“Guilty until proven innocent”
The impact of non-native species is frequently complex and poorly understood and the “guilty until proven innocent” philosophy of invasion biology does not solve that problem. In fact, it extends the prejudicial nature of much of the debate by appearing to encourage what it actually precludes. As we saw in the previous section, prominent invasion biologists often defend their field by stating that they are not against all introduced species—just against the ones with a negative environmental impact (Simberloff 2003; Simberloff et al. 2011; Richardson and Ricciardi 2013). And yet there is in fact a shift of the burden of proof onto the so-called “invasive” species (or their supporters) that requires them to prove that they are not harmful.
The practical problem is that the nature of the environmental impact can often be just as difficult to classify in an objective and clear-cut way as the native or non-native status of various species (the difficulties associated with establishing native or non-native status will be analyzed in the “Additional problems in invasion biology” section of this article). Usually, species—native or introduced—have a variety of environmental impacts—some positive and some negative, depending on our points of view and which particular aspects and interactions we choose to focus on. There has been a general tendency in invasion biology to focus mainly on the negative impacts of species perceived as introduced. The positive impacts of non-native species, and the beneficial contributions these species can bring to ecosystems and native species, are frequently either minimized or ignored altogether in the invasion biology literature (Rodriguez 2006; Goodenough 2010; Guiaşu 2016; Schlaepfer 2018). Furthermore, if one sets out to “prove” at all costs that a species has some sort of a negative impact on the environment simply because the species exists—and therefore it must feed, occupy a certain space, use some resources, etc.—then one can “prove” that about any species anywhere, of course (Guiaşu 2016). Both native and non-native species can have complex ecological impacts and interspecific interactions, which are poorly understood in many cases due to insufficient research and mainly short-term rather than long-term studies (Willis and Birks 2006; Guiaşu 2016). Goodenough (2010) pointed out that “the ‘native good, alien bad’ maxim does not convey the complexity of invasion ecology: alien species do not axiomatically pose a threat to native biota.” Davis et al. (2011) also emphasized that both native and non-native species can have a variety of positive and negative effects on the environment, and, therefore, using the assigned native or non-native status of a species to determine (or pre-determine) its potential ecological impact is not very useful.
Aside from this practical concern there is a logical problem of equal import. Ruesink et al. (1995) deliver the clearest statement of this problem. For them, nonindigenous species represent a “major threat to the integrity of natural systems”. Their discussion does help to clarify how they would understand ‘threat’ and ‘major threat’, and in light of this discussion they express concern about a policy that they take to be “innocent until proven guilty,” where “taxa with no record of damage are generally considered acceptable imports.” But the difficulty of developing tests for such species and the high degree of uncertainty they see surrounding them push the authors to the other extreme: guilty until proven innocent. As they themselves admit, “even if we know the intimate details of species’ natural history and attributes and how these interact with new physical and biological surroundings, we cannot accurately predict the outcome of every introduction because the environment is complex.” This needs to be matched against their own “guilty until proven innocent” recommendation to fully see the import of what they are suggesting. They judge the most effective way to reduce harm is to assume that all nonindigenous species are threats. The authors thus recommend: “Keep nonindigenous species out of the United States until a species is known to be safe” (our italics). On the face of things, this seems like a clear requirement with a condition that must be met. But in practice, it is no condition at all because on the terms that the authors themselves have established it is a condition that can never be met. Recall: because the environment is complex, “we cannot accurately predict the outcome of every introduction” even when we have intimate knowledge of how an introduced species interacts with the environment. At some future time it is logically possible that a problem can emerge. Thus, a species can never be known to be safe with the degree of certainty those who propose the “guilty until proven innocent” policy require. And thus further, nonindigenous species cannot be proven innocent and so must always be deemed guilty.
There are various ways of presenting the fallacy at the heart of this policy. There is an element of “poisoning the well,” whereby an arguer presents a position in terms that predispose an audience against accepting it (Walton 1996). And there is the sense of a “false dilemma,” whereby an issue is presented in terms of a stark dichotomy without any appreciation of the middle ground between the extremes (Tomic 2013). However we understand this, we should appreciate the logical problem of presenting a condition that can never really be met. In drawing attention to this, we equally draw attention to the need for serious work to be done in the middle ground between the two extremes of assuming complete innocence and complete guilt.
