Global Trade Spreads Invasive Species
The biggest influence on how many destructive exotic species a country has acquired is the amount of international trade it's involved in.
A study of nations around the world finds that the value of goods imported is the factor most linked with the dispersal of invasive organisms.
The proportion of imports that are agricultural or wood products has little bearing on the number of harmful species moving into a country. Instead it's the overall value of imported merchandise that propels introductions. The Great Lakes form one example, where 60% of recently established species have arrived on commercial ships.
The volume of international transportation likely determines how many opportunities plants and animals have for hitchhiking abroad. Plenty of opportunities simply result in more creatures of all kinds moving to new locations.
Another notable driver in the distribution of invasive species, at least on a country-wide scale, is biodiversity. Countries naturally rich in flora and fauna are more susceptible to being engulfed by new species.
Many factors, surprisingly, do not show up in this analysis as having large-scale influence on spreading species. The proportion of species that are endemic or unique to a country has no bearing on the extent that invasive species become established. Whether the country is an island, the length of its borders, its latitude, and type of ecosystems also have no affect. Neither does the degree of human disturbance as indicated by urbanization, population density, road density or rate of deforestation.
This research used the Global Invasive Species Database put together by the World Conservation Union (IUCN) that lists 357 species which seriously threaten native ecosystems. The country in the database with the most alien species is the United States at 293. The US has nearly twice the amount of the next most-invaded country, Australia with 156 new species. Out of 227 countries, New Zealand and Canada rank third and fourth in their number of introduced species.
Michael I. Westphal, Michael Browne, Kathy MacKinnon and Ian Noble. 2007. The link between international trade and the global distribution of invasive alien species. Biological Invasions.