Environmental t News It’s in the microwave popcorn, not the Teflon pan
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KELLYN BE T TS
esults of a study by the U.S. Contaminants (2005, 22, 1023–1031). rectly convert 1.4% of fluorotelomer Food and Drug AdminisThe scientists found that a sigalcohol to PFOA. Another 7% is metration (FDA) published in nificant percentage of the fluorotabolized to intermediate acids that October reveal that compounds telomers migrated from the bags to are also expected to eventually deknown to break down into the susthe popcorn oil, resulting in levels grade to PFOA (Chem.-Biol. Interact. pected carcinogen PFOA (perfluoof 3–4 mg/kg (11 µg/dm 2). These 2005, 155, 165–180). So a conserrooctanoic acid) may be served up concentrations are hundreds of vative estimate for the conversion to millions of unwitting consumers times higher than the amount from fluorotelomers to PFOA is 1%. in bags of microwave popThis means that a person corn. The family treat could eating a whole bag of popaccount for >20% of the avcorn could take up 0.017 ppb erage PFOA levels now meaof PFOA. sured in the blood of U.S. A person would have to residents. eat about 300 bags of micro Most Americans carry wave popcorn over 5–10 4–5 ppb of PFOA in their years (about a bag a week) if blood, according to the U.S. the average 4 ppb of PFOA in EPA’s draft PFOA risk astheir blood came from the sessment, but its source snack. Toxicologists say that has been unknown. Prod5–10 years is an appropriate ucts used in the home are timescale because PFOA is thought to play a role, inreported to have a half-life cluding nonstick cookware New research shows that the grease-repelling fluorotel- in humans of ~4 years. Consuch as Teflon pans, which sumption of just 10 bags of omer chemicals used to treat some microwave popcorn bags can migrate into the popcorn oil. are produced by a process microwave popcorn a year that uses PFOA. But several could contribute about 20% studies, including this one, sugof PFOA that could migrate from of the average blood PFOA levgest that nonstick cookware is not nonstick cookware the first time it els, say the scientists interviewed a major source. is heated >175 °C. Because the suranonymously for this article. The FDA team investigated face area of a microwave popcorn “This dose is certainly not insigconsumer products that contact bag is ~1000 cm 2, a person connificant,” Martin says. “Scientists food—nonstick pans, food wraps, suming a bag’s worth could take should be, and are, considering and papers—as potential sources, up to 110 µg of fluorotelomers, acpolyfluorinated precursors [such says FDA chemist Timothy Begley, cording to three toxicologists who as the fluorotelomers] as a potenthe study’s lead author. Some of performed these calculations on tial human exposure pathway to the papers used for packaging food the condition of anonymity. perfluorinated acids, including are treated with grease-repelling Toxicologists commonly conPFOA,” he adds. fluorotelomer coatings. Microwave vert such an exposure into a huMicrowave popcorn bags probpopcorn bags have the most of any man dose by dividing by the ably represent the worst-case scefood wrappers—about 4000 mg/kg average adult body weight, 65 kg. nario for getting PFOA precursors in the coating or 25 mg per square This means that the average dose into foods, Begley notes. This is bedecimeter (dm) of paper, the auof fluorotelomers from each bag cause the amount of fluorotelomers thors note. of popcorn is 1.7 µg/kg. Children in the coatings is high and because Many fluorotelomer paper coatwould get a higher dose. popcorn bags heat up to >200 C in ings contain mixtures of C6, C8, Scientists don’t currently know just a minute or two. However, fluoC10, and C12 fluorochemicals. Prehow readily humans can metaborotelomer coatings are not used in vious research suggests that the lize fluorotelomers to PFOA, says all microwave snack-food packaging, C8 and higher fluorotelomers can University of Alberta (Canada) bioaccording to Begley, who is still redegrade to PFOA, Begley and colchemist Jonathan Martin. But he searching other papers and coatings. leagues write in Food Additives & has found that rat liver cells can di—REBECCA RENNER n Environmental Science & Technology / january 1, 2006
© 2006 American Chemical Society
News Briefs
Benjamin Bostick
Microbes doing the otherwise beneficial job of degrading organic waste in landfills could be releasing arsenic from the surrounding rocks and sediments by creating reducing environments that mobilize the metal, according to new research published in this issue of ES&T (pp 67–73). The findings suggest that, in many parts of the U.S. and the world, landfill leachate can create an arsenic problem in groundwater that moves through rocks containing normal amounts of iron oxides and arsenic. Data from Superfund sites reveal the magnitude of this problem, says the research’s corresponding author, geochemist Benjamin Bostick at Dartmouth College. Arsenic is a groundwater contaminant of concern at more than a third of U.S. Superfund sites—even though arsenic was never dumped, deposited, or used at many of these sites. Microbial degradation is the cornerstone of the increasingly popular practice of monitored natural attenuation (MNA). The Superfund sites that use MNA are nearly 50% more likely to have excess arsenic contamination than sites that use other methods, Bostick notes.
