Alien Invasion? An Ecologist Doubts the Impact of Exotic Species
Many conservationists have dedicated their lives to eradicating invasive plant and animal species, but Mark Davis wants them to reassess their missions
Credits: BRENDAN BORRELL
Kudzu Killer For ecologist Dan Simberloff of the University of Tennessee Knoxville, understanding and fighting invasive species has been a lifelong project, and he doesn’t buy Mark Davis's arguments. "He's contrarian," Simberloff says. "He holds a radical view that very few people favor." Simberloff points to the brown tree snake, Brazilian pepper and the kudzu [background] as invasive species that he will never learn to embrace.
Water Sucker The tamarisk tree has been called the "second-worst invasive species in the United States," and millions of dollars have been spent on its eradication. Early studies had reported that tamarisks used more water than native vegetation, and one author wrote that they sucked up "almost twice as much water per year as the major cities in southern California." Forgotten in the hubbub were later measurements that failed to replicate those studies, and no one has established whether the tamarisk causes soils to go salty or if it simply has a higher tolerance for salty soils.
University of Nevada, Reno
I Heart Tamarisk Matthew Chew of Arizona State University stands in front of the tamarisk tree, or salt cedar, a nonnative species vilified for displacing native vegetation and sucking up scarce water resources. "I'm skeptical of some of the basic assumptions of invasion biology," Chew says. "Some of it makes sense, but I don’t think the sky is falling."
Fast and Loosestrife Davis's doubts date back to early attempts to eradicate purple loosestrife from around Minnesota lakes and waterways. Eradication proponents argued that the imported plant, which has spread across North America, was seriously displacing the native plant species that muskrats and ducks eat, thereby displacing the animals. The overall result was reduced plant diversity and wildlife. One author said that dense stands of the plant were the equivalent of a "parking lot," and the National Invasive Species Council put it on their top 10 invaders list. But as government agencies and conservation groups launched major efforts to eliminate the plant before all the data were in, recent studies from Canada have failed to substantiate these fears. "A lot of energy and money and time has been allocated to try to eradicate or control species simply because they are not native," Davis complains.
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Naysayer Mark Davis of Macalester College has been one of the most vocal critics of invasion biology alarmism, but his perspective is still controversial in a field that considers all-native ecosystems to be the pinnacle of restoration. "My view is that a lot of priorities haven't been based on sound science, and that's not in the best interest of conservation," he says, "I'm trying to contribute in a positive way, where I see the field has made some missteps."
Toad Busting Australia is one big island—a continent, actually—and ever since Europeans arrived there in the 18th century, it has been wracked by outbreaks of nonnative rabbits, mice and even camels. One well-known case involves the introduction of the cane toad. Originally brought to Australia to fight a sugar cane pest, the amphibians are now a threat to native reptiles and marsupials that try to eat the toads' toxic flesh. Volunteer groups like the Kimberley Toadbusters go on "toad busting" missions to prevent the animals from spreading to Western Australia. The group claims it has been able to slow the cane toad's spread, but some observers doubt the group's effectiveness. More importantly, herpetologists like Richard Shine of the University of Sydney wonder if fears about the toads' impact have been overblown.
Snakes Alive! After World War II, the brown tree snake arrived on the U.S. Pacific island of Guam, 6,100 kilometers west of Hawaii. It has since extirpated 10 of the island's 12 native forest bird species, and ecologists are now studying the wider impacts of these bird extinctions on the forest. Islands represent one ecosystem where scientists agree that invasive species are nearly always devastating. Princeton University ecologist David Wilcove says that 98 percent of imperiled Hawaiian birds are impacted by invasive species, and the same is true of imperiled Hawaiian plants. But as Davis points out, "It's easy to find nonnative species causing extinctions on islands or insular lake environments, but on continents and marine systems, there are very few."
United States Geological Survey
Mussel Madness For ecologists Mark Davis of Macalester College in St. Paul, Minnesota and Matthew Chew of Arizona State University, rhetoric over invasive species has crossed the line from science to propaganda. Certainly, fighting invasive species can have a kind of populist appeal: ecological righteousness that anyone can undertake in their own backyard. Conservation groups and government agencies have fueled this xenophobic mentality by producing FBI-style "wanted" posters for some of the most notorious invasive species. Consider the case of zebra mussels, which first arrived in 1988 from the Black and Caspian seas in Asia to the U.S.'s Great Lakes region, after possibly hitching a ride in the ballast water of ships entering through the Saint Lawrence Seaway. Today, the mussels clog the region's power utilities and water treatment plants, damaging nautical equipment, as well. Some have estimated that the mussels cost such facilities $500 million per year, but Chew says the mollusks' impact has a silver lining: "They’ve clarified the water a bit."
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