Ecologists weigh in on native-exotic debate

Charlie Rohwer, a frequent guest contributor to the Garden Professors, brought to my attention a recent letter in Nature by Mark Davis and 18 other ecologist entitled, “Don’t judge species on their origins.” Davis is a leading authority on invasive species and author the book Invasion Biology. In their article, Davis and his co-authors make many of the same points that I’ve made here on the blog (Are natives the answer? Dec. 14, 2009; Restoration ecologists you need us – part 2. Aug. 9, 2010) regarding the native/exotic debate. The main point is that we need to develop a more pragmatic approach to managing landscape systems and this includes natives and exotic species; it’s not an either/or question anymore, not that it ever really was.

A couple of excerpts from their letter:

“It is time for scientists, land managers and policy-makers to ditch this preoccupation with the native–alien dichotomy and embrace more dynamic and pragmatic approaches to the conservation and management of species — approaches better suited to our fast-changing planet.”

“Clearly, natural-resource agencies and organizations should base their management plans on sound empirical evidence and not on unfounded claims of harm caused by non-natives. Another valuable step would be for scientists and professionals in conservation to convey to the public that many alien species are useful.”

The pragmatic approach that Davis and his co-authors (and I) advocate recognizes a several realities:

Exotics can fill many of the same ecological roles and niches as natives, Doug Talamy’s book notwithstanding.

Any inherent “ecological superiority” of natives over exotics has long since been negated by anthropogenic land-use change, alien pests, and climate change.

Attempts to eradicate alien species are largely warm-fuzzy exercises with little likelihood of success without liberal use of industrial-strength herbicides.

29 thoughts on “Ecologists weigh in on native-exotic debate”

  1. I enjoyed that article — people who think that invasives are destroying the world really need to read Mark Davis’ material — especiall his book which you mentioned above.

  2. I agree there is no need to go overboard, but certainly there is some agreement about which plants really should not be sold. That is what bugs me–the lack of information for the consumer. The entire burden should not be on me when I buy a plant, and I really do want to be responsible about what I plant.

  3. Tom,
    I posted an lengthier excerpt on today’s post. Linda and I may be able to access since we’re on university systems that have subscriptions.

  4. Val,

    There is no doubt there are plants that cause many problems. But we need to remember a couple of things: One, invasiveness is function of the plant AND its environment. A plant may be invasive in Florida but not in more northern locations. This is the problem with ‘blacklisting’; especially when lists are copy-pasted from agency to another. The other point more directly related to this article, is that a plant’s function (and also its problems) are not dependent on whether it’s native or exotic. We can have well-behaved exotics and we can have weedy natives.

  5. The letter by Davis et al. Is unfortunate in many ways. It is simply not a coherent argument, being full of logical errors, but the more insidious problem is the way that Davis (and on this page, Cregg) misrepresent both scientific data and the positions of those with whom they purport to disagree. I don’t know whether to hope the misrepresentations are intentional or inadvertent.

  6. Indeed, Vincent, please elaborate on your concerns. Our purpose in writing was to highlight and broaden an ongoing discussion, not to end one.

  7. My criticisms of the Davi
    s et al. comment in Nature are several, and I hope that Bert will not mind me posting them here at great length even though I’ll refer to pieces of the essay that he hasn’t quoted.

    My primary objection is that the Nature comment suffers from an excess of straw man attacks, berating some unspecified opponent – for example – for having “vilified” non-native species as the culprit in driving their “beloved” native species to extinction and doing so with rather inflammatory language.

    I know that space is short in a Nature comment, but Davis et al. did themselves no favor by setting up “scientists, land managers and policy makers” as being pre-occupied, stagnant, and unpragmatic. I suggest that the authors have constructed an inaccurate caricature of their opponents merely to serve as a rhetorical device. Are there land managers who actually have a policy of eradicating all non-native species from all environments at any cost? Really?

    And what scientist, for example, actually believes that introduced species “pose an apocalyptic threat to biodiversity”? This would seem like the kind of allegation you could find a source for if, as Davis et al. seem to believe, the bias against non-native species is so pervasive.

    This leads to my second major objection, which is that Davis et al. misrepresent (directly or contextually) many of their sources. This most often takes the form of over-generalizing the (typically) narrow findings of the research they cite.

    For example, take the paragraph which opened with the hyperbolic “apocaolyptic threat”.

    After this strawman attack, Davis et al. mentions a 1998 paper which concluded “that invaders are the second greatest threat to the survival of threatened or endangered species after habitat destruction.”

