The authors' exhaustive analysis, which drew on data from the 15th century to the 20th century, revealed an unsettling conclusion. For every four animals that made the transatlantic journey, one became invasive. Jeschke notes, "Our data indicate that once introduced, vertebrates have a 25% chance of becoming invasive. This figure, which appears to be true for other animals as well, is significantly higher than the 1% probability that dominates invasive species risk assessment. The 1% probability is based on plant invasions. Introduced animals do not act like introduced plants-- they appear to have a much higher invasion success rate."
Given that humans are the primary vehicles for transporting animals across the ocean, it's not surprising that animal introduction patterns between Europe and North America mirror immigration patterns. Overall, a higher proportion of European animals entered North America than vice-versa, with introductions peaking in the 19th century and decreasing thereafter. This decline can be attributed to a reduction in immigration following WWI and increased U.S. regulations on wildlife imports.
Conversely, North American introductions to Europe have been on the rise throughout the 20th century as more Americans immigrate to European countries. In many parts of Europe, regulations on imported wildlife are not as strict as the U.S. and Canada. Canadian goose, gray squirrel, and northern cottontails already populate the European countryside. In the absence of precautions, Jeschke speculates that North American animal introductions may become more common.
The bottom line-- vertebrate animals have a high rate of invasion success. Once established, they can act as biological pollutants that result in ecological and economic damages. "The best way to combat invasive species is to prevent them from being in
Contact: Lori M. Quillen
Institute of Ecosystem Studies