Defined by the authors as animals that have established and spread outside of their native range, many invasive species become economic and ecological burdens. Successful invaders can displace native animals through resource competition, predation or disease-- ushering in biodiversity loss. Globally, invasive species are the second leading cause of animal extinction, preceded only by habitat loss.
Not all introduced animals become invasive. When moved into a foreign environment, many animals find conditions unfavorable and fail to establish. In an effort to gain insight into how frequently introduced animals become invasive, Drs. Jeschke and Strayer analyzed bird, mammal and freshwater fish introductions between Europe and North America. Jeschke comments, "We focused our study on larger vertebrates because there are better historical records about their introductions, which were often done purposefully for human use."
While motivations vary-- some animals make great pets, others have valuable pelts-- humans have been moving animals between Europe and North America for hundreds of years. Enterprising furriers imported American mink into Europe, where the animals escaped from captivity and spread prolifically. European mink have been suffering ever since. Other animals have been introduced unintentionally. Rats were stowaways on the first vessels sailing from Europe to North America. They now flourish in urban and agricultural areas, causing approxima tely $19 billion dollars worth of damages annually in the U.S. (Pimentel et al. 2000. BioScience 50, 53).
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 introduced. As global trade increases, precautions like port inspection and exotic wildlife regulations are essential. Consumers also need to be educated; many exotic animals that are legal as pets could be ecologically lethal if released into the wild."
Future research will explore why, once introduced, animals are more likely than plants to become invasive.