    In an apparent rebuttal of Wilcove, Davis cites himself in an assertion that “invaders do not represent a major extinction threat to most species in most environments.” This, of course, has no relevance to the findings of Wilcove et al. whatsoever. Wilcove specifically investigated only those few species that have already been identified as imperiled, a tiny subset of “most species in most environments” to say the least, and did so in an effort to understand WHY they were imperiled.

    Davis is not actually wrong, of course: most introduced species don’t cause the extinction of native species. But is that an argument for treating native and non-native species as ecologically equivalent.

    In another example, Davis summarized the recent paper by Gleditsch et al. this way: “In Pennsylvania, more non-native honeysuckles mean more native bird species. Also the seed dispersal of native berry-producing plants is higher in places where non-native honeysuckles are most abundant.” Neither of those two summary statements is true.

    Gleditsch found that it was the abundance of fruit-eating birds, not the number of native bird species which was correlated with the amount of available fruit in the landscape.

    And Davis completely glosses over the fact that the study was limited to frugivores, and only during a two month window designed to coincide with peak fruiting of the honeysuckles. In other words, the study was essentially rigged. It was no secret that fruit-eating birds will eat the fruit of honeysuckles (that’s one of the factors that makes the honeysuckle so invasive to begin with), and the study counted only birds most likely to benefit from honeysuckle abundance and counted them only during the time period in which honeysuckle had the greatest advantage.

    The Gleditsch data are not wrong, per se, but they don’t begin to approach a level of comprehensiveness which would tell us whether a non-native honeysuckle monoculture is better or worse OVERALL than the native ecology which the honeysuckle has displaced. And when you ignore – as Gleditsch et al. and then Davis et al. do – the wealth of evidence that non-native plants produce far less insect biomass than natives do, and ignore the fact that most frugivorous adult birds were STRICTLY carnivorous baby birds once, then you end up with a mostly silly result.

    In the end, the advice offered by Davis et al. (that we should “. . . . organize priorities around whether species are producing benefits or harm to biodiversity, human health, ecological services and economies.”) is hard to disagree with almost to the point of being a tautology. The paper has generated publicity for the authors, it is true. but was so unnecessarily acerbic and unbalanced that I’m not sure it will lead to better policy decisions.

  8. Vincent raises several important issues. It is certainly true that we had very little space that restricted the possible level of nuance, and Nature’s editors were quite assertive about what they wanted to see in that space. We were also limited to 10 citations. Nevertheless, we the authors are responsible for what actually appeared. That said: * Nativeness as it has ALWAYS been defined has negligible ecological content. It signifies only an absence of evidence for dispersal by humans. For more see Chew & Hamilton 2011 as cited in Nature. ** Wilcove, et al 1998 is an opinion poll; its authors disclaimed virtually all of their data as anecdotal, but quantified their findings in terms of threats, not opinions about threats. Those findings have further transmogrified via egregiously sloppy citation into an axiomatic cliche. What interest does that serve? *** The Nature essay authors are aware there are varieties of professional opinion on the ‘alien problem’; but we are also aware from long, intensive experience that categorical anti-alien sentiment has both a rich history and a robust presence in current thought. Its theoretical weakness is obvious, as is its wide appeal; an interesting combination. **** At no point did we promote a ‘honeysuckle monoculture’; THAT is a straw man argument, as is invoking the mythical, money-is-no-object manager who can actually put a desire to maintain a fully native assemblage into practice. ***** Appealing to ‘native ecology’ carries the same liability as appealing to ‘native species’. It reduces to preference for a configuration that no longer exists, and that we have no capacity to re-impose other than briefly and superficially. ****** The purpose of an opinion paper is, of course, to pre
    sent an opinion. The effect of presenting our opinion on policy and the practice of science remains to be seen.

  9. One area where I think that Matt and I may share some ground is in the conviction that more research specifically on the ecological function of species is worthwhile. The reality is, however, that very little research of that type has been done and even less has been done well enough. It is a challenging research problem even for a species in its native ecosystem. If you want to evaluate the role of an introduced species, you cannot study it directly until AFTER you introduce it. And so this is where I disagree with Matt: I argue that the native/alien distinction DOES have significant ecological content because it is legitimate proxy measure of ecological function. If you are a landowner or policy maker and you know nothing else about a species besides its “nativeness” to a particular ecosystem then the only rational response is to use the information that you DO have to make the best decision you can. And that decision, in the absence of other information, will be to favor native species over introduced ones. Data about nativeness is relatively inexpensive to acquire and yet highly predictive. Would more data, of the kind that Davis et al. advocate, be better? Obviously, and if the rant as published in Nature sparks more of that kind of research then I think we’ll all be thrilled.

  10. Vincent, I found your comments and this debate interesting but what is your background with this issue? This is just out of curiosity not for verification of your view points.

