Earth Island Institute logo, tap or click to visit the Institute home page

Go Back: Home > Earth Island Journal > Issues > Winter 2012 > +/-


Harm Is in the Eye of the Beholder

Mark Davis is the DeWitt Wallace Professor and Chair of Biology at Macalester College, St. Paul, MN. He is the author of Invasion Biology (Oxford U Press, 2009).

For 25 years, the American public has been inundated with horror stories involving non-native species. Think: snakehead, kudzu, Asian carp. This has largely been the result of selective communication from scientists and a media that too often have been more than eager to promote these stories without engaging in any critical analysis or research of their own. Usually provided with just a single perspective, the public largely accepted the idea that non-natives, as a group, are noxious and undesirable.

In fact, this is anything but true. Non-native species are just species. Like native species, some of them produce effects we like, some produce effects that we don’t like, and most are comparatively benign. Have some introduced species caused changes that most everyone would agree have been very harmful? Absolutely. By killing timber trees, gypsy moths and the emerald ash borer have caused, and continue to cause, enormous economic damage to the United States. Introduced pathogens that threaten human health are also clearly harmful species. At the same time, the danger posed by non-native species as a group often has been exaggerated and misrepresented. For example, ecologists and conservationists often describe non-native species as the world’s second greatest extinction threat, despite the fact that existing data shows this clearly not to be the case. It is true that introduced species can and have caused many extinctions in insular environments such as oceanic islands and freshwater lakes. But they have caused very few extinctions on continents or in marine systems. In fact, the primary regional biodiversity effect of introduced species is to increase species diversity. Due to the introduction of thousands of plant species, the United States has approximately 20 percent more wild plant species than it did 500 years ago.

While extinction threats should not be our only concern when it comes to non-native species, if we are going to label a species as “harmful,” that characterization needs to be based on good science. Efforts to vilify non-native species by misrepresenting their effects are ultimately counterproductive, as is the use of pejorative language to describe them: e.g., calling them “invaders” or “biological pollution.” Once harm is claimed, society is generally obligated to try to reduce or eliminate the harm. Also, if harm is misrepresented, scarce societal resources can end up being spent on needless and futile management programs. Good (or bad, depending upon your point of view) examples of such efforts include the persistent efforts by federal, state, and local agencies to eradicate or significantly reduce non-native herbs and shrubs in forest environments in the US.

Most of these efforts have been ecologically misguided since they typically have not addressed the underlying causes for the spread of these new species. Because many native species began to decline at about the same time that non-native species began to become abundant, ecologists and conservationists prematurely assumed that the non-native species were the cause for the decline of the native flora. Once good scientific studies were conducted on these systems, the data frequently revealed that other changes in the environment were the primary cause of the decline of the native species, and that the non-native species were simply better adapted to the new environment. A good example is garlic mustard, an introduced species usually described as an aggressive invader that chokes out native plant species. In fact, recent scientific studies of this species have concluded that garlic mustard is less an agent of change than a consequence of it. Because these non-native plants often have not been the primary drivers of native plant declines, efforts to reduce the abundance of the non-native species frequently do little to make the environment more suitable for the native plants. Besides their frequent long-term ineffectiveness, these management efforts often have involved extensive use of pesticides and/or mechanical removal, interventions that commonly produced considerable collateral damage, negatively affecting other species and the environment. Too often these eradication and control efforts have illustrated the adage, “a poor way to get what you want is to try to get rid of what you don’t want.”

Take a closer look at the non-native plant control efforts in eastern American forests that have focused on species such as garlic mustard and several species of non-native honeysuckle. These species are not threatening human health or causing any significant economic harm. Nor are these species threatening any native species with extinction. Nevertheless, many ecologists and conservationists have decided to characterize the ecological changes associated with the spread of the new plant species as harm. It is true that the relative abundance of species is changing in the US. Species that were once common in some areas are becoming less common while other species that were uncommon, or not even present, are becoming abundant. However, this change becomes harm only if someone declares it so.

