Aliens come in many guises. There is the extraterrestrial kind, variously envisaged over the years as little green men, wrinkly creatures homesick for their own planet, curious kidnappers and genocidal invaders. There is the human kind -- people who fetch up in some foreign country and find themselves officially designated "aliens," outsiders, disenfranchised, suspect. And then, as we were reminded last week by Tuesday's observation of International Biodiversity Day, there is the nonhuman kind, the so-called invasive alien species that conservationists say pose a growing threat to the diversity of life on Earth. These are the ones we should be worrying about.

Such invasions are nothing new. Plant and animal species have been traveling to distant places for as long as humans have wandered the planet, often as part of migrants' or explorers' or traders' luggage, sometimes just on the soles of their shoes. But such movement was a mere trickle in the context of Earth's teeming ecosystems, and oceans and mountains acted as natural barriers to other means of dispersal. Things picked up as transportation evolved. A mosquito brought to Hawaii in a sailing ship's water barrels in 1826 triggered the spread of avian malaria throughout the islands and the extinction of some 10 native bird species. Colonists took their cats, rabbits, pigs and garden plants along with them, unleashing ecological disasters in countries from Africa to Australia as pets went feral and usually docile plants grew a mile a minute in hot climates. Now, potential pests travel conveniently in the wheel-wells of jet planes, as Southeast Asia's brown tree snake may have done on its first, fateful trip to Guam half a century ago.

The World Conservation Union's list of the 100 worst invasive alien species reveals that, though all these disasters were unintended, many invaders were brought to their host destinations deliberately, usually for one of two reasons: either to prey on some local pest or, in the case of plants, as crops or ornamentals. Hence the fiasco caused by the Western mosquito fish, introduced into waterways worldwide to control mosquitoes but doing a lot more to kill off indigenous fish; the stranglehold of the Brazilian strawberry guava in Mauritius and Hawaii; and the virtual takeover of Tahiti by the ornamental Miconia, a gardener's favorite that has devastated native species. In Japan, as in many other countries, the small Indian mongoose is now a big problem: Introduced to help control rats and snakes, it is endangering the rare Amami rabbit here and has killed off several bird, reptile and amphibian species elsewhere.