    Matt, your diction seems excessively professerial for a blog for the general public. “transmorgified via egregiously sloppy citation into an axiomatic cliche”… really! You must be right if you can say it so ornately.

  11. Well done dialogue! Fascinating and provocative. One added thought: Isn’t the main issue not so much a species’ historical “native-ness” but its “invasiveness”? Whatever its origin, when something takes over a plot of land aggressively and to the exclusion of just about anything else, that can’t be good. Surely by that measure we can agree that highly invasive behavior is to be combatted when possible…though any frustrated land manager knows it isn’t always possible.

  12. The discussion always comes back to function. Tallamy certainly justified his argument for Bringing Nature Home using natives. But there is a comprehensive lack of study for non-natives and function, specifically herbivory by insects and effect on soil. In the end, we will have a large percentage of non-natives, some of them invasives, in our landscapes. We will not win the war against invasives but can win battles if they are focused. These discussions (arguments in the classical Greek sense) are necessary to stimulate the research needed to sort these issues out and resolve them. What irks me is the emotional language and rhetorical challenging from otherwise levelheaded researchers and practitioners. The environmental field needs a more unified vox populi if it wants the average consumer to take it seriously. Monsanto doesn’t have this problem.

  13. @Michael – My view is that the debate is unlikely to be cleanly resolved to any single issue (e.g. “invasiveness”) because we really have many different levels of concern in this debate. I haven’t yet figured out an elegant way to describe them all discretely – they all intersect to varying degrees – but I usually thing about them as being roughly: 1) the scientific (descriptive) question of biological/ecological function; 2) the ethical (prescriptive) question of how humans “should” aspire to interact with the Earth and other species; and 3) the policy question of how to best allocate scarce resources (e.g. time and money) for the betterment of society given 1) and 2) above. From a purely ecological perspective, for example, invasiveness may not necessarily be a bad trait. For pioneer species after a natural disaster (flood, fire, tornado, etc.) it may be ecologically critical. One reality that makes this debate so ugly sometimes is that the classic (aka Federal) definition of “invasive species” is really a somewhat sloppy mishmash of all three aspects: it is a classification scheme that relies both on ecological function and policy (i.e. it requires “harm” which can either be environmental OR economic) but also on an anthropocentric ethical function (i.e. it requires the “harm” to be the fault of man).

  14. OK, enough abstract verbiage, I’m feeling lost in a kudzu thicket- and BOTH of you are sounding like lawyers to me- how about specific examples of plants not worth the attempt of eradicating or, better yet, ones on the cusp of either eradication or reluctant acceptance. I understand the idea that plants on such lists may vary regionally. Maybe this will show where you two are on common ground or on divergent paths.

  15. Vincent: whoever you are, you have my undying appreciation! You have voiced what I only held in my mind. I especially like the use of Logic to deconstruct some of Davis’ method and approach. All of these “new normal”, “Brave new world” articles spend almost half the time on rhetoric, native vs. non-native, and all of the inflammatory language, and not enough on science. One article implied that, since zebra mussels clean Great Lakes water to good clarity, not all non-natives are bad. Also, no one ever gives you the “new pallette” of what species are to be accepted, and which ones are definitely to be eradicated, and why. It is all up front castigation, with no really useful advice on how to deal with this ‘brave new world’ Vincent, if I ever run into you, lunch is on me for your well- reasoned piece. It is an long overdue rebuttal that has been incubating to my head.

  16. Alright, I’ll jump into the fray. I study geoscience, so I find myself more than bemused by the “native-alien” distinction. After all, barely anything is “native” anywhere in North America when compared with conditions a mere 10,000 years ago, at the last glacial maximum. The period since the last glacial maximum represents less than 0.002% of the Earth’s history since the Cambrian explosion, when most phyla appeared on Earth. ~ ~ I don’t want to sound flippant about this. We are currently seeing a rate of extinctions on this planet that rival any in the rock record. However, I find the native-invasive dichotomy to be less than helpful in viewing the situation. ~ ~ Someone suggested providing an example, and I’d like to do so: . The idea is that hybrid animals, occurring in the north as the climate warms, are “bad” and should be culled to protect the gene pool of the native arctic species. Although the article acknowledges that is impossible, the scientists still wish it could be so. It seems to me, precisely for the reasons of protecting genetic diversity, the hybrids represent a system that preserves the polar-adapted genes to the greatest extent possible. This is particularly in the face of the diseases that the arctic species are particularly prone to, as described in the same article. Hybrids will continue to carry the genes from the arctic animals. Total loss due to disease of the arctic animals will result in no preservation of genetic information. ~ ~ Again, speaking as a geoscientist, we are in the midst of one of the great transitions in the Earth’s history. These things happen remarkably abru
    ptly in the rock record, and apparently that is on a time scale that is even abrupt on human terms. ~ ~ Unlike the previous extinction events, however, this one is largely being brought about (has been brought about?) by human activity. And, likewise, there may still be some things we humans can do to protect genetic diversity both for ourselves on the human time scale as well as the larger time scales. But the native-alien discussion seems unlikely, to me, to help find those answers. ~ ~ The actual answers seem obvious to me. Stop converting land to uses that serve only the greatest invader of them all: us.

