19 Nov 2009: Report

Courting Controversy with
a New View on Exotic Species

A number of biologists are challenging the long-held orthodoxy that alien species are inherently bad. In their contrarian view, many introduced species have proven valuable and useful and have increased the diversity and resiliency of native ecosystems.

by greg breining

When biologist Mark A. Davis talks about exotic species, he eventually comes to LTL, his shorthand for Learn to Love them. Flying in the face of the conventional wisdom among biologists that exotic species are harmful to native ecosystems, Davis and a small cohort of biologists espouse a heretical viewpoint: Exotic species are here to stay, so get used to them, and forget about ripping out the fast-spreading shrub, buckthorn, on a large scale or throwing Asian carp on the bank to die.

If the newcomers are only changing the ecosystem but “not causing significant harm,” then “altering one’s perspective is certainly much less costly than any other sort of management program,” Davis writes in his recently published book, Invasion Biology.

“It’s amazing how extensive the indoctrination has been: ‘Non-native species are bad — we’ve got to get rid of them,’” says Davis, chairman of the biology department at Macalester College in St. Paul, Minnesota. “Boy, if you want nature to stop, you’re going to be miserable.”

Davis and his like-minded colleagues contend that the rigid attitudes, and militaristic metaphors, that characterize the debate about exotic species make for poor science and policy-making. Assuming that exotic species are

Today, Americans continue to wage war on alien species that have taken root by accident or design.

inherently bad, that ecosystem “integrity” can be measured by the number of alien species, or even that newly arrived species are functionally different from longtime residents, simply isn’t supported by science, says Davis. Many introduced species, he notes, have proven valuable and useful, including crops (from apples to wheat), horticultural plants (hostas and Norway maples), and game species (ring-necked pheasants and brown trout).

He holds firm to that position, even while conceding that some newcomers, such as kudzu (“the vine that ate the South”) can be terribly destructive.

“I’m very careful to say that lots of invasive species are causing great problems,” says Davis, who says he supports control programs where damage is great and controls stand a chance of success. One highly destructive exotic species causing great economic harm, and for which Davis supports eradication programs, is the emerald ash borer, a beetle — recently introduced into the U.S.’s upper Midwest from Asia — that has already killed 2 million ash trees in Michigan and nearby states.

“All I’ve been arguing for is a more nuanced characterization of what’s been happening,” says Davis. And he claims that he’s finding traction for his ideas: “People are thinking more carefully about the words they are using, the assumptions they might be bringing in.”

Davis’ work is in the tradition of some scientists — including the late evolutionary biologist Stephen Jay Gould — who have argued that the movement of species around the globe should be viewed as part of the tumultuous evolutionary process, and therefore not necessarily a destructive force. Gould wrote in 1998 that the discussion about native plants “encompasses a remarkable mixture of sound biology, invalid ideas, false extensions, ethical implications, and political usages.”

“But we will not achieve clarity on this issue,” Gould continued in the journal Arnoldia, “if we advocate a knee-jerk equation of ‘native’ with morally best, and fail to recognize the ethical power of a contrary view, supporting a sensitive cultivation of all plants, whatever their geographic origin, that can enhance nature and bring both delight and utility to humans.”

Today, Americans continue to wage a war on alien species that have taken root by accident or design. Zebra mussels, Dutch elm disease, cheat grass, purple loosestrife, Eurasian water milfoil, spiny water fleas, gypsy moths — the United States is beset by exotics invading native forests, spreading across prairies, and clogging streams and lakes.

But Dov F. Sax, assistant professor of ecology and evolutionary biology at Brown University, says a growing number of scientists are listening to iconoclasts like Davis. James Brown, professor of biology at the University of New Mexico, says he generally agrees with Davis’s view of alien species but acknowledges that “almost certainly most ecologists and conservation biologists would not agree with either one of us.”

One of those scientists, Dan Simberloff — a professor of ecology at the University of Tennessee, the director of the Institute for Biological Invasions, and one of the most prominent voices in the field — called Invasion Biology “a really good book.” He added, however, “I’m going to say it has a number of peculiar aspects to it. They almost all revolve around Davis’s odd views that invasions aren’t really so problematic and there’s something xenophobic about people who worry about them.”

