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Why I was not one of the 141 scientists who objected to Davis et al.’s invasives comment in Nature

Last month, Mark Davis and 18 ecologists argued in a Comment published in the journal Nature that the native-versus-alien dichotomy in conservation is not only increasingly impractical, but potentially counterproductive. The authors acknowledged that while some invasive species (e.g. zebra mussels) have widely-documented negative impacts, the application of the “invasive” label can also result in mismanagement of conservation funds and efforts. I won’t spell out the Davis et al. argument here (it’s short, to the point, and well worth reading), but when I read the paper, I was compelled by the fact that they not only outlined a potential problem (over-simplistic thinking in ecology), but also offered a way forward (a new guiding principle that embraces the idea of novel ecosystems in a highly managed world). “We are not suggesting that conservationists abandon their effors to mitigate serious problems caused by some introduced species, or that governments should stop trying to prevent potentially harmful species from entering their countries,” the authors argue, but instead they “urge conservationists and land managers to organize priorities around whether species are producing benefits or harm to biodiversity, human health, ecological services, and economies.” The take-home message is that in the highly-anthropogenic geographies of the present, ecologists should focus on the functions of species in new systems, rather than where those species originated.

I was impressed by the list of authors– among them Geerat Vermeij, James H. Brown, Scott P. Carroll, Steward T. A. Pickett, Katherine N. Suding, the late Joan G. Ehrenfeld, and J. Philip Grime; all are ecologists who have devoted their careers to understanding ecological change from a wide range of time scales. As a paleoecologist, I am occasionally frustrated by the short-term perspectives of neo-ecologists, and have often privately wondered what a deeper-time perspective could offer invasion ecology. Paleoecologists are comfortable thinking in terms of millennial timescales, and a natural result is that we tend not to put too much weight on a particular moment in time. Most ecologists are aware of the modern pest-driven decline in hemlock, for example, but not everyone knows that hemlock experienced an earlier range-wide reduction around 5,200 BP; for a thousand years, hemlock pollen is virtually absent from eastern North American pollen records, probably due to a combination of a paleo-pest and climate change. Studying the past, we hope to better understand the future.

I attended (and tweeted) a talk by Ariel Lugo (one of the “et al.” of Davis et al.) at my department last winter, in which he presented compelling evidence that novel ecosystems may arise as a natural response to environmental change (in this case, anthropogenic). I was excited by his results, but not surprised. My advisor’s lab researches the drivers of novel ecosystems of the past (communities with no modern analogue), and certainly the concept of change through time is nothing new to a student of the Quaternary. I’ve wondered if many (though certainly not all) of our conclusions about invasives have been based on short-term studies or unusually tenuous systems (like islands). So, I was honestly surprised to see the reactions that followed on the Ecological Society of America’s list-serv. Understandably, emotions run high when you think someone might be attacking your life’s work, or your livelihood. A response by Dan Simberloff, co-signed by 140 ecologists, criticized the Davis et al. piece as “attacking straw men,” and a number of people raised concerns that the first Nature letter would confuse the public, or was seriously downplaying the danger of invasives.

I disagree. First, I don’t think Davis et al. are attacking the legitimacy of concerns over specific invasive species per se, but rather the false native-invasive dichotomy– many natives act invasive in disturbed settings, for example– so what’s the guiding principle there? We live in a changing world, and certainly Gleason’s dynamic, individualistic model of ecology seems more likely to to be relevant in such a world. Davis et al. acknowledge the destructive power of invasive poster-child species, but suggest that, as landscapes are increasingly altered by humans, we need to re-evaluate our prejudices against the concept of the novel ecosystem. Many of the landscapes we hold dear are, arguably, novel; the forests of New England changed considerably even centuries before European arrival, and have certainly since. Every species in Hawaii was, at one time or another, an invasive.