By sampling wells at the Coakley Superfund site in New Hampshire, Dartmouth College researchers, including Carl Renshaw and Jamie deLemos, learned that arsenic was leaching into groundwater.
Bostick and his Dartmouth colleagues show in a combined field and laboratory study that common soil microbes are oxidizing organic carbon and reducing Fe(III) that occurs in poorly crystalline iron oxide minerals. This process also releases arsenic that is adsorbed to the iron oxides and can result in high levels in the underlying groundwater down-gradient from a landfill site. It is the same mechanism—albeit on a much smaller scale and with far less dire consequences—thought to be responsible for the high arsenic levels in drinking water in Bangladesh. The phenomenon is likely to be widespread at landfill sites, says hydrogeologist Charles Harvey at the Massachusetts Institute of Technology, who is studying the problem in Bangladesh. “You don’t need hazardous organic waste from a Superfund site. Every landfill has garbage, and that’s organic waste.” “This problem is probably widespread,” agrees U.S. EPA environmental engineer Robert Ford. “It’s been identified in the Northeast [U.S.], but it will be recognized more and more in other parts of the country,” he predicts. Indeed, every scientist contacted for this article agreed that Bostick has identified a little-known problem likely to be occurring throughout the world. For 10 years, Dartmouth scientists have monitored the Coakley Landfill, located near the town of Rye in southern New Hampshire. Coakley was listed as a Superfund site in 1983 because of leaking VOCs. It was also contaminated with arsenic and other metals, although the source of the arsenic was unknown. The landfill was capped in 1998 to limit the spread of the contaminants and encourage MNA. Since then, natural pro-
Solar cycles impact warming
Roughly 10–30% of the rise in global surface temperatures during the last two decades may be due to increased solar output, according to new research from Duke University. Nicolla Scafetta and Bruce West based their research on an earlier study that found that a two-year gap in satellite data had missed an increasing trend in solar luminosity from 1980 to 2002. Scafetta and West analyzed 22 years of satellite data to determine how the atmosphere would respond to increasing solar irradiance. Although the Intergovernmental Panel on Climate Change has concluded that the sun has not contributed to the 0.4 °C rise in global surface temperatures during the past two decades, global climate models need to account for solar activity, the authors say. (Geophys. Res. Lett. 2005, 32, L18713)
Trouble looms Down Under
Climate change will damage health. That’s the conclusion of a new report by the Australian Medical Association and the Australian Conservation Foundation, which predicts that average temperatures will rise by 1–6 °C by 2100. In Australia, up to 15,000 people could die every year from heat stress by 2100, up from about 1000 a year at present, while dengue fever and other mosquito-borne diseases could spread as far south as Sydney. Dengue fever in Australia is currently confined to the country’s tropical and sparsely populated far north. Australia has not ratified the Kyoto Protocol on Climate Change treaty. The report is available at www.ama.com.au/web.nsf/doc/ WEEN-6GFAZM.