Among the widely accepted precepts that Davis challenges are the following:

Because native species evolved in a specific ecosystem and exotics didn’t, natives are better suited to their niche. In fact, says Davis, a scientist who didn’t know the history of individual organisms would have difficulty distinguishing natives from non-natives. The Galapagos Islands — a world

Assemblages of exotics and natives quickly adapt and perform about as well.

heritage site of biodiversity and the scene of Darwin's important discoveries leading to the theory of evolution — provided just such an example. For years conservationists had fretted over invasive weeds overrunning the islands. Because the weeds were out of control, conservationists assumed they were non-natives, recently introduced by human traffic. Research showed, however, that the plants were native. They belonged. In fact, they had been present since prehistoric times.

Diverse communities of native species resist invasions by alien species. While carefully controlled experiments on small plots have confirmed this age-old belief, studies in the tumult of natural forests and grasslands show quite the opposite, says Davis. Natural disturbances, such as disease or fire, and fluctuations in resources in diverse communities offer plenty of footholds for invaders.

Pristine ecosystems are highly evolved and well-ordered. Again, ecologists have little evidence for this, Davis says. Ragtag assemblages of exotics and natives quickly adapt and perform about as well. “If you view [nature] as a continually changing tumult, the introduction of new species isn’t necessarily looked at as a huge threat.” He cites the example of regeneration of forests in Puerto Rico, where non-native species are facilitating the re-establishment of native species.

Which brings up the dire warning of orthodox conservation biologists: The spread of exotic species threatens to drive natives extinct. Rarely happens, Davis says, except on islands, in lakes, or in other insular environments. Usually it’s the opposite — the appearance of exotics increases species richness, he maintains. Throughout the United States, local ecosystems have perhaps 20 percent more plant species than they once did because of the addition for foreign species. “How many species of plants in the U.S. have gone extinct because of the thousands of nonnative plants that have been introduced?” asks Davis. “Zero!”

Finally, exotics signify a “degraded” ecosystem. “There isn’t such a thing as a healthy ecosystem or a sick ecosystem,” Davis says. “Ecosystems are just out there. There’s no particular goal or purpose. They’re just the species and the physical and chemical processes taking place.”

Other ecologists beg to differ. Dan Simberloff counters that invasions of exotic species do threaten native species with extinction. The chestnut blight, caused by an introduced fungus, swept across the eastern U.S. a century ago, virtually exterminating the native chestnut tree. In addition, Simberloff says, “We know it caused total global extinction of at least seven species of moths that were host-specific only on American chestnuts.”

Even if exotics don’t drive native species to extinction, they can completely transform ecosystems, many biologists argue.

“There are some biotic communities that have entirely disappeared from the U.S. because of invasive species,” says Simberloff. “There are many

Land managers can’t begin to control the thousands of exotics that reach our ecosystems.

others that have drastically changed over large areas.” For example, he notes that Eurasian cheat grass now dominates millions of acres of Western range, displacing native bunch grasses and reducing the value of the range for livestock. Asks Simberloff, “If thousands of people work to deal with the impact of these, even aside from any moral or aesthetic issue, is Mark [Davis] saying they’re all nuts?”

James Carlton, professor of Marine Sciences at Williams College and a Pew Fellow in Marine Conservation, attacks Davis’s assertion that biologists can’t distinguish natives from exotics. “We can easily distinguish natives from many (not all!) nonnatives many times in many ways, based very much on how they function in an ecosystem. Native species are often finely tuned physiologically to the environment, whereas nonnative species may possess a repertoire of adaptations that clearly do not match the environment.”

Still, a sizable minority of biologists are more sanguine about the impact of exotic species. Dov Sax says he began to question exotic species orthodoxy as an undergraduate at the University of California, Berkeley. A professor leading a field trip described the Bay Area’s abandoned plantations of Australian eucalyptus trees as a “biological desert.” Says Sax, “There was all kinds of stuff growing in there. I found there were really a similar number of species in both [native oak and eucalyptus] woodland types. Exotics weren’t always doing the awful things people seemed to think they were doing.”

Sax says that land managers can’t begin to control the thousands of exotics that reach our ecosystems through globalized trade and travel. “A lot of conservation biology in the past has been built around the idea of preventing change,” says Sax. “That old mantra is going to get thrown out because it’s going to be impossible to prevent change.”

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The future landscape will be home to “novel ecosystems,” never-before-seen agglomerations of species, of which exotics will be a key — and often valued — component, Sax and others say. “If we lump them into this category of all being evil or awful in some way,” he says, “we may blind ourselves to those situations where they’re actually providing a benefit either for humans or for biological conservation.” For example, a recent study of two nonnative wetland plants, phragmites and hydrilla, suggests they provide waterfowl habitat, biomass production, and nitrogen retention that equals that of native species.