Simberloff (2003) stated that the main reasons for concern about introduced species are that such species “can threaten the existence of native species and communities and that they can cause staggering damage, reflected in economic terms, to human endeavors.” Some introduced species may do that, but, then again, such problems may be caused just as readily by certain native species. For example, as Davis et al. (2011) pointed out, the native mountain pine beetle (Dendroctonus ponderosae) probably kills more trees in North America than any other insect species. Also, Venter et al. (2006) found that interactions among native species were more of a threat than introduced species to endangered species in Canada. In fact, introduced species were the least important threat among the six categories examined in that study. So, destructiveness and damage to the environment or the economy are not attributes associated solely with, or identifying features of, non-native species (Davis et al. 2011; Guiaşu 2016). Many introduced species are not guilty of causing any such environmental or economic problems.
In a survey of 422 invasion biologists—chosen because they acted as peer reviewers for the journal Biological Invasions—Young and Larson (2011) found that even among invasion biologists, the vast majority agreed that “There should be more scientific objectivity and less emotional xenophobia regarding invasive species.” An even greater majority of the invasion biologists surveyed agreed that scientists working in this field should “gather, interpret and communicate information” about invasive species “as accurately and objectively as possible.” These findings suggest that even many invasion biologists are aware that invasion biology is currently plagued by too much subjectivity, and presumably also too much unwarranted or insufficiently substantiated negativity regarding non-native species.
The presence of bias
In a remarkably one-sided dismissal of the criticisms of what they called “invasion science”, Richardson and Ricciardi (2013) referred to the critics of their field as “a relatively small but vocal number of scientists and academics—naysayers in various guises” and declared that “many of the criticisms against invasion science simply do not withstand scrutiny.” It should be noted that this article was not a detailed review paper, but a rather brief editorial. This editorial was titled Misleading criticisms of invasion science: a field guide. Within the article, Table 1, which is labelled as “A field guide to misleading criticisms of invasion science”, summarizes six major criticisms of invasion biology and the so-called rebuttals to the criticisms, and provides a fairly small number of references, or sources, for each side of the argument. While, at first glance, this may appear to be an even-handed examination of these issues, it is quite clear that the authors of the editorial are far from neutral—in fact, they even refer to the articles they criticize as being part of “a cottage industry of criticisms”. The rebuttals are given in considerably more detail than the actual criticisms, and there is no way to decide, based on the limited information provided, whether the sources cited in support of the rebuttals provide comprehensive and convincing responses to each of the criticisms of the field. The impression given is that each of the criticisms listed in the table is without merit, and has been fully addressed and debunked in the relevant literature. However, that is not the case. In other words, the table about the supposedly “misleading criticisms” appears to be quite misleading itself. At least some of the already meagre sources cited for the rebuttals are very brief opinion pieces which do not offer any new data—for example, the Wilson et al. (2009) reference is a one page article published in the Letters Response category of the journal Trends in Ecology and Evolution, and the Simberloff et al. (2011) source is a very brief letter to the editor. Furthermore, plenty of additional studies and examples could have been selected, and a more detailed and nuanced description of the criticisms of invasion biology could have been included, of course. Another table could have been put together just as easily, showing studies and data which contradicted the rebuttals presented by Richardson and Ricciardi (2013).
