Global trade—and now global warming—are making the problem of invasive species ever more challenging. From surveillance to cooperative management, Rebecca Epanchin-Niell explores options to control these damaging invaders.
In 1909, Tokyo Mayor Yukio Ozaki presented the US government with 2,000 young cherry trees to be planted around Washington, DC’s tidal basin. The gift was part of a beautification effort for the National Mall.
There was one problem. When the trees arrived in Washington in early 1910, inspectors discovered they were infested with damaging roundworms and insects. The trees would have to be destroyed. US Secretary of State Philander Knox informed Japanese Ambassador Yasuya Uchida of the bad news:
The United States has suffered immense damage to its trees and its agriculture generally by various injurious insects not indigenous but introduced from foreign countries, and . . . the introduction of any new kind might result in the future in the enormous detriment to fruit growers and agriculturists of the country. From this point of view, the Department of Agriculture seems to have no choice but the painful duty of ordering the destruction of the trees.
Skillful diplomacy smoothed over any potential hurt feelings, and a new shipment of pest-free cherry trees arrived in 1912, the same year that Congress passed the landmark Plant Quarantine Act, among the first federal legislation dealing with importation of exotic species.
More than 100 years later, exotic pests remain very costly to the American economy, imposing billions of dollars in damages on crops and ecosystems. Invasive species are now a staple of news reports:
- The Burmese python, likely introduced as a pet and now taking up residence in the Everglades, dines on everything from small mammals to endangered birds and even alligators.
- The Asian tiger mosquito—thought to have arrived in the Port of Houston in 1985 in a shipment of used tires—is now a backyard menace in 26 contiguous states and Hawaii.
- The zebra mussel, most likely brought over from Russian freshwater lakes in ballast water, is now driving native mussel species to near extinction and clogging water intake pipes at electric utilities from the Great Lakes to the Mississippi basin.
- The emerald ash borer, probably introduced to the United States in the 1990s in wood packaging material, is responsible for the loss of more than 100 million ash trees since its first detection in 2002, with devastating economic and ecological impacts.
How Invasives Arrive
Invasive species are yet another manifestation of human impacts on the global environment, as human activity is far and away the main driver of species spreading to new areas. Many invasive species have even been introduced intentionally by individuals unaware of the potential negative consequences. To take a notorious example, the European starling arrived in 1890 as part of Eugene Schieffelin’s effort to bring every bird mentioned in Shakespeare’s plays to America. Many other invasive plants and animals were initially introduced as part of the horticultural or pet trade and subsequently became established in the wild.
Another important way that new invasive species arrive is by hitchhiking on other shipments, as in the case of the pests that accompanied the first gift of cherry trees from Japan. The increase in global trade and travel has exacerbated this phenomenon, with pests arriving on agricultural products, in packing material, in ballast water, and in passenger baggage.
Trade in live plants is a particularly important pathway, as it not only directly introduces plant species that have the potential to become invasive, but more importantly, it also is the most frequent medium for introduction of non-native pests of agricultural and natural resources worldwide. Of invasive forest insects and pathogens taking root in the United States in the last 150 years or so, an estimated 70 percent are thought to have arrived on imported live plants.
But not all alien species qualify as invasive. At a minimum, the species must be able to take hold and flourish in its new surroundings. Most introduced species are not able to do so, but a small percentage can, benefiting from the lack of natural controls like predators, competition, and climate fluctuations that would otherwise keep their populations in check. From a policy perspective, the species must also be harmful to be counted as invasive. The US government’s official definition is an alien species “whose introduction causes or is likely to cause economic or environmental harm or harm to human health.”
And although not all introduced species are harmful—in fact, of the food crops grown in the United States today, only a handful are actually native—enough are damaging to create serious risks for many parts of the economy. For example, the emerald ash borer alone is estimated to cause $850 million in local government control expenditures annually, as communities treat or remove urban ash trees devastated by this pest.