Overselling the threat exotic species pose is bound to lose credibility as exotics make up ever more of the biota around us, says Davis. And it will lead to misguided spending on projects as fruitless as ripping out buckthorn from thousands of acres of parks, when we should focus instead on disease organisms, agricultural pests, and other more pressing threats, Davis believes.

“It’s very important,” he says, “to distinguish harm from change.”

POSTED ON 19 Nov 2009 IN Biodiversity North America

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COMMENTS

My reaction is largely the same as what Dan Simberloff was quoted saying in the article — [Simberloff] called Invasion Biology “a really good book.” He added, however, “I’m going to say it has a number of peculiar aspects to it. They almost all revolve around Davis’s odd views that invasions aren’t really so problematic and there’s something xenophobic about people who worry about them.” -- I found many of the points from the book, as recounted in the article, to be perfectly sensible, but…

Here’s one such peculiar viewpoint -- For example, a recent study of two nonnative wetland plants, phragmites and hydrilla, suggests they provide waterfowl habitat, biomass production, and nitrogen retention that equals that of native species. -- While these measures may be reported accurately, they overlook the “elephant in the living room” that within dense populations of Phragmites or Hydrilla, it is difficult to find any native wetland plant diversity whatsoever, not to mention any of a large proportion of the specialized, and even generalist, native invertebrate, and indeed vertebrate, life associated with the native plants.

Example after example of this sort of narrowed thinking popped out at me as I read the article. It is my belief that the authors’ apparently favored children of agri-business and horti-business will continue to do just fine, no matter what is done by society to fight what I call the “invasive, displace-ive” species. The scientific, aesthetic, spiritual, and possible future agricultural, horticultural and medical benefits of preservation of diverse native biological diversity should not be pitted, in a false dichotomy, against the commodity-producing agriculture and horticulture.

James C. Trager, Ph. D.
Biologist - Naturalist
Shaw Nature Reserve
Gray Summit MO 63039

Posted by James C. Trager on 19 Nov 2009

When mass opinion turns orthodox, we are at risk of group think and may become blind. Cheers to Davis for bringing up a valid and thoughtful viewpoint.

Posted by Claire Howard on 19 Nov 2009

Agreed: Nonnatives can be beneficial, neutral, or highly destructive.

Agreed: Some nonnatives are here to stay, and money and time spent trying to remove them is wasted.

Bone of contention: How to stop the next highly destructive nonnative invasive BEFORE it arrives. What legal controls prevent importing the next gypsy moth, the nest kudzu, the next zebra mussel? What legal penalties for those responsible for importing? What responsibilities for those importing a nonnative biological control for a nonnative invasive species?

Posted by John Barr on 20 Nov 2009

Perhaps the view of global homogenization of species is the view of inevitability. But let's not undervalue the losses that go with it. Read Dr. Douglas Tallamy's book Bringing Nature Home for a view of coevolution on a biochemical level between native insects and native plants.

While we may have to learn to tolerate aggressive exotics because we simply don't have the tools or money to do otherwise, there are some exotics that have a definite impact on other species. Certainly not all exotic species are in that category, but that does not mean that we can give a blanket shrug of the shoulders and say "Oh, well...LTL."

The deeper issue is what will this planet be like when we are knee-deep in generalist species and have lost the narrow niche species?

Posted by Patricia McGhan on 20 Nov 2009

“I’m very careful to say that lots of invasive species are causing great problems,” says Davis. I'm glad to hear that, because if an exotic species is not causing or likely to cause "great problems," it's not an invasive species.

Invasive species are destructive by definition. The generally accepted definition of invasive species is "an alien (or non-native) species whose introduction does, or is likely to cause economic or environmental harm or harm to human health."

If an exotic species is not harmful or likely to be harmful, it's not invasive by definition.

As in many arguments, this one seems to be based on a confusion over the definitions of words.

Of course, there can be uncertainty involved in determining whether an exotic species is invasive.

Furthermore, if human beings place a value on any species, community, or ecosystem, for whatever reason (not necessarily economic), we can rightly say whether it is healthy or "sick." It would be irrational to do otherwise.

Posted by Bill Jacobs on 20 Nov 2009

Coming from New Zealand, I find all this bizarre. Introduced species have decimated New Zealand's endemic fauna.

Posted by John Ashton on 24 Nov 2009

A great way to get lots of press is to make outlandish claims that go against rational interpretation. I think Davis needs to go sit in a field of yellow star thistle and contemplate his philosophy.

Posted by Judith Myers on 24 Nov 2009

I've seem (invasive) thistles in the San Francisco Bat Area that attracted hoards of monarch butterflies to an urban lot. I could never understand all the excitement about native species when such neglected urban lots had a wildness and beauty of their own, so very, very much preferable to the awful, sterile "development" that was steadily erasing them. In those cases, inappropriate development, not invasive species, should have been the priority issue.