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Arsenic and old landfills
Environmentalt News cesses have reduced most of the organic contaminants to levels considered safe. For example, concentrations of benzene, the only source of which was the landfill, decreased after capping. So did the concentrations of all the metals—except that of arsenic, which increased as much as 10-fold in some wells. This demonstrates that the arsenic isn’t coming from the landfill, Bostick says. Instead, the arsenic is leaching out of the clay soils on which the landfill sits. This clay has concentrations of 20 ppm arsenic with 5% iron—most of which is in amorphous iron hydroxide minerals. Although these arsenic levels are high, they are not unusually high, Bostick says. He believes that
benzene accumulated in the clay layer because it strongly partitions to clay. Capping the landfill set off the arsenic release process by limiting the supply of oxygenated water and thus creating reducing conditions, he says. Laboratory experiments further confirm the group’s hypothesis. “It’s very significant to note that the landfill is not the direct source of the arsenic—it’s natural,” says University of Arizona environmental chemist Wendell Ela. “It means that many leaking landfills have, and will have, arsenic come out into groundwater,” he says. Natural attenuation can still be an appropriate remedy for organic contaminants, Ela says. But when making such a decision, one needs
California’s shifting sands using LIDAR (laser imaging detection and ranging), a laser scanning technique more precise than the analyses of aerial photographs that researchers had used previously. By comparing the volume of sand that fell from the cliffs with estimates from other studies University of california , San Diego, Jacobs School of Engineering
Southern California’s golden beaches, immortalized by the musical group the Beach Boys, may not last forever thanks to erosion. Even the widely held assumption that rivers are the main source of the beach sands appears to be crumbling away. Instead, two new studies indicate that the dramatic sea cliffs, which Californians fight to save, supply about half of the beach sand. In southern California, beach cities have spent millions of dollars on “beach nourishment” projects that dredge and replace sand that has been swept away. At the same time, seawalls, rock piles, and other protective structures keep coastal properties and highways from tumbling into the sea in some places. The “armoring” of the coastline has pitted private property owners against environmental groups who argue that the barriers are ugly and damage the beaches. To better understand these processes, University of California, San Diego (UCSD) structural engineer Scott Ashford and his Ph.D. student Adam Young spent 6 years measuring the cliffs on a 50-mile stretch of coast north of San Diego. They collected data by
New research shows that California’s dramatic sea cliffs are just as important as its rivers for supplying sand to the state’s fabled beaches.
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to consider the iron content of the surrounding rocks, the mineralogy, and the likelihood that the subsurface geochemical environment could become anaerobic. “In some cases, we may find that natural attenuation needs to be augmented—for example, by aerating the site,” he says. Janet Hering, a geochemist at the California Institute of Technology, agrees. The suitability of MNA for remediation of a particular site depends on a lot of things, she notes. MNA could even be suitable for arsenic remediation in situations in which the aquifer is oxidized and its arsenic concentration is low, says Hering. “MNA is not an easy solution,” she adds. “It has to be carefully evaluated.” —REBECCA RENNER
of the volume deposited by river sand, they concluded that roughly half of the beach sand probably originated from the cliffs. LIDAR has been used previously to map beaches, but this is the first time it’s been used to monitor beach erosion, says Young, who presented the study results at the American Shore and Beach Preservation Association meeting in October. UCSD geologist Neal Driscoll and graduate student Jennifer Haas came to the same conclusion by using a mineralogical fingerprint technique, according to Driscoll. Haas spent three years examining thousands of sand grains along the same stretch of beach. She found that the beach sand is dominated by clear quartz grains—the same kind of grains that are abundant in coastal cliffs. In river and dredge sands, she found mainly frosted grains. Haas estimates at least 50% of the beach sand comes from the cliffs. Mineralogical fingerprinting is a “great way” to estimate sources, says U.S. Geological Survey sedimentologist Jonathan Warrick, because it is a simple method that measures what’s actually in place. It does not rely on any calculated estimates. The researchers from both