In taking issue with the criticism that some control programs may be directed at non-native species that do not really cause any, or much, harm, Richardson and Ricciardi (2013) wrote:
In reality, managers are constrained by limited resources and seek to prioritize species that are likely to become problematic. However, this effort is hampered by several facts that are generally ignored by the naysayers: (1) the impacts of most invasions have not been studied, and so important effects may remain undetected, (2) invaders that are apparently innocuous in one region can be disruptive in other regions, (3) subtle impacts that may be unrecognizable without careful technical study can produce enormous ecosystem changes over time, and (4) many non-native species that currently appear innocuous may become damaging many years later – when it is no longer feasible to eradicate them. (Richardson and Ricciardi 2013, p. 1462)
The four items in this paragraph seem to fit within the earlier mentioned “guilty until proven innocent” philosophy. How do we decide which species “are likely to become problematic”, before any problems actually occur? What if we rush to judgement, and end up eradicating a species which would not have caused any harm, and may even have had a net positive impact, if we left it alone? Let us analyze the four so-called “facts” mentioned by Richardson and Ricciardi (2013), in the order in which they were listed above. (1) If the impacts of most invasions have not been studied, why should we necessarily assume that the effects of these introduced species will be important and negative? All that really means is that we don’t know anything about these species and their impacts. Consequently, unless we belong to that half of invasion biologists who believe invasive species have a negative impact by definition (Young and Larson 2011), we probably should not even call these species “invasive.” (2) Yes, theoretically, “invaders” may be harmless in one region and “disruptive” somewhere else. However, that clearly does not mean we have to consider non-native species automatically as problematic when they appear in a new area, particularly if these species are not known to have had a negative impact anywhere. It is also possible that even introduced species which have acquired a bad reputation elsewhere may cause no problems in the regions we are interested in. This is a two-way street. (3) “Subtle impacts that may be unrecognizable without careful technical study” may actually remain subtle, or minimal. “Careful technical study” may mean that we are trying a bit too hard, perhaps, to find some type of negative impact at all costs. This may not necessarily be the most efficient use of the “limited resources” mentioned by Richardson and Ricciardi above. Subtle impacts may, theoretically, produce larger impacts over long periods of time, but just how long would these projected periods have to be before we decide we must act now? If the impact of an introduced plant or animal will not be significant for hundreds or even thousands of years, it is obviously not of immediate concern. Who knows what current ecosystems will look like centuries or millennia from now? Many other factors will affect these ecosystems over such long periods of time. Furthermore, sometimes the impacts of invasive species become less important over time. So, again, this works both ways. In some cases, the impact of an invasive species may be most significant immediately after the introduction. After a few years, this impact may decline naturally, without our interference. This highlights the need for thorough and objective longer-term studies about the impact of non-native species, and suggests that we should be careful not to jump to (possibly premature) conclusions based only on very preliminary observations or short-term studies of such species (Guiasu 2016). And we should not start with the assumption that the “ecosystem changes over time” caused by non-native species have to be undesirable and harmful. (4) Yes, some non-native species may become “damaging” many years after their introduction, but, again, this is a theoretical concern that could be applied to any species. It is an a priori charge of “guilty”, or “potentially guilty one day”, without any supporting evidence. Changes in environmental conditions may cause a native species to become much more abundant and widespread than before as well. If we really care about managing our “limited resources” wisely, and if invasion biologists are really not against all introduced species (as Simberloff 2003; Simberloff et al. 2011; Richardson and Ricciardi 2013 stated), then it does not make sense to eradicate “non-native species that currently appear innocuous.” Do invasion and conservation biologists consider all non-native species to be problematic or not? Let us make up our mind, and let us all indicate clearly where we each stand on this. We cannot have it both ways. Quite simply, the “guilty until proven innocent” approach of some invasion biologists to non-native species, in general, seems incompatible with the assertion that these biologists are not against all such species.
Simberloff et al. (2011) stated that: “Pronouncing a newly introduced species as harmless can lead to bad decisions about its management.” So, does that mean that we can never declare a newly arrived species to be “harmless”, even in the absence of any evidence that would indicate otherwise? The assumption that a currently harmless species should be controlled anyway could also lead to potentially bad management decisions, resulting, for example, in the squandering of limited resources to fight against a species which should not be of concern and the damage to the environment that may be caused by such misguided control programs. Sometimes, actions taken against non-native species can have a variety of unintended consequences and may end up harming native species as well (Guiaşu 2016). For example, in New Zealand, poisons used to eliminate introduced mice also killed the North Island saddleback, a rare native forest bird (Davidson and Armstrong 2002).
In an article which attempted to find the middle-ground in some of the current debates about invasion biology, Shackelford et al. (2013) wrote: “Those who defend the removal of non-native species have been accused of xenophobia and those who are more ambivalent are charged with biological homogenization. Both sides have merit.”
In trying to analyze some current debates in invasion biology from a more neutral perspective, Shackelford et al. (2013) mentioned that the field “has shifted its rhetoric in recent years to reflect a focus on species with the greatest impact.” These authors also added:
However, how much of this shift reflects a change in attitude toward non-native species, rather than just limited resources and political appeal, is unclear. This point sits at the center of much recent controversy. Without resource and methodological constraints, many, if not most, conservationists would still probably prefer to rid systems entirely of non-natives regardless of impact. (Shackelford et al. 2013, p. 56)
In other words, if more resources were available, there is a risk that there could be more control and eradication programs against plant and animal species that may not have any proven significant negative impact, simply because these species are suspected of being non-native. Furthermore, the very concept of “impact” can be subjective in this field. If a species—for example, purple loosestrife—has been declared to be a problem, in general, it is very likely that control or eradication campaigns will be attempted against that species at many locations where its impact may not be negative overall. It is doubtful that conservationists or managers in charge of individual conservation areas are always conducting their own thorough independent research to determine whether the introduced species present within their jurisdictions are having mostly negative, or positive, impacts. Once a species is put on a list of undesirable “invasives”, it will probably stay there for a long time, even if convincing new exculpatory evidence may emerge. These lists are not always based on the best and most up-to-date science (Guiaşu 2016). And getting rid of non-native species entirely is clearly not a realistic goal, even assuming we could somehow identify all the non-native species in a given ecosystem, which would also be extremely unlikely.