One ecologist on the Ecolog list-serv quipped that the Davis et al. letter “was loaded with lurid language that has no place in science.” I found nothing lurid in the letter, personally, but the phrase made me wonder if his objection was really to the fact that Davis et al. are explicit about the fact that ecologists make value judgements every day. The invasive-native dichotomy is a normative directive– it’s a set of explicit instructions, seemingly backed by objective science, that gives conservationists a guideline to follow when interacting with the natural world. Without objective rules, ecology and conservation biology become very messy indeed. The problem is that ecologists, like all scientists, are subjective. We manage landscapes based on a very human decision of what those landscapes “ought” to look like, and how they “ought” to behave. Those “oughts” are informed by science, certainly, but– and here’s the paleoecologist speaking– at the end of the day, human beings are still making decisions about which point in time they’d really like the natural world to resemble. We’re driven to reach some ecological baseline, but the baselines have always changed. The very act of privileging one particular landscape over another is, in and of itself, based on cultural values; it’s a normative decision, regardless of how much science you throw at a problem. Davis et al. pointed out the famous 1998 study that concluded that “invasives are the second-greatest threat to the survival of threatened or endangered species after habitat destruction,” and that “little of the information used to support this claim involved data.” Maybe we could throw some more science at this problem before we unilaterally conclude that Davis et al. are wrong.

Ultimately, I think the scientific community, as well as the public, are best served when we acknowledge complexity and avoid binary thinking. As a global change scientist, I often struggle to convey the concept of uncertainty to stakeholders, and I think that the longer we avoid tackling that problem, the more damage we’ll do. The urgency of ecology (and climate science) as crisis disciplines means we’re going to be asked for simple answers to complex questions. We’ll be tempted to portray urgent problems as simply as possible, in order for our sciences to be useful (or even heard) by the public we ultimately serve. However, from where I sit, I see species individualistically responding to environmental change, constantly coming together and separating into what, at any particular snapshot (and I use the term deliberately to denote both the moment in time and the boundaries in space), looks like something you could call a “community,” or an “ecosystem.” Millennia, or even minutes, later, you’ll get a different snapshot. At some scale, all ecosystems are novel. This isn’t to say that we should throw our hands up and declare the natural world a lost cause. Rather, we ultimately do a disservice to the landscapes we hold dear whenever we tell them that old yearbook cliche’, “Don’t ever change.”

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Jacquelyn Gill

7 replies

  1. I think what is not being acknowledged in this discussion is that most managers of invasive species are making risk management decisions with limited available information and resources. Impacts in of invasives to ecosystem functioning is the driving concern. However decisions need to be made today regarding the risks of substantial future impacts of new and emerging species, before they are well established and when the costs of control are low. Non-nativeness is one of many factors considered in invasive risk assessment systems aimed at helping land managers minimize future invasive species impacts. The “native-good and non native-bad” dichotomy is a simplistic representation of these complex real world decision making processes. It is not widely held among invasive management professionals and Davis mischaracterizes this in his straw man argument.


    1. I’d argue that we don’t have a very good handle at all on the impacts of invasives on ecosystem function and absolutely agree that this kind of research should be done, and should be an integral part of decision-making. I don’t think the dichotomy is a straw argument, personally — and certainly folks like Ariel Lugo, Katherine Suding, Joan Ehrenfeld, and Stewart Pickett who have served both in both science and policy or management roles were concerned enough to add their names to the letter. I do agree that the precautionary principle dictates action even when the potential outcomes of that action remain uncertain, but novel ecosystems are an emerging reality. Do you disagree that “conservationists and land managers” should “organize priorities around whether species are producing benefits or harm to biodiversity, human health, ecological services, and economies?”


  2. Seems pretty likely to me that the moderate, more functionally oriented approach that Davis et al are summarizing in this piece will make it more easy to advocate for removal of those species that are especially frustrating to conservation projects. The ones that most of us seem concerned about are the ones that radically alter systems – zebra mussel, buckthorn, barberry etc.


  3. Lots of terrific points in this discussion. I used to consider the “oughts” a lot during my PhD studies on invasives but many folks clung tenaciously to the idea that there is some Utopian ideal ecosystem towards which all disturbed ones should be restored. Clearly not practical. Thanks for a thought provoking post!


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