  17. Denise, thanks for saying (better and in more detail) what was on my mind–that “invasion of species” has been happening since there was life on this planet (sometimes more euphemistically termed “migration”), and it’s therefore a moot point to discuss it in terms of “right” or “wrong”. A bigger point of discussion should be, what to do (or not do) with the information we gather about this phenomena.

  18. Denise, don’t be swayed by those who offer up the “ecologies are constantly evolving” trope. It is a red herring, true but irrelevant. It seems to me that the core point Davis et al. were trying to make is that the “native/alien” classification scheme is not a useful one. In formulating their message in such an extreme way, they are obviously quite wrong if the point is taken literally. I’m willing to cut the authors some slack on a personal level, since the Nature editors constrained them in some important ways, but the article itself has to stand on the words that were published. And it misses the boat.

  19. I guess I must read (sound) like an idiot because I am invisible in this discussion, but I really fail to see why specific examples can’t be used to bring the topic down to earth. I assume that it is the practical implications that inspire this controversy, so what are they? Without specific examples of how the theory here is applied it is meaningless to me- an empty debate where only the jousting is the point. How can I even tell if the disagreement isn’t primarily semantic?

  20. Alan, I’m honestly not sure this debate is primarily motivated by specific examples. There are plenty of cases at the extremes (i.e. alien species that pose little or no ecological threat, and alien species that are unquestionably devastating), but the real challenge are the cases in the middle of the continuum: species whose ecological impact is either not known or is ambiguous. The honeysuckle example is one of these, and I discussed it earlier. Davis et al. read the Gleditsch paper, which found that some birds will eat the fruit of non-native honeysuckle some times, and chose to highlight the “positive effects of many invaders.” In this specific case, Davis et al. focused on the slight positive effects of the species and ignored an entire body of literature describing the many negative effects of the same species. So the problem really is not the example, per se, but rather of the big picture approach: good decisions cannot be made by focusing exclusively on the cost or on the benefits or any particular species, but rather on balancing both. Davis et al. paid lip service to that approach, but in every single example they mentioned they failed to do it themselves.

  21. Thank you Vincent, that did help clarify things. Examples are useful though, because the result of policy is the implementation of specific actions- in this case to either attempt to delay or stop the spread of a given species or not. Your complaint seems to be that not enough research has been done to even begin executing such policy decisions in a responsible manner.
    No? But this would seemingly leave land managers in a quandary of what to do in the meantime. Whack the honeysuckle or not.

  22. Alan, in general the lack of research on ecological function is a real problem. And even if that research WERE done, we’d STILL face a big problem: valuing that function. But back to the pragmatic: my sense is that most land managers (the good ones, anyway) have a good sense for which invasive species pose the biggest threat in their particular ecology. Land managers generally are trying to do a lot of work with a little money, and so they are quite adept at prioritizing. If Davis et al. want to argue that a more disciplined framework would be better, I wouldn’t argue that it wouldn’t IF it were possible. Until it is, and in the absence of information about the particulars of any situation, I stand by my view that the general heuristic of favoring native plants over alien plants is better than the alternative of not caring about origin at all. The tone of the Nature comment could easily lead one to think that Davis et al. are suggesting that “nativeness” is an ecologically meaningless attribute, yet every rational ecologist knows it isn’t.

  23. Vincent, there are many types of land managers. Many people who participate in this forum only manage their own land. My business has me on many estates and although my work there is not managing their land but their orchards, they often come to me for advice on how to maintain the wild parts of their property, which may be just a few acres or a thousand. Private land owners need practical guidance on sound stewardship of their land holdings no matter how much land that involves. I think the decisions end up being based on a similar criteria as that of saving endangered species. Just as the attractive animals get the most active protection, the ugly invasive plants are the ones the we most try to eradicate. Most of the time this ugliness is manifested by the tendency of a plant to overwhelm a landscape. I guess those plants tend to be the most dangerous at disrupting the ecology as well, but I’m sure this isn’t always the case.

  24. I 100% agree that there is no need to go totally overboard, but I think there is somewhat an agreement about which plants shouldn’t be sold That’s what annoys me-the lack of information for the us – the consumer.

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