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Deep Sea Invasion

Battling Introduced Wildlife

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The aquarium-bred "killer algae" that have run rampant in the Mediterranean represent just one of hundreds of introduced species that have wreaked havoc around the world, disrupting economies, damaging habitats, and forcing native plants and animals into extinction. Here, Dr. Daniel Simberloff, an invasive species expert at the University of Tennessee, talks about some of the worst invaders and what can be done to stop both them and those that are sure to follow.

NOVA: How does the infestation of Caulerpa taxifolia in the Mediterranean rank on the list of destructive invaders?

Simberloff: Well, it's a bad one. Alex [Meinesz, the biologist who first identified the "killer algae" problem] sees it as one of the world's worst, of course. It's not surprising why he feels that way. The continental shelf area in the northwest Mediterranean is very narrow. Almost all of the life in that very biotically rich area is on these very narrow shelves. The latest update is that around 13,000 hectares [32,000 acres] have been completely infested by Caulerpa taxifolia along the coasts of Spain, France, Monaco, Italy, Croatia, and recently in Tunisia, and it's still spreading. It completely changes the habitat, with subsequent impacts on creatures that would otherwise have bred in the seagrass meadows that are no longer there. The alga has also gotten into areas near San Diego and Los Angeles, where there is a major campaign now to stop it.

So it's definitely a bad invasion, but if you were to ask me how many species has it already caused the extinction of, or how many acres has it completely changed, it wouldn't be one of the worst. There are species like the small Indian mongoose, which has been introduced in various places for biocontrol of rats and mice and has already caused the extinction worldwide of at least 30 species, some endemic to various islands. If you look at plants like yellow star thistle or cheatgrass in the American West, they've completely destroyed native grasslands over far more than 13,000 hectares.

NOVA: Do you have a classic case of a nasty bioinvasion that you cite?

Simberloff: There are so many to choose from. I guess one that I often cite along with the small Indian mongoose is the rosy wolf snail. It was introduced in the Pacific Islands to control the giant African snail, another introduced species. It never did control the giant African snail, but it caused the extinction of something like 50 endemic species of snails.

But boy, there are so many. Indeed, bioinvasion is the second greatest cause of species extinction and endangerment worldwide after habitat destruction.

NOVA: What about in this country? Which invaders do you consider the most damaging?

Simberloff: Well, remembering that Hawaii is part of this country, both the small Indian mongoose and the rosy wolf snail are in this country and are very damaging. If you're talking about the continental United States, I guess I would say some of the introduced trees in Florida, which have destroyed hundreds of thousands of acres. The big three are melaleuca, Brazilian pepper, and Australian pine. Also, tamarisk in the American Southwest. It's just an awful invader that has completely changed the habitat and species along waterways.

“There are no birds in the forests of Guam. It’s very eerie.”

NOVA: Is there an exotic that you fear most might gain a foothold in this country that hasn't yet?

Simberloff: I've often thought about the brown tree snake getting not only to Hawaii from Guam, where it's been introduced, but to the continental United States. The brown tree snake is native to Australia and some of the islands near it, and it got to Guam from the Admiralty Islands in the 1940s or '50s. It has destroyed all the forest birds in Guam. There are no birds in the forests of Guam. We're talking about approximately 12 species or subspecies that were extinguished there. It's very eerie. You're in a forest where there are no birds.

The brown tree snake has gotten to Hawaii at least eight times. It was intercepted at the commercial airport in Honolulu seven times and once at a military airport, also on Oahu. It also got to Corpus Christi, Texas, once. It gets into cargo bays and wheel wells of planes, so there's a major effort to keep it out of the runway area at the airport in Guam, and especially in Hawaii to be vigilant about what comes out of wheel wells. But there's so much air traffic that it's likely to happen again. I only hope it doesn't escape, because it climbs trees, and if it got established it would have just a horrible impact in the Hawaiian islands and large parts of the United States.

NOVA: Are you worried that genetically engineered organisms could generate invasions?

Simberloff: Yes, I'm very worried about it. It could happen. After all, to the extent that we know what limits certain native species, it's often a fairly minor feature, like inability to tolerate a temperature beyond a certain amount of hot or cold. Or it could be the presence of some pathogen that it simply can't surmount. Much of genetic engineering involves changing these tolerances, whether to produce longer growing seasons or to offer resistance to some pathogen. It's fairly ominous.

For example, a number of plants are now being engineered with Bt, Bacillus thuringiensis, a natural insecticide that kills insects, especially lepidopterans [butterflies and moths]. The target plants that the gene is put into, which are generally crop plants, probably won't become pests. But it is known that genes get from one plant to another by various means. The Bt gene may get into weeds that would then be tolerant of biocides. It could be that some of these weeds are currently controlled by lepidopterans. That's just one scenario. So I think it's very much something to be afraid about.

NOVA: Is the government concerned about this potential?

Simberloff: I have to say that both the federal government and industry have successfully managed to keep this concern from impeding their activities much. I don't want to overestimate the concern. I think these matters can be fairly well controlled with judicious expert examination of various proposals. But it hasn't been much on the radar screen of regulatory agencies yet. To a large extent, the industry and academic scientists who work in this area have succeeded in keeping the analogy of genetically engineered organisms to introduced species from being widely accepted.

