One of the less-noted ecological consequences of globalization is the fact that other species tend to travel alongside all those people and cargo hopping from continent to continent. By some estimates, 7,000 species—plants, fish, mammals, you name it—get moved each day into new habitats thanks to human activity. Carl Zimmer of The New York Times has a provocative article today trying to figure out what the rise of "invasive species" means for biodiversity. There's no doubt that some of these invasions are extraordinarily destructive, often in unforeseen ways:

Botulism, for example, is killing tens of thousands of birds around the Great Lakes. Studies indicate that two invasive species triggered the outbreak. The quagga mussel, introduced from Ukraine, filters the water for food, making it clearer. The sunlight that penetrates the lakes allows algae to bloom, and dead algae trigger an explosion of oxygen-consuming bacteria. As the oxygen level drops, the botulism-causing bacteria can multiply. The quagga mussels take up the bacteria, and they in turn are eaten by another invasive species: a fish known as the round goby. When birds eat round gobies, they become infected and die.

"If you pour on more species, you don't just increase the probability that one is going to arrive that's going to have a high impact," Dr. Ricciardi said. "You also get the possibility of some species that triggers a change in the rules of existence."

Cases like these get all the headlines, and rightfully so. Invasive zebra mussels are wreaking havoc in waterways across the United States.* After Nile perch were introduced into Lake Victoria in 1954, they nearly wiped out several hundred types of native fish. It's a bad scene. But, it's worth noting, invasions aren't always destructive—two scientists, Dov Sax and Steven Gaines, recently looked at New Zealand, whose 2,065 native plants are outflanked by 2,069 immigrant species that have been naturalized, and found that in many cases the ecosystem has thrived. A separate study in Hawaii found that the five native species of freshwater fish have adapted to the introduction of 40 new species. Sometimes the invasion of a new organism can even promote biodiversity, rather than wiping it out:

In Australia, the introduction of cane toads in the 1930s has also spurred evolution in native animals. "Now that you have cane toads in Australia, there's a strong advantage for snakes that can eat them," said Mark Vellend, of the University of British Columbia. Cane toads are protected by powerful toxins in their skin that can kill predators that try to eat them. But in parts of the country where the toads now live, black snakes are resistant to the toxins in their skin. In the parts where the toad has yet to reach, the snakes are still vulnerable.

Dr. Brown argues that huge negative effects of invasions are not documented in the fossil record, either. "You see over and over and over again that this is never the case," he said. Species have invaded new habitats when passageways between oceans have opened up or when continents have collided.

Of course, what's happening today is vastly different from anything you'd find in the fossil record—as McGill's Anthony Ricciardi notes, the pace of ecosystem change that's going on right now is unparalleled: "Invasions will interact with climate change and habitat loss. We're going to see some unanticipated synergies." Indeed, anywhere from one-third to one-half of all species are expected to go belly-up if greenhouse-gases continue to rise and the earth warms an additional 2-3 degrees C, which alone makes it hard to imagine these "unanticipated synergies" are going to turn out well.

(Photo credit: EPA. And yes, that's what happens when you leave a shopping cart alone with a young band of hooligan zebra mussels for a few months.)

--Bradford Plumer