Thanks to global shipping and trade, species of exotic fish are fording into new waterways worldwide, shoving native species toward extinction and costing countries billions of dollars each year as fisheries collapse and governments fight to stem the tide of aquatic interlopers. According to a new study, however, the success of these invaders depends less on ecology and more on economies.

The news has come as a surprise to ecologists, who have long debated the conditions that make a habitat vulnerable to invasion. One hypothesis, popularized 50 years ago by British ecologist Charles Elton, is called biotic resistance. Elton believed that robust ecosystems had too many native occupants to make room for anything else. Essentially, invading species seeking a niche were met with a “no vacancy” sign. But in recent years, a counterhypothesis of “biotic acceptance” has emerged, contending that healthy habitats are equally alluring to both native and invasive species. As Jonathan Levine, an ecologist at the University of California, Santa Barbara, explains, “it’s just as you might go to a restaurant because there are a lot of customers there, thinking it’s indicative of high-quality food.”

A team of researchers has muddied the long-standing debate in the February issue of PLoS Biology. The investigators looked at data from 1,055 river basins covering 80 percent of the earth’s land and found six “global invasion hotspots,” where more than 25 percent of freshwater fish are nonnative migrants. The six hotspots encompass large networks of river basins in western Europe, North and Central America’s Pacific coast, southern South America, Australia and New Zealand, South Africa, and Central Eurasia. The high number of invasive species, says Fabien Leprieur of the Paul Sabatier University in Toulouse, France, and lead author of the report, coincides with maps of the world’s largest gross domestic products, greatest amount of urban development and highest population densities. Perhaps most troubling, the hotspots also boast the greatest number of threatened native fish species.

At least on the scale of entire river basins, Leprieur’s findings support neither biological hypothesis, says Levine, who researches how international trade spreads exotic plants. Evidently, human activity enables invasive species to get established in any kind of ecosystem.

Leprieur also expresses amazement “that natural processes are blurred by human activities in controlling the richness of nonnative freshwater fish species.” But, he says, it is not hard to see how humans help invasive fish get a fin up—the more economically active a nation is, the more likely it is to engage in international shipping, which transports stowaways in ballast water, and to have large aquaculture and pet industries, where escaped fish are common. What is more, booming economies often come with dams, bridges and other environmental disturbances that could facilitate the spread of exotics.

Rochelle Sturtevant, an ecologist who studies the Great Lakes for the National Oceanic and Atmospheric Administration, hopes Leprieur’s study can serve as a cautionary tale as developing nations join the global market and undertake activities that threaten to introduce exotic species into their relatively pristine ecosystems. Unfortunately, she points out, the conclusions drawn in the paper are too broad to help conservationists create concrete solutions. She thinks that more specific investigations may find evidence that biological processes actually do play a role in invasive dispersal. And, Sturtevant adds, terms such as “GDP” and “urbanization” should be fleshed out to include the specific human activities that drive exotic species invasions in a given region. Once conservationists uncover such details, perhaps they will be able to head off the next invasion.