I don't know if you've heard the slogan, "Regulate the product, not the process." That's pretty widely accepted. In my mind, it's a gross oversimplification. It's certainly true that by conventional breeding or by natural processes, genes get from one species to another. But the genes from human growth hormone, for example, don't get into, say, plants or bacteria. So I think there's a potential problem here.

NOVA: Are there any general rules that govern successful invasions, either by natural or bioengineered species, that authorities and biologists can use to predict future invasions?

Simberloff: By far the best predictor is whether a species or a near relative has become an invasive problem elsewhere. Within narrowly defined groups, like all species of pines, for example, there appear to be some pretty good predictors. A lot has to do with seed size and length of the seedling period. But there seem almost always to be exceptions, and for most groups there hasn't even been that kind of study to isolate those factors. So prediction hasn't really been very good.

NOVA: What about using one introduced species to combat another? For invasions that get out of hand, do you believe in biocontrol?

Simberloff: You make it sound like a religion—"Do you believe?"! I believe biocontrol has worked sometimes, and I can prove it. I also believe biocontrol has not worked many more times, and I can prove that too. I also believe that biocontrol introductions have sometimes attacked non-targets, including species of conservation concern, and I can prove that. All of those are facts. So when you say "Do you believe in biocontrol?" I guess the better question is, "Under what circumstances, if any, do I think it should be used?"

“I think biocontrol should be used with our eyes wide open — and as a last resort.”

My answer would be, I think it still has a role to play. But I think it usually should be the last resort, rather than what it often is now, the first resort. I think it's important that it's approached with full knowledge of the fact that most biocontrol projects don't work, that a substantial number even of failed biocontrol projects do leave populations of newly introduced species (namely the biocontrol agent that didn't work), and that there are a growing number of cases of non-target impacts. So I think biocontrol should be used with our eyes wide open—and as a last resort.

I guess I say "as a last resort" because it's often seen as a "green" alternative to chemical pesticides. But you can always stop chemicals if they don't work. If you've introduced a species and it's established itself, you can stop introducing it, but it will go on. Also, all living organisms have certain traits that make their subsequent trajectory unpredictable: they reproduce; they disperse autonomously; they evolve. So this is pretty risky technology.

NOVA: One of the ways that some of the worst invaders like the zebra mussel have arrived is in the ballast tanks of ships. Can anything be done to protect against that means of entry?

Simberloff: Yes, many things. First of all, the zebra mussel probably came in ballast water. It might have come attached to hulls. There's some dispute.

But undoubtedly ballast water carries an awful lot of organisms, and quite a bit could be done—for example, purging ballast tanks in deep water. The species that are picked up near ports [where ballast tanks are often filled] are adapted to life in shallow water, basically over the continental shelves. Most of them wouldn't survive in deep water. So picking up and purging ballast water far out to sea would help.

In general the industry has fought regulation, however, advocating instead for voluntary measures, especially this deep-water ballast filling and purging. But a number of studies have shown that compliance is not very high. We're dealing with a political and cultural problem—the power of a lucrative and politically important trade group. They have lobbyists—they actually have a member right on the National Invasive Species Council's Advisory Committee—and they don't like the idea of regulation. So in spite of various technologies that could be used, until this political problem is solved, the ballast problem will only grow worse.

NOVA: Does the concept of "free trade" and the globalization of the world economy only increase the chances that bioinvasion will happen?

Simberloff: It does, but it doesn't have to. That's a very interesting question. In the era of bilateral agreements between nations, before GATT [the General Agreement on Tariffs and Trade] and the WTO [the World Trade Organization] and NAFTA [the North American Free Trade Agreement], for example, when trade between nations was a bilateral issue, a country like the United States could simply refuse to accept a species that another country wanted to ship or a product that might carry a species that was viewed as potentially too dangerous. And that was the end of it.

“The Canadians wanted to ship frozen trout to Australia, and the Australians refused to accept it. They weren’t just whistling Dixie.”

But this approach has been superseded by the multilaterals. That greatly complicates the issue, because all the multilaterals, including the WTO treaties and NAFTA, have a different procedure. When an importing country wishes to exclude an import on the grounds that it is too dangerous, either because it is a species (or could carry a species) that might be dangerous, the exporting country can file a complaint that this is simply economic protectionism masquerading as environmental concern. This appeal is heard not by the importing country but by the adjudicating body of the multilateral treaty. So in the case of the WTO, the WTO decides what's safe and what isn't, not the United States, for example. All the multilateral treaties have shown themselves to be relentlessly hostile to environmental exceptions of any sort, including introduced species.

NOVA: Can you give an example?

Simberloff: Yes, and it's quite current. The Canadians wanted to ship frozen trout to Australia, and the Australians refused to accept it on the grounds that it could carry parasites, and the parasites could ultimately get into Australian waters and kill endemic Australian fish, many of which are already threatened.

