Harm is in the eye of the beholder.
Protecting ecosystems from non-native species has been a priority among ecologists and conservationists since at least the 1980s. Horror stories such as the Burmese python slithering around the Everglades swallowing endangered birds, the Asian carp decimating native fish populations in the Great Lakes, or kudzu taking over just about everything are, in fact, worrisome. But do all non-natives deserve a bad rap? Aren't ecosystems dynamic by nature? Mark Davis says biologists and government officials need to be more careful about assigning blame. Daniel Simberloff argues that preventing invasions is better than trying to address them later.
For 25 years, the American public has been inundated with horror stories involving non-native species. Think: snake-head, kudzu, Asian carp. This has largely been the result of selective communication from scientists and a media that too often have been more than eager to promote these stories without engaging in any critical analysis or research of their own. Usually provided with just a single perspective, the public largely accepted the idea that non-natives, as a group, are noxious and undesirable.
In fact, this is anything but true. Non-native species are just species. Like native species, some of them produce effects we like, some produce effects that we don't like, and most are comparatively benign. Have some introduced species caused changes that most everyone would agree have been very harmful? Absolutely. By killing timber trees, gypsy moths and the emerald ash borer have caused, and continue to cause, enormous economic damage to the United States. Introduced pathogens that threaten human health are also clearly harmful species. At the same time, the danger posed by non-native species as a group often has been exaggerated and misrepresented. For example, ecologists and conservationists often describe non-native species as the world's second greatest extinction threat, despite the fact that existing data shows this clearly not to be the case. It is true that introduced species can and have caused many extinctions in insular environments such as oceanic islands and freshwater lakes. But they have caused very few extinctions on continents or in marine systems. In fact, the primary regional biodiversity effect of introduced species is to increase species diversiW. Due to the introduction of thousands of plant species, the United States has approximately 20 percent more wild plant species than it did 500 years ago.
While extinction threats should not be our only concern when it comes to non-native species, if we are going to label a species as "harmful," that characterization needs to be based on good science. Efforts to vilify non-native species by misrepresenting their effects are ultimately counterproductive, as is the use of pejorative language to describe them: e.g., calling them "invaders" or "biological pollution." Once harm is claimed, society is generally obligated to try to reduce or eliminate the harm. Also, if harm is misrepresented, scarce societal resources can end up being spent on needless and futile management programs. Good (or bad, depending upon your point of view) examples of such efforts include the persistent efforts by federal, state, and local agencies to eradicate or significantly reduce non-native herbs and shrubs in forest environments in the US.
Most of these efforts have been ecologically misguided since they typically have not addressed the underlying causes for the spread of these new species. Because many native species began to decline at about the same time that non-native species began to become abundant, ecologists and conservationists prematurely assumed that the non-native species were the cause for the decline of the native flora. Once good scientific studies were conducted on these systems, the data frequently revealed that other changes in the environment were the primary cause of the decline of the native species, and that eradications are improving rapidly, helping to minimize cost and the impacts on non-target species. Short of eradication, several technologies are available to lower invasive populations: biological control (introduction of a natural enemy), chemical control, and mechanical and physical control. Each has contributed to successes, though none is a silver bullet.
Best of all would be a two-fold proactive approach: Minimize inadvertent introductions of new species that hitchhike rides into the United States (e.g., in untreated wooden packing material, as with the Asian long-horned beetle, or in ballast water, as with the zebra mussel), and subject any planned introduction (for example, of an ornamental plant or a sport fish) to rigorous examination by experts, as New Zealand does. An ounce of prevention is worth a pound of cure.
Critics contend that native species also sometimes become invasive, so there is no legitimate reason to be particularly worried about introduced species or to regulate them. This erroneous stance would prove extremely costly Some native species do cause problems similar to those of invasive non-natives. However, such cases almost always involve other human activities (e.g., grazing or changing natural fire or hydrological regimes) that trigger the expansion of the native, and there are many fewer "native invaders" than non-native ones. In the United States, the likelihood that an introduced plant will become invasive is approximately 40 times greater than that for a native.
The naysayers are unlikely to make much scientific headway, given the demonstrated costs of invasions on the ground and the remarkable findings of hundreds of scientists worldwide who have turned their attention over the last two decades to detailed study of invasions. However, it doesn't take much for a few credentialed scientists to influence policymakers, particularly when the poficymakers are glad, for political reasons, to be able to justify not acting. One need only think of the fringe scientists who question the impact of anthropogenic climate change and the views of Governor Rick Perry, who says scientists are "coming forward daily" to disavow a "theory that remains unproven."
Mark Davis is the DeWitt Wallace Professor and Chair of Biology at Macalester College, St. Paul, MN. He is the author of Invasion Biology(Oxford U Press, 2009).