The value of informed dissent
In their editorial about the supposedly “misleading criticisms of invasion science”, Richardson and Ricciardi (2013) wrote: “To suggest that non-native species are not unequivocally a major concern for the conservation of biodiversity and ecosystem services is to ignore decades of peer-reviewed science.”
However, according to many ecologists (for example, Williamson 1996; Primack 2012), only a small minority of non-native species become invasive and cause problems, so why are we not allowed to think, or suggest, that many non-native species are not a concern for the environment or human interests? Moreover, researchers should not be accused of being ignorant of the relevant literature, willfully or otherwise, if they entertain such thoughts or make such suggestions and back them up with proper references and data. The word “unequivocally” seems rather misplaced in there as well. Again, the very definition of the term “non-native” is often unclear in this field, as will be discussed further in the next section, and even when we can actually determine that a species is indeed not native, the impact of that species is often ambiguous or poorly understood. In many cases, such species may have some negative short-term effects as well as some positive ones, and the long-term impacts may be unknown. So, to suggest that anyone who does not consider non-native species in general to be unequivocally bad or capable of causing major problems is ignoring “decades of peer-reviewed science” seems rather strange. As mentioned, the field of invasion biology is young, so we don’t have much more than about two and a half decades worth of peer-reviewed studies on display. And a thorough review of many of these studies, such as the one undertaken by Guiaşu (2016), can actually reveal that there is quite a bit of information that would contradict the assertion made by Richardson and Ricciardi (2013). Not all of these peer-reviewed articles point in the same direction or lead to the same conclusions. Perhaps it could be said that Richardson and Ricciardi (2013) are ignoring some of the peer-reviewed literature in their field when they are making such general statements. And some of the invasion biology papers, published in peer-reviewed journals, are mere opinion pieces. Such papers clearly add to the volume of the literature in this field, but they do not contribute any new evidence or data.
In their survey, Young and Larson (2011) found that 37% of invasion biologists agreed that “exotics are an unnatural, undesirable component of the biota and environment.” However, 34% of invasion biologists disagreed with that statement. This shows there is no unanimity, or even a strong consensus, in the field with respect to the presence and role of non-native species. Furthermore, in the same survey, 37% of invasion biologists agreed that “the term ‘invasive’ should not be used to connote negative environmental impact”, while 46% disagreed with that assertion. Young and Larson (2011) also found that there was almost equal support in the field for two different definitions of invasive species—one which stated that such species have negative environmental or socio-economic impacts and another one which did not. These findings contradict the assertion made by Russell and Blackburn (2017) that there is a “scientific consensus on the negative impacts of invasive alien species”. As discussed, species can often have a variety of impacts—some positive and some negative—and choosing which particular impacts to focus on may depend on our particular points of view or interests.
Therefore, it is perhaps unjustified and unfair to refer to critics of invasion biology as merely a few “naysayers”, who are ignoring the relevant scientific literature (Richardson and Ricciardi 2013), or members of a small “contrarian minority” (Simberloff 2011b), or individuals who are engaging in “science denialism” by rejecting “undisputed scientific facts” (Russell and Blackburn 2017). Invasion biologists themselves appear to be quite divided on some important issues and definitions in their own field.
Furthermore, not only is the notion of “scientific consensus” regarding the status and impact of non-native species far from clear, but the majority view at a particular point in time is not always invariably correct and should certainly not be above scrutiny.