They weren't just whistling Dixie. As you may know, the native rainbow trout fishery of several Western states, most notably Montana but also Colorado and Utah to a great extent, has been destroyed by a disease called whirling disease, which is a parasite that was imported to the United States in frozen trout sent from Sweden to Pennsylvania. The trout were sold in a supermarket in Pennsylvania. Some people cooked them on outdoor grills. The remains somehow got into streams in their backyards. Those streams drained into a larger stream that happened to go to the largest trout hatchery in the United States. The fingerlings then got infected with whirling disease, and they were shipped all over. And that was the end of the rainbow trout fishery in Montana and almost now in Utah.

So you can see exactly what the Australians were worried about. They were right. They've been turned down twice now, however, and the Canadian appeal has been sustained by the WTO. The reason was that the Australians could not produce a quantitative risk assessment. All the multilateral treaties demand risk assessments accompanying any claim of an environmental problem. The Australians couldn't provide those. They could document cases like whirling disease and show that their notion of the real threat was plausible, but they couldn't quantify it. No one can. It's impossible.

NOVA: So that was an easy way out for the WTO.

Simberloff: Very easy way out. And that's what they've taken. So now the Australians will either have to pay a very stiff fine or accept what I think they rightly consider a very dangerous import.

Now, there are technologies that could allow free trade but cause it to be inspected and monitored and conducted under certain regulations that could greatly lessen the threat. New Zealand, for example, passed what it calls the Biosecurity Act of 1993, which regulates but does not really impede free trade. It costs someone something, of course; inspection isn't free. But they recently issued a report on the performance of the act, and it's worked quite well. It has greatly cut down on the flow of introduced species to New Zealand. Other countries could do this too.

“The catchphrase ‘Think globally, act locally’ applies very well with invasive species.”

NOVA: Might the U.S. adopt such measures?

Simberloff: Well, it will take some Congressional action. There's interest in Congress, but not enough. There have been various attempts. The last major one that I know of was in 1992, a bill that died in the House Merchant Marine Committee. But sooner or later there will be more serious steps. I only hope it doesn't require some horribly damaging invasion to foster enough interest.

NOVA: So until then, what can average citizens do help control bioinvasives?

Simberloff: Well, there are two different levels of action. The catchphrase "Think globally, act locally" applies very well with invasive species. Locally, personally, you can avoid buying exotic pets like Madagascan hissing cockroaches or landscaping with invasive species or with species whose invasive possibilities aren't known.

You can also be aware that this is an issue and force politicians to pay attention to it. You can also volunteer in various ways. All around the country there are groups that do things like pull exotic weeds. In the political realm, you really can think globally. You can force state legislators as well as your congressional representatives to view this as a really important issue. A lot of them are aware of it, and most of them do view it as somewhat of an issue, but it's not right at the top of their agenda.

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Dan Simberloff feels that sooner or later Congress will enact legislation akin to New Zealand's pioneering Biosecurity Act of 1993, which protects against bioinvasions without impeding free trade. "I only hope it doesn't require some horribly damaging invasion to foster enough interest," he says.


At least 32,000 acres of seabed, mostly in the Mediterranean, are now infested with the aquarium strain of Caulerpa taxifolia.

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Windswept tamarisk

Also known as saltcedar, tamarisk was intentionally introduced to the U.S. to control erosion and to serve as windbreaks or ornamentals. But it did much more, overrunning riparian ecosystems throughout the nation's desert regions.

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BT corn

Simberloff worries that exotic genes introduced into crop plants, such as these corn plants bioengineered with the natural insecticide Bacillus thuringiensis, may "jump" to plant species in the wild, with unforeseeable consequences.

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Sea slug

Marine biologist Alexandre Meinesz has proposed using a tropical sea slug (above, eating C. taxifolia) as a biocontrol agent to curb the "killer algae."

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Zebra mussel

The zebra mussel reproduces at an extraordinary rate: A single female may release more than five million eggs in a year.

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Rainbow trout

Wild stocks of rainbow trout have all but disappeared in three Western states because of a parasite that arrived from Europe in frozen trout and later infected native trout in streams.

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Hissing cockroach

One thing citizens can do to combat alien species, Simberloff says, is to avoid buying exotic pets like hissing cockroaches from Madagascar (above) or plants known to have invasive tendencies.

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Deep Sea Invasion Web Site Content
Chronology of an Invasion

Chronology of an Invasion
Follow the alarming spread of Caulerpa taxifolia around the world.

The Impact of Invasive Species

The Impact of Invasive Species
Alexandre Meinesz on the degrees of menace posed by alien invaders.

Battling Introduced Wildlife

Battling Introduced Wildlife
Daniel Simberloff on strategies for combating weed-like species.

Matching Aliens With Impacts

Matching Aliens With Impacts
Match 10 successful invaders to the damage they've caused.

Interview conducted and edited by Peter Tyson, editor in chief of NOVA online.

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NOVA Home Find out what's coming up on air Listing of previous NOVA Web sites NOVA's history Subscribe to the NOVA bulletin Lesson plans and more for teachers NOVA RSS feeds Tell us what you think Program transcripts Buy NOVA videos or DVDs Watch NOVA programs online Answers to frequently asked questions