in many forms: as random genetic mutations that are passed on, or as new behaviors which turn out to be beneficial for survival. But adapting also includes physically moving to an environment that better suits one’s current biology. This is the case especially if the rate of environmental change or intensity of competition pressure doesn’t allow a species time to evolve.

F.O.11 This is the scenario today: For many species the only option is to physically move and encounter species they haven’t competed with. If successful, they will carve out a niche in the landscape maybe at the expense of other species, but maybe to the benefit of local biodiversity if their immigration encourages the presence of other species (Sax & Gaines 2003). If they aren’t successful, they may be forced to move or perish.

F.O.12 While change is almost certain, new species aren’t inherently worse than resident species and it’s not a given the newcomers will damage the landscape. Nevertheless, scientists try to identify how an invasive species is having a negative impact on the native community. There isn’t a shortage of studies concluding that they do. The Invasive Species Specialist Group outlined the world’s top 100 worst invasive species, claiming each brings nothing short of calamity (Global Invasive Species Database).

F.O.13 Purple Loosestrife is a wetland plant notoriously invasive to the northern USA where it displaces native species and alters habitats. Researchers concluded that its presence signified an ecological disaster (Thompson et al. 1987). But what if invaded habitats were assessed over longer time spans? While local biodiversity may fluctuate after the emergence of new species, it’s unlikely to remain in decline. In fact, a meta-analysis found no evidence of local plant biodiversity loss over time (Vellend et al. 2013). Habitats are instead undergoing species diversity change, not net loss (Sax & Gaines 2003; Dornelas et al. 2014).

F.O.14 Surely not all invasive species are harmful. Where would we be without wheat, rice, corn, or soy? What about chickens or cows? Humans domesticated and introduced them and they have significantly altered native communities, yet we don’t consider them invasive even though by definition we should. Here the prevailing bias comes into full view. Studies exploring the benefits of invasive species are practically non-existent. But, the absence of evidence isn’t evidence of absence. Ken Thompson, in the book Where Do Camels Belong, presents a strong case for exactly that. He points instead to a lack of interest and funding for studying species that don’t appear to be a threat, while simultaneously downplaying the potential benefits of new species.

F.O.15 Landscapes change and species move in response to environmental conditions, which are affected by humanity. Not only are we making the unreasonable assumption that invasive species are bad and represent a threat (they only represent a threat to our idea of a static environment), but also the findings of studies are dependent on their spatial and temporal scope. Time and scale are limiting factors in understanding how species in a landscape affect one another and an ecosystem. Such complex interactions can only be understood from the point of view of evolutionary time.

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).

F.O.17 As long as we view the natural world as static and immigrating species as harmful, we will continue to spend billions to control them. This simply doesn’t make sense. Once we discard these incorrect notions, we see the futility of controlling the inevitable movement of species. Frankly, it’s not possible and it’s the wrong approach. Boivin and her colleagues declared in a 2016 paper: “The impact of human agency on ecosystems is neither completely avoidable nor entirely undesirable. . . Rather than an impossible return to pristine conditions, what is needed is the historically informed management of emerging novel ecosystems to ensure the maintenance of ecological goods and services.”

F.O.18 We should spend our money, time, and resources to address encompassing and imminent threats like climate change, pollution, and human population growth. If we address those top-tier issues, we will mitigate a host of additional problems that affect biodiversity, ecosystem functioning, species extinctions, and human health and safety. Let’s focus on the underlying sickness rather than the benign symptom of species on the move.

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.

A.O.3 The next barrier is reproduction, which often depends on how many individuals are introduced, their sex ratios, and reproductive strategies. For example, if enough males and females are introduced, there is a greater chance for them to successfully reproduce. In some instances, pregnant females are introduced (e.g., a crab carrying eggs) and in this case there is no need for a male. Consequently, if a species reproduces asexually (e.g., a plant that can reproduce via budding) then the likelihood of that species spreading successfully is high, as it takes only a single individual to reproduce.

A.O.4 Once a species has survived and is able to reproduce successfully, it is known as an established alien species. After establishing a population in the new environment, if the species is able to disperse from the point of introduction, then it is classified as an invasive species. Invasive species are, therefore, those alien species that produce self-sustaining populations, spread from their point of introduction, and create impacts that threaten ecosystems and species in recipient regions.

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?

A.O.6 This evidence certainly exists. The positive impacts, which are far less documented than the negative, can often be considered transient, whereas negative impacts can be permanent (Richardson and Ricciardi 2013). Negative consequences of invasions manifest in a variety of ecosystems including terrestrial, marine, and freshwater environments. The increase in human population, trade, and developments in technology has changed the ways in which species are transferred, causing a rise in species introductions (Seebens et al. 2017). Initial introductions were land-based and generally intentional, for consumptive, medicinal, or ornamental purposes. With the development of ships and commercial trade, species were able to cross oceans. Advances in development allowed for several pathways to emerge, including railways, roads, shipping, aircraft, aquariums, and even the pet trade. As a result, most native ecosystems are exposed to introductions.

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

Terrestrial
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.

