In an essay published in June in Nature entitled “Don’t judge species on their origins,” Mark Davis (professor of biology at Macalester College) and 18 other colleagues argued that conservation too often unthinkingly vilifies non-native species and devotes precious resources to automatically targeting them for removal, resources that would be better spent on other issues. Reaction to the piece was quick and charged, both without and within the Conservancy; it included a letter to Nature by Daniel Simberloff, the Gore Hunger Professor of Environmental Studies as well as director of the Institute for Biological Invasions at the University of Tennessee-Knoxville, and 140 co-signers accusing the Davis piece of attacking straw men.

I asked a panel of Conservancy scientists and policy analysts who work or have worked on invasives issues to compose some questions to pose to Davis, Simberloff and TNC Chief Scientist Peter Kareiva (who has blogged about the Davis piece on Cool Green Science) about when, if and how conservation should deal with invasive species. Thanks to Davis, Simberloff and Kareiva for responding; their answers follow. —Bob Lalasz

Jump to Question #1: The Davis et al 2011 essay is being read in many quarters as a broadside against control of any non-native species. In your view, what criteria should trigger intensive management of a non-native species? And what are some appropriate metrics to assess whether a species should be considered ecologically harmful?

Jump to Question #2: When a species has both positive and negative impacts (i.e., the lantana cited in the Davis et al. 2011 paper) how should natural resource managers weigh these impacts to make management decisions?

Jump to Question #3: Given the well-recognized expense and difficulties with eradication, what do you think are effective strategies in addressing a highly damaging invasive species? Are the impacts of any species so severe that it would be worth continued investment just to constrain its abundance rather than reduce it significantly?

Jump to Question #4: Look out 10 years from now. Are invasives going to be considered a bigger problem than they are today by conservation scientists and by the general public? Why or why not?

Question #1: The Davis et al 2011 essay is being read in many quarters as a broadside against control of any non-native species. In your view, what criteria should trigger intensive management of a non-native species? And what are some appropriate metrics to assess whether a species should be considered ecologically harmful?

Mark Davis: I agree that some people have interpreted the Nature essay as a broadside against the control of any non-native species, although it is difficult to understand how they could have come to this conclusion given that we clearly stated that this was not our message: “We are not suggesting that conservationists abandon their efforts to mitigate serious problems caused by some introduced species, or that governments should stop trying to prevent potentially harmful species from entering their countries.” Species that threaten human health or significant economic harm should always trigger intensive management efforts.

However, the metrics used to define strictly ecological harm need to change. Currently, changes in relative species abundances and ecosystem processes are considered harm by many in the conservation field. For example, among its definitions of harm from non-native species, the U.S. National Invasive Species Council states that “harm includes significant changes in the composition and even the structure of native plant and animal communities.” I believe the bar to initiate management of non-native species that are producing solely ecological effects should be set very high. Society seldom has the luxury (resources) to manage nature according to personal preferences for appearance, species composition or ecosystem processes.

Daniel Simberloff: That’s not the right question. Many non-natives remain restricted and innocuous for years, even decades, then spread and become damaging. Brazilian pepper, a Florida scourge, is a good example. Another reason that isn’t the right question is that some non-natives have highly consequential impacts, but these are sufficiently subtle that we don’t recognize them for years — for example, some plants that affect nutrient cycles. So we shouldn’t necessarily wait to see something happen (your “criteria”) before deciding to try to eradicate an introduced species or, failing that, to attempt maintenance management. Davis et al. inveigh against wasted efforts managing non-native species that aren’t very damaging. I could come up with many examples of “Damn, we should have kept it out or eradicated it ten years ago! Now we have a much more difficult problem.”

Peter Kareiva: We should do everything possible to keep non-natives out, and then if they begin to establish a population, we should go full speed ahead on eradication because early detection and early eradication is the most cost-effective approach to mitigating damage from invasive species. I think what your question is trying to get at is this: once a non-native has proven invasive, and is in fact really widely established and abundant over a broad area, do we engage in triage and learn to live with it? The reality is that TNC stewards routinely face this question and make choices about which invasive weeds in prairies, for instance, to focus on. Those decisions ultimately should be based on a return on investment (future damage averted/ management cost), and it is my experience stewards make judgment calls about this every field season. We could get more scientific about it — especially if we had better measurements of treatment effects using before/after-treatment control designs.

