Tamarisk, a Eurasian shrub, is your classic invasive species—designated one of America's "least wanted" plants by the National Parks Service. In recent decades, it has spread along Southwestern riverbanks, replacing native trees such as willows and cottonwoods. For nature lovers in the region, tamarisks (also known as saltcedars) rank somewhere between Land Rovers and James Inhofe. Measures to thwart them include burning, herbicides, and "tammy whacking" (physical removal sometimes done by freelance volunteers). A few years ago, the USDA let loose thousands of leaf-eating Asian beetles in order to sic them on tamarisks, which die from the defoliation.
But these efforts to oust the intruder have encountered a glitch. It turns out that a charismatic endangered bird—the southwestern willow flycatcher—is known to nest in the offending shrubs. Last March, the Center for Biological Diversity sued the government, charging that indiscriminately killing tamarisks jeopardizes the flycatcher. A recent article in the journal Restoration Ecology goes even further in the weed's defense. Tamarisks are widely thought to hog water and drive out other vegetation, but the authors dispute that theory. In their view, the newcomer may just be better suited than the natives to an environment altered by human activities.
These controversies highlight a broader debate within "invasion biology," a field that emerged in the 1980s. Some scientists—such as Matthew Chew, Dov Sax, and Mark Davis—are challenging what they consider old prejudices about "alien" species. They point out the inevitability of change and the positive roles that non-natives can play in ecosystems, while describing eradication projects as often wasteful and even counterproductive.
The outlook of the more traditional camp goes something like this: While most non-natives are harmless, a minority—about 10 percent—cause serious damage. Some pose threats to human health (H1N1 is an invasive species), while others, such as agricultural pests, wreak economic havoc. A third category dramatically changes landscapes. (Think kudzu, the creeping vine that has conquered the Southeastern United States.) And since these effects are unpredictable—sleeper species can seem benign for decades—all exotics are suspect. The hard-liners promote a "guilty until proven innocent" approach to biological foreigners, including a strict "white list" of those allowed to enter each country. As globalization accelerates the movement of species, vigilance is more important than ever. Climate change adds another wildcard, making the behavior of organisms all the harder to foresee.
Now a growing contingent of scientists is advocating a more neutral attitude. Certainly, they say, non-native plants and critters can be terribly destructive—the tree-killing gypsy moth comes to mind. Yet natives such as the Southern Pine Beetle can cause similar harm. The effects of exotics on biodiversity are mixed. Their entry into a region may reduce indigenous populations, but they're not likely to cause any extinctions (at least on continents and in oceans—lakes and islands are more vulnerable). Since the arrival of Europeans in the New World, hundreds of imports have flourished in their new environments. Common wildflowers such as Queen Anne's lace and certain kinds of daisies are "naturalized" aliens. The storied apple tree originally hailed from Asia.
Even when species are destructive, there's the tricky question of what to do about them. We may all agree that a particular plant or animal is loathsome, but eradication isn't innocuous. Such plans tend to cost millions of dollars and often rely on toxins that bring collateral damage to other species. Biological solutions—like leaf-eating beetles and root-boring weevils—are usually considered more benign, but as the case of the tamarisk shows, they, too, can pose problems. Once an ecosystem has absorbed a new species, any targeted intervention is likely to have significant ripple effects.
The past few years have also seen active debates about the field's terminology. Some criticize the term "invasive" as too value-laden and imprecise. One widely cited definition comes from a 1999 executive order, issued by President Clinton, that aimed to ward off "alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health." But what exactly constitutes "environmental harm"? It's easy to see when a power plant is hemorrhaging pollutants into the air and water—but how do you make the call when the agent of harm to nature is … also nature? Critics such as Mark Davis say we need to distinguish "harm" from "change." If an introduced species causes natives to become less abundant, does that constitute harm? Many people would say yes, but Davis doesn't think so.