A.O.8 Studies have reported that more than 90% of livestock farmers find Prosopis to be problematic and the invasion has resulted in serious and violent conflict among communities in Ethiopia and Kenya. This phenomenon is related to the scarcity of natural resources as a result of the Prosopis invasion, which dramatically affects the farmers’ livelihoods (Shackleton 2016). This invasion, therefore, displays clear negative impacts on biodiversity and socio-economic stability.

Marine
A.O.9 The marine environment is not devoid of invasive species that cause detrimental impacts. Along the coasts of Tasmania and mainland Australia, the invasion of the northern Pacific Sea Star, Asterias amurensis, has caused noticeable changes in marine communities, significantly reducing densities of native mollusk species. Since some of the native fish species are dependent on mollusks for food, competition for resources with the Sea Star resulted in declines of fish populations (Parry and Hirst, 2016).

A.O.10 Furthermore, native mollusks were unable to recognize the invasive Sea Star as a threat (Hutson et al., 2005), supporting the theory that escape responses of marine invertebrates have evolved in relation to predation risk. This type of prey naivety puts native marine species particularly at risk from invasive predators. The Sea Star is also able to shift its diet to feed on other species when the main component of its diet occurs at low densities, putting other species at risk of depletion and further affecting prey availability for native predators. In addition to the environmental impacts, the Sea Star also affects commercial bivalve farms, since many of the mollusk species that it feeds on are used in the bivalve farming industry (Ross et al. 2002).

Freshwater
A.O.11 Similarly, freshwater invasions have displayed serious consequences to native species and the economy. One example is that of the parasitic Sea Lamprey, Petromyzon marinus, one of the oldest living vertebrate lineages alive today. This invasion goes back to the early 1920s, when modifications to a canal that links lakes in the United States resulted in the movement of Sea Lampreys into the Great Lakes. They parasitized large predatory fish species, such as Lake Trout, by attaching their sucker-like mouths directly onto the fish, feeding on their blood and bodily fluids, ultimately resulting in the hosts’ death. The invasion resulted in the collapse of commercial and recreational fisheries of the Great Lakes, causing devastating economic impacts with cascading effects on food webs within the ecosystem (Smith and Tibbles 1980; Miller et al. 1989).

A.O.12 Often, when a native predator is removed from a food web, prey species can dramatically increase in numbers because the absent predator can no longer naturally regulate their population size. This effect results in overconsumption of resources further down the food chain, which can include other prey species, as well as primary producers such as plants or algae. There results an imbalance in the ecosystem, and since so many ecosystems are interconnected, the effects can further influence the ability of the system to recover.

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.

F.R.2 One can extrapolate that a species’ native environment is one where they weren’t introduced “through human means” via pathways mentioned in A.O.6, and that natural movement means species are “dispersing as a survival strategy” on their own (A.O.1). However, to find species whose distributions were not influenced by humans, we need to travel back over 200,000 years in Africa — before our species evolved — and 60,000 to 2,000 years ago in other continents. Humans have been affecting species’ abundances and ranges since the dawn of our evolution (Boivina et al. 2016). How can we possibly define ‘native’ and ‘invasive’ based on humanity’s involvement?

F.R.3 The author also mentions that it’s challenging for species to overcome geographical barriers without human help (A.O.2). But it’s not impossible or unlikely. We only need to account for genetic and environmental variability, species interactions, and time (Case et al. 2005). A species with high genetic variability could invade a habitat exhibiting gradual environmental differences given enough time.

F.R.4 Even lacking some of those conditions, colonization is possible. Hawaii and the Galapagos are volcanic islands that haven’t been connected to other landmasses. To reach them, species overcame seemingly insurmountable barriers by sea and air. It is presumptuous to assume that species considered invasive never would have reached those locations without us. If they had, they wouldn’t be considered invasive even if they dramatically changed recipient communities.

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.

F.R.6 While the author admits that not all invasive species are bad (A.O.3), she also says invasive species “display impacts that threaten ecosystems and species in recipient regions” (A.O.4). She creates her case by saying “the majority of impacts observed have been negative” (A.O.5) and that “positive impacts. . . can often be considered transient, whereas negative impacts can be permanent” (A.O.6).

F.R.7 She doesn’t mention that most observed impacts are negative because it’s easier to secure funding for species that seem to have negative impacts (F.O.12 and F.O.14, Thompson 2014). Yet, her worst oversight is not acknowledging that any impacts can be transient. It’s complicated to determine the direction of change since it depends on perspective and time. What may seem bad now may end up being good in ways we can’t yet comprehend.

F.R.8 I am not arguing that action shouldn’t be taken to address immediate health and safety concerns caused by invasive species (in reference to A.O.7). I’m arguing for a measured approach to understanding their impacts seated within the context of broad timescales. If the impacts mentioned in A.O.9 through A.O.12 were placed within a broader timescale (e.g., centuries or millennia), we would see an accurate picture of what’s occurring and may find habitats and biological communities are not worse off after invasions despite how it may appear from time-limited studies.

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.