Question #2: When a species has both positive and negative impacts (i.e., the lantana cited in the Davis et al. 2011 paper) how should natural resource managers weigh these impacts to make management decisions?

Mark Davis: Unfortunately, there is no simple answer to this question, and in fact the question is even more difficult than it appears. In cases like this, different stakeholders will likely differ in what they consider to be positive and negative effects. This means that there is not one set of positive and negative effects, but multiple sets. Thus, those responsible for setting management policy are not simply weighing positive and negative effects but also the desires and interests of different stakeholders. Those making the decisions should solicit input from diverse groups of stakeholders. Then, based on this input, those whose job it is to make management policy should do their best to decide what decision will be in the best interest of society as a whole. These comments are based on the assumption that the management efforts are funded by public dollars. Individual owners of property, or other private ownership groups, can
make their management decisions based exclusively on their own system of values and priorities, as long as these decisions do not violate any existing laws or statutes.

Daniel Simberloff: Your question has no generic answer. Show me where Davis et al. 2011 mention lantana’s impacts and I may be able to discuss that case (I don’t see lantana in their Comment). Obviously, in any such case costs and benefits must be assessed very carefully, and ecological costs often don’t translate readily into economic ones. For an introduced species, a site-manager ought also to consider the possibility of delayed impacts, the possibility that an unrecognized impact might already be occurring, and the possibility that a population doing no significant damage at his/her site might spread to another site where it could be more harmful. It’s also important to consider who’s doing the assessment of benefits, costs and practicality. By picking flycatchers over native riverine plant communities, aren’t Davis et al. prejudging management outcome and doing some version of what they say not to: “It is impractical to try to restore ecosystems to some ‘rightful’ historical state”? They’re declaring what’s practical and also judging that keeping the system at some ‘”future” state is more“rightful.”

Peter Kareiva: “Positive and negative” are partly in the eyes of the beholder, and more importantly the measurement of positives versus negative impacts is rare, and nearly impossible in real time. Thus, I do not think managers spend much time dwelling on this question — their thinking is instead guided mainly by treatment effectiveness and treatment cost.

Question #3: Given the well-recognized expense and difficulties with eradication, what do you think are effective strategies in addressing a highly damaging invasive species? Are the impacts of any species so severe that it would be worth continued investment just to constrain its abundance rather than reduce it significantly?

Mark Davis: What constitutes an effective strategy will vary depending on the organism, its distribution, and the environments in which it inhabits. Once a highly damaging species is widespread, efforts to significantly reduce its abundance will normally be very expensive and likely have other undesirable effects. For such species, biological control will typically be the best option in terms of cost, effectiveness and minimizing collateral damage. If good biological control agents are not available, then society must resort to other measures, such as pesticides or physical removal, approaches that are usually very costly, not as effective and often produce other types of harm.

Without question, some non-native species cause such harmful impacts that society is justified in spending resources in perpetuity to at least constrain their abundance. A good example is the sea lampreys in the Great Lakes. Despite tens of millions of dollars spent annually by Canada and the US to reduce the abundance of these parasites, the Great Lakes population of lampreys still numbers in the hundreds of thousands. Nevertheless, experience has shown that the multi-billion dollar commercial and sport fishery industries can be maintained in the face of lamprey populations at this level.

Daniel Simberloff: Some eradications are not terribly expensive or difficult (especially when the species is detected early); others are. Your question has no generic answer; each case must be judged on its own merits. There are successes and failures
with all relevant technologies — biological control, chemical control and mechanical and physical control. In addition, often one wouldn’t use a different strategy “just to constrain its abundance” than to “reduce it significantly.” In other words, you’re setting up what will frequently be a false dichotomy. Often reducing a population significantly entails doing more of whatever you’re doing to constrain its abundance. Eradication, rather than just reducing a population significantly, is more likely to entail a different method, but even here that’s not always so.

