The Global Price of Invasive Species

The U.S. and China pose the greatest threat as exporters of invasive species, but other countries have more to lose

Invasive kudzu girdles a forest. Robert Michalove Flickr Creative Commons

It’s the stuff of environmental horror stories: Exotic species invades new shores, and swiftly lays ruin to the native ecosystem and its inhabitants. And it's so common it's become almost a trope. Think of kudzu vines strangling trees and shrubs in the South, Asian longhorned beetles decimating hardwood forests in the Northeast and prolific Asian carp outcompeting native fish (and terrifying boaters) in the Great Lakes.

But the devastation wrought by these invasions isn’t just environmental—it’s economic. Alien insects and pathogens cost an estimated $40 billion annually in the United States alone, in terms of the harm they wreak on crops and forests. As disparate parts of the world grow increasingly connected and thus face increased risk of new invasions, those costs will only rise.

Despite these scary stakes, researchers haven’t yet assessed the worldwide costs that these invasions cause as a whole. Instead, most research on invasive species has only been done on a singlecountry basis. That means we’ve been missing out on capturing the global nature of the problem: trade links virtually all countries, and thus all would-be invaders in a network of possible pathways into a new country.

Now, a new study attempts to fill that knowledge gap by using complex computer models to quantify the cumulative threat of 1,300 insect pests and fungal pathogens to crop production in 124 countries. The results are stark: almost a third of the countries studied had a high likelihood of imminent invasion. Developing countries stand to experience the worst impacts, while major agricultural producers like China and the U.S. pose the greatest risk as sources of invasive species, according to the findings, which were published this week in Proceedings of the National Academy of Sciences.

While the results reaffirm what researchers suspected, “this is the first work that has shown it quantitatively and at the global level,” says Dean Paini, a senior research scientist at the Commonwealth Scientific and Industrial Research Organization in Australia, and lead author of the study. Having a better idea of the risks “presents us with an opportunity to do something about it,” he adds.   

One of the key connections that enabled Paini and his colleagues to come to their conclusions was knowing the link between invasive species and global trade. While there are myriad ways by which invaders can sneak into a new port, past studies consistently found that the number of invasive species in a given country was related to that country’s trade levels. Knowing this, the researchers examined the proportion of total imports from each country’s trading partners to calculate the probability of an invasive species arriving in a given country.

Next, they estimated the chance of those pests actually establishing themselves in a new country by analyzing worldwide distributions using an artificial intelligence algorithm. That method generates likelihood indices of pests settling down and proliferating in areas where they do not already occur, based on where they are found already and how they interact with other species. Finally, researchers looked at annual crop production in each country and calculated the invasion threat to those crops, depending on which pests eat what and who trades with whom.

The results indicated that a third of the 124 countries faced a very high risk of being invaded, while only 10 countries faced a very low risk of invasion. In terms of absolute cost, countries that are major agricultural producers—the U.S., China, India and Brazil—stand to lose the most. But in terms or relative cost, developing countries, particularly those in sub-Saharan Africa, including Malawi, Burundi, Guinea, Mozambique and Ethiopia, were the most vulnerable. Trade patterns, pest presence and species analyses also revealed that the U.S. and China pose the greatest threat in terms of unintentionally delivering their potential invaders on other countries.

While Paini and his colleagues ran statistical tests to confirm that their results were robust, uncertainty always surrounds work with computer models. “I think the study was conducted well as an overview of the global threat to agriculture,” says Daniel Simberloff, an environmental scientist at the University of Tennessee, Knoxville who was not involved in the study. Simberloff adds that follow-up studies should delve more deeply into the impact of specific crop pests. “This will take a lot of work, but it will be much more definitive in terms of the real probability of the various threats,” he says.

For now, Paini and others hope that the paper will prompt countries to deploy the necessary resources to protect their own environments and economies, as well as encourage more affluent countries to prevent invasive species from spreading to developing ones. The paper’s conclusions “highlight the need for a world body to address, in a comprehensive manner, the continued threat of plant pests and pathogen invasions that result in enormous economic losses in the affected countries,” says Harold Mooney, an environmental biologist at Stanford University who was not involved in the work.

Mooney, for his part, is optimistic. “There is a lot being done internationally, which is cause for hope,” he says. This new research could help: After all, perhaps the best way to get people to care about their environments is to attach a dollar amount to it.

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