By David Suzuki with Faisal Moola

We've probably all heard the urban legend about the unsuspecting shopper who takes home a bunch of bananas from the supermarket, only to have a tarantula later crawl out and terrorize the family. Well, new research shows that there could be some truth to the story.

As it turns out, spiders are excellent hitchhikers and often end up taking rides across countries, continents and oceans. According to a report published in the journal Diversity and Distributions, spiders are thumbing rides more and more often as global trade increases. And as our planet heats up from global warming, more spiders might decide to make their vacations permanent.

Researchers with the University of Bern in Switzerland looked at 87 alien species of spider from 25 different families known to have been introduced to Europe from other continents in the past 150 years. They found a near-linear progression of increased spider introductions correlating with an increase in global trade over the same time frame.

Both the volume of trade and the number of trade routes around the world have expanded greatly over the past century. At the same time, the duration of each of these trips has shortened due to more efficient shipping routes and techniques, and faster forms of shipping, such as air transport. Less time spent en route increases a hitchhiker's odds of surviving a trip, which make it more likely that the creature will be able to set up home in a new location.

Spiders are also survivors. A three-season study from New Zealand in 2002 found 31 alien adult spiders of seven different species (including several that were poisonous), plus nine egg sacs, survived trips from California in containers of table grapes — in spite of the containers having been fumigated and kept at a chilly one degree Celsius.

Notably, researchers also found that the spiders most likely to survive long-distance shipping were all significantly larger than their native counterparts of the same family. They conclude that, as a result of increasing global trade, at least one alien spider species will likely settle in Europe every year for the near future. This number could increase if global warming makes Europe more hospitable to spiders from warmer climates.

The ecological impact of alien spiders is not well known. However, as with any introduced species, spiders have the potential to displace native species and disrupt local ecosystems. Depending on the type of spider, there could be human health implications as well.

But if invading alien spiders seem like a small concern in the big scheme of things, consider that spiders aren't the only unwanted organisms that can hitchhike on our increasingly wide and dense global trade network. A recent review published in the journal Ibis, for example, reported on how the highly contagious avian bird influenza H5N1 spread across Europe. Again, the most likely culprit is global trade.

Many people originally thought that migratory birds were the vector that allowed bird flu to spread from China throughout Asia, to Africa and Europe. However, French researchers found that the pattern of spread didn't follow bird migration routes, but rather trade routes. And they cite case after case of known transmissions through domesticated birds. Their conclusion: "In summary, although it remains possible that a migratory bird can spread the virus HPAI H5N1 and contaminate poultry, the evidence overwhelmingly supports the hypothesis that human movements of domestic poultry have been the main agent of global dispersal of the virus to date."

We live in an increasingly interconnected world. That brings with it a host of new challenges and new responsibilities. Whether it's big spiders or dangerous viruses, we'd best find ways to minimize the ecological and human health threats posed by our global economy. The world is shrinking. And we'd better get used to it.

November 16, 2007
http://www.davidsuzuki.org/blogs/science-matters/2007/11/global-trade-brings-unwanted-visitors/