Peter Kareiva: Your question is a little like asking: “When did you stop beating your wife?” I am not so quick to accept the universal impracticality of eradication — having seen it work on TNC’s own Santa Cruz Island program and knowing in general that, if eradication is successful, it can accrue huge benefits compounded year after year. But skipping over your unseemly prefatory remark, managers and reserve stewards for TNC routinely make decisions to contain and constrain as a viable management approach. The ultimate success of this approach likely depends on some basic unanswered ecological questions. Most importantly, if intact ecosystems with a full complement of native species and limited degradation can resist invasive species, then the “contain-and-constrain” approach is clearly wise (because intact, undegraded ecosystems will be able to resist seeds arriving from the contain infection). But if this hypothesis proves to be widely wrong, then the contain-and-constrain strategy loses much of its appeal. There is still a lot of basic ecology that needs addressing regarding invasive species, as well as better meta-analyses of the costs and effectiveness of the many different treatment cocktails one might apply to invading species.

Question #4: Look out 10 years from now. Are invasives going to be considered a bigger problem than they are today by conservation scientists and by the general public? Why or why not?

Mark Davis: No, at least with respect to species causing only ecological effects. In the United States and most countries, the term ‘invasive’ is reserved for non-native species that cause harm. While, people usually agree on what constitutes harm when the threat is economic or human-health related, they often disagree when it comes to strictly ecological effects. For simple pragmatic reasons, conservationists will need to learn to live with many of the ecological changes they previously designated as harm. Because the criteria for defining ecological harm will change in the future, fewer nonnative species will be characterized by conservationists as “invasive.” Also, as globalization continues and people becomes more accustomed to living among new species — in the same way that they are becoming more accustomed to living among people from other parts of the world — the public will become more accepting of new species, and claims of ecological harm from non-native species will become less frequent. However, it is possible that non-native species causing economic damage and harm to human health could become a larger problem in the future. Of greatest concern are the possible introductions of new pathogens that could threaten widespread mortality of crops, livestock, timber and/or humans.

Daniel Simberloff: They’ll be recognized as a bigger problem. The number of introduced species increases yearly (for many groups, TNC’s Joe Fargione has shown a linear increase), and more recent ones are not less invasive or damaging. Also, more ecological and economic costs of established non-native species are recognized every year. Here in Tennessee our forests have long been devastated by chestnut blight and dogwood anthracnose; hemlock woolly adelgid and gypsy moth are more recent disasters; and now we’re adding emerald ash borer and thousand cankers disease. A better question might be: What will our forests be like in 10 years? Just as the impact of anthropogenic global climate change isn’t going to disappear because some scientists attract attention by saying it isn’t happening or won’t be so bad, the impact of invasions isn’t going to wane because Mark Davis says we should “Learn to love ‘em” (see his 2009 text book, p. 150, for the quote).

Peter Kareiva: Ah, this last question gets at the idea I liked in the Davis article, even though that idea may not have emerged as clearly as it should have. At one level, as we gain more and more estimates of the economic costs of invasive species and as “invaders from Hell” like the Asian carp in the Great Lakes come to our attention, we will be more confident that our anxiety about invasive species is justified. On the other hand, there will be increasingly large expanses of ecosystems that are infested with invasive species, some of which might seem like “pretty shrubs and flowers,” and that support a lot of animal species — including threatened and endangered species. Because of the expanding human footprint, these infested and novel ecosystems will be valued, and not shunned as somehow unworthy. This is something Davis hints at, although it is perhaps better developed in Emma Marris’s new book, Rambunctious Garden. So on one hand, the toll taken by invasive species will be more evident in 10 years, but the world will have faced so much change and human impact that some highly invaded ecosystems will be valued as “natural systems” in their own right.

Image: Crews search for invasive Asian carp near Chicago, Aug. 2, 2011, following several recent discoveries of their genetic material in Lake Calumet, IL. Credit: Jessica Vandrick- USACEpublicaffairs/