In the current issue of Mother Jones, Julia Whitty has a terrific piece looking at how invasive species have, for better or worse, conquered the planet—and what that might mean:
Nowadays when species obey the commandment to "be fruitful and multiply, to fill the waters in the seas, to let the birds multiply on the Earth," all is decidedly not good. Proliferation on a biblical scale generally signals biological apocalypse, what scientists call invasion—the establishment and spread of introduced species in places they've never lived before. Species have always been on the move. But they've also been held in check by Earth's geographical barriers, like mountains and oceans. Today the rate of invasions has skyrocketed because of our barrier-hopping technology—jets, ships, trains, cars, which transport everything from mammals to microorganisms far beyond their natural ranges. The process is further accelerated by global climate change, that enormous human experiment unwittingly redistricting the natural world.
The results devastate both planetary and human health—most disease organisms, from influenza to malaria, are invaders over most of their range—and few invasions can be stopped once they're successfully established. Biological invasions are now second only to habitat loss as a cause of extinction—the leading cause of the extinction of birds and the second-leading cause of the extinction of fish. Twenty percent of vertebrate species facing extinction are doing so because of pressures from invasive predators or competitors. In a classic example, brown tree snakes arrived in Guam (snakeless but for a worm-sized insectivore) sometime after World War II and systematically ate 15 bird species into extinction while consuming enough small reptiles and mammals to redesign the food web. They also began traveling an expanding network of power lines, electrocuting themselves and causing about 200 power failures annually. In all, invasive species are estimated to cost $1.4 trillion each year.
Very much worth reading. Here's another recent post on invasive species, noting that invasive species aren't always destructive. Biology can be complicated. There's evidence that—in a few cases—intruders can actually promote biodiversity (some black snakes in Australia, for instance, have evolved to become immune to the venom of the cane toad, an invasive species that hit the shores in the 1930s). It does appear, though, that the less-desirable side-effects tend to carry the day.