measurements of sediment load reliably reflect the amount of sand deposited in the coastal zone. This is an enormous amount of sediment, but it may not get to the beaches. Because river water thick with sediment is often denser than seawater, it may sink to the bottom and escape the coastal zone that replenishes the beaches. “This is our hypothesis—but we don’t have any observations,” he says. Ashford and Driscoll plan to map sediments offshore by using sonar profiling in an effort to answer this question. —REBECCA RENNER
Guibin Jiang to direct ES&T’s first Asia office Guibin Jiang, deputy director of the Chinese Academy of Sciences’ Research Center for Eco-Environmental Sciences—an institute with more than 300 staff members—has been appointed ES&T’s first Asiabased associate editor. The new office in Beijing officially opens in January 2006. In making the announcement, Jerald Schnoor, ES&T’s editor, hails Jiang as a distinguished scientist and researcher who has made signifiGuibin Jiang cant contributions in the area of environmental organic chemistry. As an associate editor, Jiang will handle papers submitted to ES&T from around the world on topics such as emerging chemical contaminants, herbicides, organometallics, green chemistry, and ecotoxicology, Schnoor says. Jiang earned his Ph.D. from the Research Center for Eco-Environmental Sciences and has conducted research at Shandong University (China), the National Research Council of Canada, and the University of Antwerp (Belgium). His research interests include speciation of organometallic compounds; biomarkers for screening of endocrine disrupters; and the analysis, fate,
and contamination status of persistent organic pollutants. Both Schnoor and Jiang see the new Beijing office as an important opportunity for the journal and for the environmental sciences. “The number of manuscripts that we are receiving from China, South Korea, and south Asia are increasing rapidly, about 20% per year,” says Schnoor. “Because [the research efforts in] environmental science and engineering are improving rapidly in these countries, we hope that an ES&T Asia office will help to focus attention on the importance of the environment in these countries, help improve the science there, and encourage more papers to be submitted [to] and accepted in ES&T.” Schnoor also hopes that the new office will expand the journal’s reviewer database and forge new connections with Asia’s research community. Jiang echoes these hopes, saying that he expects the new office to promote the progress of environmental science and technology studies in Asia, especially in China, and act as a bridge between the journal and the environmental science community, organizations, and researchers in this region. “The ES&T Asia office is a window for Chinese people to know the journal better,” says Jiang. —ALAN NEWMAN
News Briefs Great Lakes are suffering
The U.S. EPA could do more to ensure that states protect the five Great Lakes, the Government Accountability Office (GAO) notes in a report. GAO analysts reviewed how successful the states bordering the lakes and EPA officials have been in implementing the Great Lakes Initiative (GLI), a federal program designed to control toxic releases and protect aquatic life and wildlife. GLI has some drawbacks, the analysts found, primarily because it asks states to regulate only point sources, whereas nonpoint sources, such as air deposition and agriculture runoff, produce more pollution. EPA Needs to Better Ensure the Complete and Consistent Implementation of Water Quality Standards recommends that EPA issue a mercury permitting strategy, fully develop the partially operating GLI Clearinghouse to allow states to share pollution control data, and collect information on the success of current schemes designed to control releases into the Great Lakes.
More fish contaminated by mercury
States in the U.S. reported 395 new fish consumption advisories in 2004, bringing the nationwide total in effect to 3221, according to the U.S. EPA’s annual National Listing of Fish Advisories database. Nearly 65% of the U.S. coastline is under advisory, with 100% of the Gulf coast and 92% of the Atlantic coast blanketed with warnings. The percentage of lake acres affected has risen from about 8% in 1993 to 35% in 2004. More monitoring by states accounts for most of the growth in advisories, EPA says. Contamination of fish by mercury leads the way, followed by PCBs, chlordane, dioxin, and DDT.
