The Cold War divided the people of Europe for nearly half a century, and it turns out humans weren't the only ones stuck behind the Iron Curtain. Trade blockades led to vastly different numbers and types of invasive birds in Western and Eastern Europe, new research reveals. The findings, say experts, highlight the dramatic impact human activity can have on the success of alien species.
After World War II, political divisions split Western Europe and the United States from communist Eastern Europe and the Soviet Union. Democratic Western Europe became closely linked to North America and Africa, while Eastern Europe was largely isolated from global trade and suffered economically under communism.
This disparity, researchers argue  in the February issue of Biological Conservation, had a profound effect on invasive bird species. The data come from more than 150 years of bird-introduction records--including government reports, scientific papers, and observations by local experts--which were collected and analyzed as part of a European program called Delivering Alien Invasive Species Inventories for Europe.
For example, Western Europe saw the introduction of 96 species of birds during the Cold War, while Eastern Europe only saw 24. The relative freedom of movement and high levels of global trade in the West account for the difference, says co-author Susan Shirley, a wildlife ecologist at Oregon State University, Corvallis. "Trade is an important factor in the movement and establishment of alien species across the world."
The way the Cold War carved up the globe also impacted the type of birds that were introduced. There was a rise in North American bird species, such as the ruddy duck, intentionally introduced to Western Europe many times between 1945 and 1989, but not much of a rise in the East. At the same time, people from former French and British colonies immigrated to Western Europe, toting along 23 African bird species. "They brought their caged pet birds with them--if not physically, then they brought the demand," Shirley says.
While connections between Western Europe, the Americas, and Africa boomed, trade across the Iron Curtain withered: Exports from Western Europe to the East represented less than 5% of Western Europe's total trade volume. The few invasive species that established themselves in Eastern Europe during the Cold War tended to come from other parts of Eastern Europe, or from Asia.
Since the Cold War ended in 1991, the pace of bird introduction events has picked up. Looking at records from 1989 to 2000, the study's authors found more than 600 instances of alien species released into the wild in Eastern and Western Europe, versus almost 900 for the roughly 40 years of the Cold War. Trade and movement across the former Iron Curtain and rising prosperity in Eastern Europe has made the problem of invasive species worse, they say. "It's speeding up exponentially, not just for birds but for many other groups, like plants, mammals, insects and fish," says team leader Francois Chiron, a researcher at the Hebrew University of Jerusalem when this research was conducted.
The findings are more than a historical curiosity, says Diederik Strubbe, an ecologist at the University of Antwerp in Belgium. Ecologists, he says, have long explained the success of invasive species by pointing to attributes of the species themselves, such as aggression or rapid reproduction, or to environmental factors such as climate. But the number of arrivals matters even more: "Whether a species succeeds or not is a probability game," says Strubbe, "but the main determinant of success is the number of times you try."