Nature is dynamic. Organisms are opportunistic. Native birds and insects feed on the fruits, the leaves, and the nectar of non-native plant species. Non-native honeybees pollinate many native plants as well as providing an enormous ecological service by pollinating agricultural crops and orchards. After a while, the value distinction between native and non-native begins to blur. Few people would disagree that control measures should be undertaken to reduce the abundance of non-native species that threaten human health or cause great economic harm. But when there is no demonstrable harm – and instead simply ecological change – a much more sensible approach to non-native species is to learn to live with them. Like other species, we too need to learn to adapt to change. In many instances, rather than trying to manage nature, we would be much better off managing our reactions to it.

For an opposing view, read what Daniel Simberloff has to say …


Email this article to a friend.

Write to the editor about this article.

Subscribe Today
cover thumbnail EIJ cover thumbnail EIJ cover thumbnail EIJ cover thumbnail EIJFour issues of the award-winning
Earth Island Journal for only $10



Hello to all readers,

Mr. Warner said he read my remarks, but his comments don’t indicate that he did. Misrepresenting and misstating people trying to bring some common sense into this discussion of non-native species only illustrates the ideology that so many people have adopted.  Belief is the enemy of science.  As a bumper sticker I saw last summer so trenchantly suggested, ‘the less you know, the more you believe’.  Or, as H. L. Mencken said, “For every complex problem, there is a solution that is clear, simple, ... and wrong.”  People love to hate an enemy.  I believe that if we didn’t have an enemy we’d create one.  It is especially easy to hate newcomers and blame them for our problems, i.e., they are a simple and clear enemy, and if we could just get rid of them, everything would be fine.  While there is no question that some new species cause severe problems, as I clearly emphasized in my comments, many new species have been vilified based only, or primarily, on belief, and not on good evidence.  Readers might be interested to know that for every email I have received on this topic during the past several years that has been critical of my message, I have received several emails expressing gratitude for bring some rationality to the discussion of non-native species.  For example, despite Mr. Warner’s claims, many insect herbivores of garlic mustard have been documented.  We certainly have insects that feed on our garlic mustard in the Twin Cities area.

Remember that with enough focused research, it is always going to be possible to find evidence that any species, native or non-native, negatively affects some other species. But, the fact that you can document some negative effect does not necessarily mean that there is any significant ecological effect in the field.  This past summer, we documented that the growth of newly emerged ash seedlings growing in garlic mustard soil was reduced by 10% (we didn’t document any effects on the growth of elm, red oak, or white oak seedlings).  Documenting a 10% reduction in growth in first year seedlings in a pot experiment does not mean that ash tree recruitment in the field is actually being affected by garlic mustard.  Documenting a statistical significant effect is not the same as documenting something of ecological significance.

By using garlic mustard to support his case, Mr. Warner couldn’t have done more to support my message.  The belief that garlic mustard is a doing all kinds of bad things to native forest species and ecosystems has been dogma for the past 20 years, but this belief is not backed up by science.  At least half a dozen research papers that have been published in the past few years, conducted by some of the most respected garlic mustard researchers (I assume that these researchers would satisfy Mr. Warner’s criteria for knowledgeable people), have in fact found that garlic mustard is likely having very little actual impact in Midwestern and eastern North American forests. 

Here are a few quotes from some of the papers:  “Invasive plants are conspicuous and are often assumed to be the agents of change; however, our study in conjunction with the work of others indicates that … invasive plants are the beneficiaries, rather than the agents, of change” (Nuzzo et al. 2008, Conservation Biology); “Declines [of native species] were not significantly different in stands with or without invasive plants. ... our data are more consistent with the hypothesis that the invasive plants are not drivers of species loss but are merely passengers” (Rooney and Rogers 2011, Invasive Plant Science and Management); “There were no significant treatment effects [removing garlic mustard] in any year on native vegetation” (Bauer et al.2010, Restoration Ecology). 