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studies hastened to note that their work doesn’t address the value of armoring, although it does show that cliffs are important in saving beaches. Their findings do highlight the current scientific ignorance about California’s beaches in particular and coastal zone processes in general. “If we have been overestimating the contribution of river sand, then we don’t understand beaches as well as we think we do,” says Ashford. The new work also casts doubt on the validity of river sediment estimates, Warrick says. These estimates assume that stream-gauge
Environmentalt INTERVIEW Judith Curry
courtesy of JUDITH CURRY
In recent months, two studies in Science and one in Nature have found that hurricanes are growing more intense. Because so much research is drawing the same conclusion, one might surmise that humans will eventually experience some negative weather-related consequences due to climate change. But with the devastation of Hurricane Katrina filling nightly television news shows, many critics saw any attempt to link hurriJudith Curry’s recent study on hurricanes ap- canes with climate peared this September change as in the journal Science. just another sign of global-warming “hysteria”. “So many people have a vested interest in this global-warming thing—all these big labs and research and stuff,” said hurricane expert William Gray of Colorado State University in a recent interview with Discover magazine. “The idea is to frighten the public, to get money to study it more.” In order to try to make some sense of the media controversy over the science, ES&T spoke to Judith Curry, chair of the school of earth and atmospheric sciences at the Georgia Institute of Technology and coauthor of one of the Science papers on hurricanes and climate change (Science 2005, 309, 1844–1846). She serves on a variety of panels related to climate, including the National Academies’ space studies board and the National Oceanic and Atmospheric Administration’s climate-research working group. On October 25, the American Meteorological Society hosted a briefing on Capitol Hill about hurricanes. Along with Curry, Kerry Emanuel of the Massachusetts In-
stitute of Technology (Nature 2005, 436, 686–688) and Kevin Trenberth of the National Center for Atmospheric Research (Science 2005, 308, 1753–1754) presented their published findings on hurricanes and global warming. The briefing was well received, although an aide from the office of Sen. James Inhofe (R-OK) protested during the proceedings that the panel lacked “balance.” When he asked the panelists what gave them the right to present their research without contrary scientists present, Trenberth quipped, “Because we’ve published.”
You had a recent paper in Science that looked at tropical cyclones. Tell me a little about what you found. Okay. What we looked at was the global data set that is available from 1970 through 2004, and it’s a satellite-based data set, so we’re able to look at every single tropical storm and hurricane. And what we looked at was the frequency, intensity, and number of hurricane days for each ocean basin where they have hurricanes. We looked concurrently at the sea surface temperature over that same period for each ocean basin. What we find—again, the increase of tropical sea surface temperature in these regions is well known—is that there was an increase in the frequency, almost a doubling, of the most intense hurricanes—the category 4s and 5s. And a similar increase in the number of hurricane days.
Now this is across multiple ocean basins? This was the Indian Ocean, the South Pacific, the North Pacific, Atlantic . . . the whole works.
People can criticize the paper because you only went back to the 1970s. Can you actually see a pattern with such limited data? We do not have global data prior to 1970. We have data from 1945 to 1970 from aircraft in the North Pacific and the North Atlantic. Prior
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to 1945, we only have statistics on landfalling hurricanes. Now, events in the Atlantic comprise only 11% of global hurricanes, and U.S. landfalling hurricanes only comprise 1%. So trying to draw inferences about global hurricane activity from these statistics just doesn’t work. Using the sampling data from the Atlantic to understand what’s happening globally is like only sampling California voters to try and infer U.S. presidential preferences. If you look at landfalling hurricanes, the statistics are really just looking at California voters over 65 [laughs]. It’s a sampling error. Just to give you a counterexample, during the same time period, landfalling hurricanes in Australia have actually gone down. So if we had only relied on landfalling hurricanes we would have a different story. But clearly the Australian story doesn’t tell you anything about what is going on globally.
In recent months, we’ve seen three papers that have come out in science journals on the subject of climate change and hurricanes. Do all these papers line up in saying that we have an increase in hurricane intensity? Yes, all papers agree on that point.
What are the differences? Kerry [Emanuel] computes something he calls the destructive power. This involves a calculation, but he uses the number of hurricane days and the intensity—wind speed. We just looked at the more physical parameters, the category— which is related to wind speed— and the number of hurricane days. So it is different ways of presenting essentially the same data. Kerry did do an adjustment to the wind speed on his data set. Now a criticism has been made that he over-adjusted, and he agrees with that. Everybody agrees that the data needs to be adjusted, but most of that adjustment would occur before 1970. So it doesn’t particularly affect our study.
