Daniel Simberloff is the Nancy Gore Hunger Professor of Environmental Studies at the University of Tennessee. He is editor-in-chief of Biological Invasions and senior editor of the Encyclopedia of Biological Invasions (University of California Press, 2011).
Obviously not all non-native species deserve a bum rap. Invasion biologists have long known this and worked hard to determine which species to try to keep out and, failing that, which to manage at low densities and which to ignore, given the limited funds available. But the idea that resources are squandered fighting harmless species is a straw man argument. Hundreds of non-native species – including many that initially seemed innocuous – have caused staggering ecological, economic, and public health damage. The only logical policy is to work hard to prevent their entry in the first place and, if unsuccessful, to find them quickly and eradicate or limit them before management becomes impossible. A small group of critics brandishing groundless arguments and catchy sound bites can get a lot of attention and impede control efforts and the research that underpins them. But they cannot make the many problems go away.
At least four factors should make us cautious about any introduced species, and about not acting when we see a small, seemingly innocuous population of some non-native. First, even widely used risk assessments to determine which plants to allow in – such as the Australian Weed Risk Assessment adopted not only in Australia but also in several other countries – are far from infallible. Some species judged likely to be harmless turn out to be problematic. The number of surprising ways introduced species can cause trouble is enormous. For example, pupae of flies introduced in the American West to control spotted knapweed (which they failed to do) provided a food bonus for overwintering deer mice, greatly increasing their populations with myriad possible follow-on effects.
Second, non-natives often remain restricted and harmless for years, then quickly spread and wreak havoc. Brazilian pepper, on the IUCN (International Union for the Conservation of Nature) list of the world’s worst 100 invaders, now infests 700,000 acres of South Florida. Yet it was narrowly distributed for many decades. In this case, we believe the change had to do with water withdrawals for agriculture and other purposes, which lowered the water table just enough to create ideal habitat for Brazilian pepper. Some other lags before invasion are mysterious, but there is no doubt that they often occur.
A third reason we shouldn’t be complacent about an apparently harmless non-native is that some of them have highly consequential impacts that are so subtle that we don’t recognize them for years. For example, some non-native plants affect soil biota or nutrients, particularly the nitrogen and phosphorus regimes, with far-reaching effects that are not quickly apparent. So it is not prudent to wait to see if something happens before acting against some non-native species.
Mark Davis and his colleagues lament the expense of efforts at managing relatively innocuous non-native species. Surely in many more cases we regret not having acted earlier, when we could have eradicated or greatly limited an invasion that turned out to be disastrous. Common crupina, a type of daisy, sat benignly in an Idaho field for a few years while authorities dickered about whether to do anything about it and who should pay. By the time they decided they should eradicate it, it had spread to California, Oregon, and Washington, occupied 60,000 acres, and could no longer be eradicated because the necessary herbicide would have threatened salmon runs. Koster’s curse, also known as soapbush, infests more than 200,000 acres in Hawaii. For over a decade, it was restricted to fewer than 200 acres and could have been eradicated; now it is viewed as Hawaii’s second worst weed.
A fourth reason why it might not be smart to wait and see what an introduced species does is that it is often possible to eradicate it at modest cost when it hasn’t yet spread widely, whereas eradication after it has spread may be impossible or extremely expensive.
Many populations of harmful animals have been eradicated, often on islands but sometimes on mainlands. Feral goats and pigs were recently eliminated from an island of 120,000 acres in the Galapagos. Fewer problematic introduced plants have been eradicated, but some have. The technologies for both animal and plant 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 policymakers 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.”
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