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.

The point being stressed by conservationists last week was that many of these worst-case scenarios have taken decades to unfold. What they fear is that, with the explosion of global trade and travel, the weakening of customs and quarantine controls and the growing porousness of national borders generally, such accidents will be nothing compared to what happens next if these warning examples are ignored. Unfortunately, signs of oblivion abound: Biological control of pests using imported predators is still practiced -- the mosquito fish being just one example -- and the problem has hardly been broached in the international pet and garden-seed trades. Already, invasions by alien species are cited by the World Conservation Union as second only to clearance of native vegetation as the main cause of biodiversity loss worldwide.

Does this matter? Or is the lament for lost species just another instance of eco-romanticism, with its view of Earth as an Eden to be preserved in the face of all precedents for change? The answer is that it does matter, on two grounds, neither of them romantic. One is economic: The annual global costs of damage from alien species invasion have been estimated at around $140 billion, for everything from weed and pest control to health costs associated with invading disease organisms. (If "weed control" sounds too much like Sunday gardening, consider the water hyacinth, an aquatic ornamental that infests waterways on five continents. Populations can double in 12 days, stopping boat traffic, commercial fishing and sports and killing submerged marine plants and the animals that eat them.)

The second reason for getting serious is self-protection -- for our own species. Scientists cite evidence that the resilience of an ecosystem, including the giant ecosystem that is Earth, depends on the self-regulating diversity of its components. Add or subtract an element, and the balanced functioning of the whole is affected. Since we can't predict whether the effects in any given instance will be good or bad, it obviously pays to be conservative in introducing changes, whether on purpose or inadvertently (which is all that "conservation" really means). With 6 billion of us on this planet, we cannot afford to mess it up by continuing to behave like the worst alien invaders it has ever seen.