Posted by TRB on 25 Nov 2009

I agree with many of the comments above: simplifying this complex matter will certainly lead us nowhere. There is a mixture of concepts: urban systems are confused with natural ecosystems, biological diversity from an ecosystem approach is confused with number of species not organized in any biological system, resilience is mentioned but not defined (how is resilience measured in cases where it is supposed to have increased because of the introduction of alien species?), serious damage is confused with simple -and inevitable!- "change".

Having worked many years on this important matter I think is doesn't help to review basic concepts on which all biologists should by now coincide, and actually do! Otherwise we keep going two steps forward. three steps backward, and the problem will never be resolved.

Posted by Victoria Lichtschein on 01 Dec 2009

I thank Davis for his views and adding balance to the long held orthodoxy. I would like to add that we can give value to the current assemblage of species or communities in any landscape and therefore qualify any change to that system as good or bad. Consequently investing in "protecting" our values withing that system from a change deemed "bad."

However, change in any system has value and is not inherently bad. I see many invasive species in a Vermont Forest landscape that are on various eradicate and watch lists but exist as a relatively benign assemblage, and some times even benefit wildlife from fruits produced.

Predictions regarding how systems will respond to "invasions" are very difficult to make when dealing with a such a short time scale and very little precedence. I have seen acres and acres of buckthorn and honeysuckle crowded out and shaded by newly competing hardwoods.

No system is the same, no orthodoxy is appropriate. Managers should continue to evaluate ecosystems with humility and responsibility.

Posted by Allan Thompson on 04 Dec 2009

Sure, many volunteer weed hackers, property managers and scientists go over the top when describing their dislike of invasive species, but that says nothing about the science that points to the conclusion that invasives species have the potential to seriously cut into biotic diversity.

As far as the argument that invasive species are more fit than natives; well those same natives displace the invasives when introduced into their (the invasive's) home country. The English sparrow is a good example of that.

New research has shown that invasives largely outcompete natives because either their pathogens, parasites and competitors have been left behind in their home country; or because the pathogens, parasites, and competitors require other hosts or environmental conditions not found in the new lands. The lack of disease causing organsims, parasites, predators, grazers or co-evolved competitors means that the invasives can divert most of the energy that would have gone into defending or repairing themselves towards more growth and reproduction.

Some case histories show that the inevitable result of species introductions will be lowered diversity on a world-wide scale. Eventually newly evolved or introduced pathogens, etc. will catch up with the invasives and then their lack of coevolution with their new biotic and physical environment will bring them down.

Look at the European honey bee as an example. Introduced to N. America about 600 years ago. By the 1800's they were common throughout N. America (read Irving Washington's account of Oklahoma and Texas for example). They likely had pushed out many (extinct or reduced greatly in population and range) if not most of N. America's native pollinators and may have caused the extinction of the Carolina Parkakeet through tree hole competition.

Now the enemies of the European honey bee have caught up with them and they are becoming rare other than in cultivation and can no longer do the job of pollination adequately. With our native pollinators having been pushed out, N. America's pollination-dependent plants are left wanting and the biotic environment is threatened with greatly lessened species diversity.

Evolution and total species diversity (world-wide) is controlled by, and is dependent upon, continental isolation. Isolation breeds diversity. The artificial linking of continents must cause a decrease in biotic diversity. The initial disruptions caused by invasive introductions can be especially severe.

I think something that needs to be looked at in depth is the reintroduction of native plants and animals pushed out by invasive species, once that invasive is brought into control through evolution of pathogens, competitors, etc. This has the potential to greatly reduce the initial shock of native species loss following non-native species introductions. This will require the maintenance of native reserve areas where invasives are removed; for long periods (century or more). Which is about all most invasive species control programs can actually accomplish.

Posted by Andy on 08 Dec 2009

How is it that humans ourselves can be left out of the conversation when defining what is an invasive species?

Posted by Julie on 08 Jan 2010

Julie: we aren't left out. See the numerous works by Paul Martin and many others that document the immediate decimation of wildlife species coincident with human colonization of various continents and islands. For example the Polynesians extirpated many species of flightless birds from each island they colonized, esp. Hawaii. Humans decimated the unique fauna of Madagascar shortly after colonizing it. Same with New Zealand, same with Australia and of course, North America. I think Jared Diamond goes into detail as does Martin in their books.

Posted by Andy on 17 Jan 2010