F.R.10 Second, prevention and eradication is shortsighted and unlikely to be successful. There are no data on the efficacy of prevention since we don’t know how many species we’ve prevented from invading or how many we could have prevented if actions had been different. Eradication takes precise planning and execution. Efforts should be underway within the first four years of a species’ introduction. But even then, only 50.9% of eradication attempts are successful (Pluess et al. 2012) and 79% of case studies reported continued spread or increasing densities despite management efforts (Shackleton et al. 2019). These are not great odds.

F.R.11 One possible explanation is that invaders and communities adapt to one another’s presence and changes in a landscape; they are immediately and intimately interwoven by the time an invasion has occurred. Ballari and colleagues (2016) even found that the eradication of a single invader can negatively affect native species because of complex food-web hierarchies and interactions.

F.R.12 The opposition’s argument operates on a narrow timeframe, perpetuating the false idea that landscapes are static. It’s based on a host of assumptions including that invasive species threaten recipient environments and our role in species’ distributions is unnatural. This view isn’t accurate or informed.

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.

A.R.2 Yes, we are a part of the natural world but simply ignoring the fact that we are causing species movements that can damage ecosystems and livelihoods, would be to say that we may as well continue burning fossil fuels. By considering ourselves equivalent to any other species, then surely our actions should not be controlled either?

A.R.3 Following the view of the author in F.O.10, the result of our resource extraction should be left as is because we are a part of the system and this is our way of competing for resources, our way of adapting and surviving, as natural selection demands. However, the very fact that we are cognizant of our actions and that we’re able to mitigate and prevent harmful consequences is what sets us apart and influences our duty to act. The author states in F.O.7 that “it’s not realistic to control species movements” but that’s exactly what we are doing, we are moving species around, causing ecosystem impacts and we can address it, as much as we do for every other threat we created.

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.

A.R.6 Furthermore, previous studies have shown that non-native species are more likely to cause negative ecological and socio-economic impacts than native species (Richardson and Ricciardi, 2013). This point is further supported by cases when invaders have demonstrated stronger impacts in their introduced ranges compared to those in their native ranges. In F.O.14, the author makes reference to the lack of studies that explore the benefits of invasive species. There are studies that address both positive and negative consequences of invasions, and for now, it appears that some invasions are initially beneficial, but if left to thrive, species can become problematic (see A.O.7). I would welcome studies that assess benefits of invasive species and their long-term effects.

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.

A.R.8 We should certainly not be excluding ourselves from the natural world, but to simply ignore our active and accelerated role in spreading species beyond their natural biogeographic ranges would be imprudent.

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.

F.C.2 Nor are human-induced species migrations a new phenomenon. By creating novel ecosystems and destroying others, humans have always affected species’ geographic distributions as they adapt to us. That’s how natural selection and evolution work. Thus, categorizing species as native or invasive based on humanity’s involvement is futile (F.O.6).

F.C.3 It’s also a prejudiced view to expect invasive species will “threaten ecosystems and species in recipient regions” (A.O.2) and that negative impacts are more permanent than positive ones (A.O.6 and A.R.5). Ken Thompson states that this view is “justified by a focus on a relatively few species that cause undoubted economic or environmental harm . . . But it ignores the vast majority that do no harm at all, or are positively useful . . .” Scientists should correct their tendency to view invaders as guilty until proven innocent.

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).

F.C.5 Therefore, it’s unrealistic to control species’ movements through prevention or eradication (F.O.7 and F.O.17). That doesn’t mean I’m “[ignoring] our active and accelerated role in spreading species beyond their natural biogeographic ranges” (A.R.8). There isn’t sufficient long-term evidence that spreading species is inherently dangerous or that changes caused by alien species damage ecosystems.

F.C.6 Focusing on invasive species distracts us from addressing more serious, yet complicated problems like climate change, pollution, overfishing, and habitat loss. We should address top-tier issues. That will mitigate impacts on biodiversity, ecosystem functioning, and human livelihoods, health, and safety more so than any attempt to control so-called non-native species.

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.1.1 Human-mediated, species range shifts are profoundly distinct from natural species dispersal mechanisms. They occur over short time periods, at increasing frequencies and move species beyond their natural dispersal abilities where other species have not evolved to survive with the newcomers (see A.R.1).
2. 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).
3. 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).

A.C.2 While my opponent introduces some interesting concepts, she fails to adequately address the arguments and fails to understand that rapid human-mediated introductions cannot be equated to the long-term range shifts and subsequent distributions she refers to (F.R.3 an F.R.4). Consequently, the author’s campaign for a broader timescale approach may not be wise, as it is neither an accurate view, nor supported by any solid evidence. By waiting to “see an accurate picture” (F.R.8) it may be a case of too little too late.

A.C.3 Our direct movement of species is yet another human-induced, environmental and socio-economic threat. This idea may be hard to grasp compared to other threats because addressing the issue involves controlling and removing species. The opposition states, “management is unrealistic” and “unlikely to be successful” and stating that “ultimately, it’s not realistic to control species,” but managing any human-induced threat is challenging, and we created the problem by being responsible for their movements in the first place. Furthermore, biosecurity measures in Australia and New Zealand prove that preventive measures can be successful.

A.C.4 As a result of these shortcomings and the lack of empirical evidence to support the opposition’s claims, I retain my position.

Debaters

For

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.

Against

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.