An international group of 26 researchers reported the new findings this week in Proceedings of the National Academy of Sciences.
Dealing with these issues will be "pivotal for policy makers and future management," the researchers said, although no easy or inexpensive solutions exist, and many nations have been reluctant to take steps that might interfere with economic growth.
"Invasive species are a continuing and extensive ecological crisis, and we're finding that human population and accumulated wealth are important drivers of this problem," said Susan Shirley, a research assistant in the Department of Forest Ecosystems and Society at Oregon State University, and co-author on this study.
"Regional patterns of species invasions are complex, and there is still unexplained variation, likely due to local scale differences in several of the ecological factors," Shirley said. "But invasive species are in large part an international trade issue, and this is an important problem we have not yet come to grips with. Next to population density, the closest correlation is to long-standing wealth, not more recent increases in income or economic activity."
Human activities often related to trade, travel and transport, particularly in the past 50 years, have caused a surge in the number of introduced species, ranging from plants to fungi, insects, fish, birds, reptiles and mammals. Some are innocuous, but many displace native species and cause a range of ecosystem disruption. As a crossroads of international travel and trade, with both a high population and high income, Europe has experienced many invasive species.
The study concluded that other possible factors, such as climate, geography or land cover, were less significant than population density and wealth capital, and that those secondary causes may have been overestimated in the past.
The mechanisms of species invasion are often associated with international trade. Invasive species can hitch-hike on imported products, be brought to new regions as pets, be associated with contaminated food, or even introduced on purpose, as in the case of some ornamental plants or new crops.
In another recent study, Shirley and her colleagues researched bird introductions in Europe, and the findings supported this premise. Trade with Eastern Europe was severely disrupted for decades during the Cold War. By the end of that long period of international tension and restricted trade, Western Europe had experienced an increase in invasive bird species, but numbers in Eastern Europe actually declined.
In the new study, researchers were able to predict the number of alien species in Europe to a reasonably high degree simply by defining the level of wealth and the number of people.
"The overwhelming effect of human factors, wealth and demography, found for several taxonomic groups translates to human activities responsible for enhancing biological invasions," researchers wrote in the study.
Solving this problem will not be easy, the study suggested.
Identifying the specific mechanisms of invasion is critical. Monitoring may need to be improved. Legislation to restrict or regulate certain imports will likely be needed, in addition to charging fees or tariffs that would help deal with invasive species when they occur. But the World Trade Organization and other international agreements "have no effective mechanisms" to address this concern, the authors said. And aside from good intentions, restrictions could be costly.
A major challenge, Shirley said, will be to understand the specific economic factors leading to introductions so they can be effectively addressed while minimizing negative impacts on international trade. These factors are likely to differ among species. For example, minimizing releases of vertebrate species might require additional regulation of the pet trade, while a focus on transport infrastructures such as roads may help control introductions of alien invertebrates.
"Nations do not have a good track record in forsaking future economic prosperity for environmental benefits," the study concluded. "Only if the true determinants are identified will it be possible to predict and manage alien species invasions adequately without adverse effects on other economic sectors."