William Gray told Discover magazine that he sees no link between global warming and hurricanes, and he made the same comments on television and during congressional testimony. Max Mayfield, the director of the National Hurricane Center, has given similar congressional testimony in recent weeks. These are hurricane scientists who don’t know a lot about global climate. Gray and Mayfield focus on Atlantic hurricanes, for obvious reasons. And their view of hurricanes is biased by the very strong variability in the North Atlantic. You don’t get simple cycles, but you do get variability in the North Atlantic that dominates variations certainly in hurricane frequency. The North Atlantic does not have anything to do with what goes on globally. So their conclusions are based on their investigation of North Atlantic hurricanes.
What is interesting is why these people are being brought out to talk to the media. We have a situation in which another hurricane expert, James O’Brien, who directs an atmospheric group at Florida State University, was hosted on October 12 at the National Press Club by the Marshall Institute, a conservative think tank, to give a talk saying that climate change has not affected hurricanes. Jim O’Brien is looking at the more limited data set of U.S. landfalling hurricanes. Like I said, this is like trying to draw conclusions from Californians above the age of 65. Now we have a data set going back into the 1800s, but again, this is like 1% of global hurricanes. In fact, landfalling hurricanes only explain about 16% of the variability in Atlantic hurricanes. So you can’t even explain what’s going on in the Atlantic by looking at landfalling hurricanes, let alone what’s going on all over the globe. [Note: During the October 12 press briefing, a reporter asked O’Brien why he was presenting data at a media conference before it was published in a peer-reviewed
journal. O’Brien appeared stunned and responded, “I haven’t decided whether to publish this story or not.”]
“global warming” in the paper. We talked about an increase in global tropical sea surface temperature.
If these people disagree with the increase in hurricane intensity analysis, why don’t they begin to publish in the scientific peerreviewed literature? You can’t find them there, only on blogs, in opinion pieces, and on news programs.
Well, we did not address that in our paper. The only answer that makes any sense, especially over the last 35 years, is greenhouse warming. Again, you cannot explain it with decadal scale variability, because [hurricanes] are different in every basin.
They haven’t had a chance to do that. Our papers have just come out. Bill Gray says that he is preparing something that he is going to try and get published. I doubt that Mayfield will try publishing anything. Gray claims that in our paper, if you include category 3 hurricanes— in other words combine categories 3, 4, and 5—you actually see a decrease. Well, we’ve done this. Category 2 and 3 hurricanes are flat over this period. So by adding them in, you dilute the effect. Instead of an 80% increase, you get something like a 50% increase increase. But you still definitely get a strong increase. So Gray hasn’t done the work, but we’ve already done it. And he claims on his website that he’s going to publish something on this.
Why don’t these people publish? They like operating in this mode. They like the media, and the media likes a debate. It sells more newspapers or gets more people interested. These people get called on because they are on the other side. Whatever their politics are, they like to operate outside the respected and time-honored [laughs] scientific tradition of presenting their data at conferences or putting it in the peer-reviewed literature. They just don’t operate this way. And each has his own individual reasons for doing this. They like media attention, and they’re able to get it without doing the hard work.
How have people misconstrued your paper? People have accused us of linking global warming with Katrina. We didn’t even use the expression
But what is causing that increase?
Do you think that the American public is starting to wake up since Hurricane Katrina? Even if this one hurricane had nothing to do with climate change, it seems like people are at least starting to pay attention to the issue. With all the confusion from the scientific community . . . the American public doesn’t read scientific journals. They listen to the media, and the media likes a good debate. So they trot out this small minority of people to present their contrarian views. And they are given just as much legitimacy as scientists with strong credentials and who publish in the peer-reviewed literature. The media gives equal weight to both sides of this. So the American public gets confused, but at the same time people are starting to be worried about this now.