A lot of time and effort and herbicides have been used to try to eradicate and control garlic mustard over the past 20 years.  Money and time well spent?  Has all that herbicide spraying and trampling been justified.  I don’t think so.  We have limited public resources to manage species.  Let’s be as discriminating as we can, and do our best to allocate these resources only where real problems exist.  Let’s do our best to base our management decisions on good science, not on belief.  And let’s remember that it is very easy to jump to premature, and wrong, conclusions.  This is the mistake that was made with so many of the non-native plants in the US eastern forests.  Ecologists, conservationists, and land managers observed two things happening at the same time: native species were declining in abundance and non-native species were increasing.  The error that was made by so many was a junior high school level mistake, attributing causation to correlation.  That mistake resulted in a the dissemination of a lot of misinformation and the wasteful expenditure of scarce public resources.

Apparently, my remarks agitated Mr. Warner so much that he felt compelled to respond before even reading Dan Simberloff’s perspective.  A question all readers should ask is why are so many people so emotional when it comes to the topic of nonnative species?  Why are so many so passionate in opposing nonnative species as a group?  A good question to reflect on.—Mark

By Mark Davis on Sat, December 03, 2011 at 3:40 pm

My apologies.  In my haste I failed to notice that Mark Davis most likely holds a Ph. D., so should be spoken of as Dr. Davis, not Mr. Davis.

By Jay_Warner on Sat, December 03, 2011 at 2:06 pm

Mark Davis’s post deserves a response, even before I read Daniel Simberoff’s views.  First, Mr. Davis seems to presume that species extinction is a primary measure of ‘invasiveness’ – damage from ‘non-native’ species.  If the chemicals generated by garlic mustard succeed in eliminating tree saplings (which I believe it does), how long will it be until a wooded river flood plain is reduced to 10%, or 1%, of the present tree population?  What changes will occur in the river as a result?  A 100% reduction – species extinction – is not the issue.

In Racine, Wisconsin, we have what fisher-people consider an excellent fishing stream, where the fish feed on many insects that in turn grow in the moist forest understory adjacent to the river.  How many insects will grow in the garlic mustard when it carpets the ground?  Likely, none - nothing eats those shiny, crinkly leaves (that’s why they are always so pristine in the late fall)  What does this absence of food supply imply for the fish population?  Yes, that is a rhetorical question.  A similar argument can be made for the excess of honeysuckle - nothing grows under them, with the implication that there is reduced food for the fish under the honeysuckle, as well. 

I would welcome a “good science” study to confirm or deny these assertions of linkages between invasive plants and fish, but until someone finds the funding, we’ll have to make our decisions based on assessments of knowledgable people.  It is arguably disingenuous to dismiss all claims not based on “good science” with which one might disagree; possibly good rhetoric, but lousy logic.

If Mr. Davis wants to argue that we aren’t sure to what “original condition” we should return the lands from the present invaded state, I agree there is a substantive debate available.  The original human inhabitants extensively managed the land for their own purposes.  Current residents too often think that the condition seen by the first Europeans was the “forest primeval.”

But Mr. Davis seems to be arguing that invasive species really don’t impact us much, so let them go.  He speaks as if the direct, visible impact on humans was the only significant measurable effect.  But not all impacts are immediately visible.  Will I suffer if the fish in the Root River decline by 90%?  No, I don’t fish, and I don’t depend on income from local tourism.  But when a seriously massive rainfall came three years ago, Racine discovered that the woods and undergrowth in the flood plains distinctly mitigated the flood damage. Most of us don’t even notice when the less serious floods come, thanks again to those wooded flood plains. Garlic mustard won’t do that. 

We need to manage our lands at least as much as our first residents did.  Everything really is linked to everything else, whether or not we have a “good science” report to specify the details of that linkage.  Naturalists who see the effects of garlic mustard or honeysuckle invasions (not to mention Phragmites australis (Giant reed grass), Teasel, and Japanese knotweed, etc., etc.) may have a hard time relating the impacts to a wallet-emptying financial impact.  Nonetheless, we may rest assured there is a linkage; the issue is how big is the impact, and how much we are willing to spend (in dollars and sweat) to minimize it.

By Jay_Warner on Sat, December 03, 2011 at 1:53 pm

Leave a comment

Comments Policy

Please enter the word you see in the image below:


Four issues for just
$15 a year.

cover thumbnail EIJ

Join Now!