For the most part, the criticisms of invasion science labelled as “misleading” by Richardson and Ricciardi (2013) seem quite legitimate and worthy of being carefully taken into account. According to some of these criticisms, as listed in Richardson and Ricciardi’s editorial, “impacts of non-native species on biodiversity and ecosystems are exaggerated”—presumably, the authors really meant to write “negative impacts”, rather than simply “impacts” in this case—and “positive (desirable) impacts of non-native species are understated”. However, the critics of invasion biology whose work we are familiar with are not saying that introduced species cannot have negative impacts or that these impacts are always exaggerated. Specific criticisms usually refer to particular species and examples. Nor can it be said that the negative impacts of non-native species are never exaggerated, or that the positive effects of these species are never ignored or understated in the invasion biology literature. Quite a few studies, including several cited in this article, would suggest otherwise. The overall scientific information on the impact of purple loosestrife in North American wetlands, for example, does not seem to justify the control programs and propaganda against this species throughout the continent (as shown in the comprehensive literature review by Guiaşu 2016). The impact of the rusty crayfish in Ontario is another relevant example. Do we know enough about it to start the anti-rusty crayfish campaigns? This species has been accused of having a negative impact on other crayfish species of the same genus. However, in a major study, Edwards et al. (2009) found that the widespread decline of all crayfish species sampled in many lakes in south-central Ontario during a couple of decades or so could not be attributed to the rusty crayfish. In fact, Guiaşu (2016) has shown that at least some of the anti-rusty crayfish information published in Ontario was based largely on some studies conducted in Wisconsin, and these studies were not always cited and interpreted in the proper context. Obviously, we should not use data about the possibly negative impact of a species in one location to demonize that species everywhere, especially since the impact of species can vary quite a bit from one location to another, as was demonstrated in the case of the rusty crayfish as well (Capelli 1982; Guiaşu 2016). Rarely is positive information about invasive species included in the brochures, information sessions, guided nature tours, or lectures associated with, or influenced by, this field. Nor do many invasion biologists or wildlife managers set out to find out just how much of a good contribution exotic species can bring to various ecosystems. What is the percentage of invasion biology studies which specifically focus on the positive attributes of non-native species? Without such detailed information, these criticisms of invasion biology cannot be labelled as “misleading.” In similar fashion, including major human diseases caused by viruses or bacteria on the lists of costs associated mainly with invasive animals and plants, as Pimentel et al. (2005) and Colautti et al. (2006) have done, could also be classified as potentially “misleading”. Non-native plants and animals generally do not have the same impacts as pathogens and dealing with illnesses which affect humans is the responsibility of medical professionals not invasion biologists. In addition, funding for health care is usually separate from funding for programs against invasive species (Guiasu 2016).
Richardson and Ricciardi (2013) tend to describe the chosen criticisms of invasion biology in very general terms. For example, one of the criticisms is described as “increased species introductions raise biodiversity (e.g. by adding to regional species pools; generating new taxa through hybridization) and therefore do not merit concern.” Well, introduced species can actually do that, and new varieties and species may eventually evolve as a result of interspecific hybridization, especially if we take a long-term evolutionary view, but that doesn’t mean the critics of invasion biology are always suggesting that invasive species are never of any concern. That is an overgeneralization, and, therefore, an exaggeration, or even a caricature, of the position taken by certain critics of the field. Just because some of us take a longer term and more generous view of non-native species and processes such as natural hybridization between native and exotic species, that does not mean we are oblivious to potential problems associated with the introduction of species. And it may be worth keeping in mind that non-native species deserve to be known and studied for more than the potential problems they may sometimes cause, and, as previously mentioned, native species are perfectly capable of generating environmental and economic troubles as well.
Richardson and Ricciardi (2013) cited only three studies (Brown and Sax 2004; Vermeij 2005; Thomas 2013) as sources for the “increased species introductions raise biodiversity and therefore do not merit concern” criticism. However, none of these three studies actually state that species introductions should not be of any concern, in general. In fact, Brown and Sax (2004) and Vermeij (2005) recognize that some non-native species can have negative effects, at least in the short term. These authors also clearly indicate that the introduction of such species should not be encouraged. And Thomas (2013) specifically mentioned that: “It is true that some invasive species damage ecosystems and can eradicate resident species.” He also added: “There are excellent arguments for conserving the wildlife we already have, but it is less clear why our default attitude to novel biodiversity is antagonism or ambivalence.” Furthermore, Thomas’s 2013 article is full of interesting specific examples which illustrate his point that biodiversity can indeed be increased as a result of species introductions. Overall, Brown and Sax (2004), Vermeij (2005), and Thomas (2013) encourage a more objective and open-minded approach to the study of non-native species and their impacts, based on evidence collected over the longer term, and not just the short term.