Before, people said the world is not warming; now, they say you can’t tie it to hurricanes. But other changes are occurring such as the melting of glaciers and ice packs in the Arctic and the acidification of the oceans. Aren’t hurricanes just one issue? Exactly. But you can’t use hurricanes to prove that there is global warming. What you can do is show an unambiguous link between the increase in hurricane intensity and the warming sea surface temperatures. And if you look for why the sea surface temperatures are warming since the 1970s, you don’t have any explanation other than greenhouse warming. —PAUL D. THACKER
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Environmentalt News PERSPECTIVE Biodiversity: Friend or foe of invasive species? dox has important implications for managing invasions, scientists say. Stohlgren and his colleagues evaluated data on the distribution of native and non-native plant species in U.S. ecosystems that range in scale from individual streams to whole regions. They found a signifijack dykinga / usda
They slip past customs officials and become embedded, unnoticed in the community for years, before one day exploding and wreaking havoc. These terrorists are the six-legged, deep-rooted, or even slithery kind known as invasive species. Lori Williams, executive director of the U.S. National Invasive Species Council (NISC), wants them uncovered and eliminated, and she’s drawing up a battle plan to do just that. Good reason for concern exists: 3 million acres are lost to invasive plants alone every year, she says. NISC is a coordinating body representing more than a dozen federal departments. “Given the shortage of resources at the federal, local, and state levels, NISC is revising and updating its national management plan to make it better able to prevent and target new invasives (both accidental and intentional), as well as to detect and respond to invasions early,” Williams says. The new plan, which is to be released for public comment early this year, links the latest research to practical tools in the field, she says. One of those tools should be biodiversity, says Tom Stohlgren, with the U.S. Geological Survey, which is a member of NISC. New research from his group presented at the Ecological Society of America (ESA) meeting last August reveals that the most biologically diverse regions of the U.S. are also the most invaded. Focusing on these areas could lead to early detection and rapid response to new invasives, Stohlgren says. But not everyone is convinced. Stohlgren’s results run counter to the long-held theory that high native biodiversity acts as a firewall to keep out exotic invaders. In addition, a host of experimental manipulations of biodiversity find that the field plots richest in native species have the fewest invaders. These seemingly contradictory findings were passionately hashed out at a special symposium at the ESA meeting. Resolving the para-
Exotic saltcedar is a common invader of species-rich stream edges throughout the southwest U.S.
cant and positive correlation between native plant diversity and the establishment of invasive species. “Stream and river areas, which are typically rich in native species, are getting pounded, especially in California,” Stohlgren says. “There is probably no direct cause-and-effect relationship between native and non-native species richness,” Stohlgren says. Rather, invasive species thrive in areas well-endowed with natural resources such as water, light, and nutrients, he says. Those same resource-rich areas also happen to be the kind of habitats that support large numbers of native species, he explains. “If you change your scale of reference, you can get a different result,” says Shahid Naeem from Columbia University. He and his colleagues manipulated 1 × 1.5 m plots of Minnesota prairie to contain 1–24 native species and ob-
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served them for 2 years. They found that the number, size, and variety of invasive species increased as biodiversity decreased, peaking in the plot that was originally dominated by one native species. Naeem speculates that the native species in the diverse plots complement each other because they differ in height, rooting depth, and drought and light tolerance. This complementarity means that resources, such as nutrients and water, are being used more efficiently in the diverse plots so that little is left over for invaders to utilize. “The results suggest that if you lose biodiversity, the ecosystem loses its resistance to invasion,” he says. The findings of Stohlgren and Naeem don’t necessarily exclude each other, because they are asking fundamentally different questions, says Jay Stachowicz, a marine ecologist at the University of California, Davis. Observational studies such as Stohlgren’s show that communities that are good for native species are good for invaders too, he says. On the other hand, experimental work such as Naeem’s shows that in the very diverse areas, loss of native biodiversity will lead to even more invasions, he says. This information has important implications for resource managers, he adds. “Biodiversity can help prevent invasions, but the trick is understanding the relative importance of biodiversity compared to other factors, such as habitat disturbance, in preventing invasions by exotic species,” says Dov Sax with the University of Georgia. “To what extent can native biodiversity be a management tool in and of itself, and when is biodiversity an indicator of where you should be spending your monitoring efforts to prevent further invasions?” he asks. The ESA symposium showed that no clear answers exist, but it also helped point out the direction of future work that needs to be done to resolve the issue, Sax concludes. —JANET PELLEY
