View slideshow of invasive species.

Gray squirrel may look cute and harmless with their twitchy noses, piercing peepers and oh-so-bushy tails. But an international team of scientists recently named the furry beasties one of Europe's 100 worst environmental offenders. Their crime? Driving the equally adorable European red squirrel toward extinction.

The problem of invasive alien species is not unique to Europe. From reindeer on the sub-Antarctic island of South Georgia to mouflon sheep in Hawaii and cane toads in Australia, biological invaders are flourishing around the globe, devastating native plants and animals in their wakes. Often, they deliver multiple punches, as in the case of the U.K.'s gray squirrels. They not only sap resources from the native reds, but carry and spread squirrel pox—a disease that is decimating their red cousins.

Initiatives, such as the European Commission's Delivering Alien Invasive Species In Europe (DAISIE) project, which seeks to identify species wreaking havoc on ecosystems across Europe, and Aliens in Antarctica, an International Polar Year (2007–2008) project, are attempting to further the understanding of biological invasions to eliminate current problems as well as prevent future ones.

Needless to say, species movements from one location to another are nothing new—the terrestrial flora and fauna on volcanic islands that rose out of the sea, like Hawaii and the Galapagos, had to come from somewhere. The difference between the natural spread of species and today's invasions is how far—and fast—species are traveling.

"The natural process happens over slow time periods," says Barry Rice, an invasive species specialist with The Nature Conservancy. "When you have human-augmented invasions, these things happen at rates that are thousands of times faster than natural processes."

Many invasive species were released intentionally, like the South Georgia reindeer that were intended to provide meat to whalers in the early 1900s. Now they have overgrazed the native plants, which serve as nesting areas for seabirds and did not evolve to protect themselves against large herbivores. Others, like the Eurasian zebra mussels that invaded the Great Lakes in the 1980s, were stowaways, using oceangoing ships to taxi to a new environment. Years later, the voracious interlopers are continuing their steady march across the nation's waterways and are overwhelming native freshwater mollusks.

Global climate change is adding other dimensions to the problem.

"South Georgia is undergoing reasonably rapid climate change and retreat of glaciers, and it's hard to say what will happen in the future," says Kevin Hughes, the British Antarctic Survey's environmental research and monitoring manager. "One of the problems with the melting of some of the glaciers is that the reindeer might be able to extend their range and get to parts of the island they're not currently able to access."

The interplay of climate change and biological invasions is not only a concern in the polar regions. Wherever more severe and frequent weather events occur, there will be an increasing number of disturbed habitats, which may be ideal for nonnative species.

"This is all unknown, but invasive species may be able to take advantage of disturbances and move into these habitats much more quickly than native species," says Steven Hess, a wildlife research biologist at the U.S. Geological Survey Pacific Island Ecosystems Research Center's Kilauea Field Station in Hawaii National Park.

Native species accustomed to a particular environment have a tough time coping when conditions change, the Conservancy's Rice says. In contrast, many of the most successful invasive species are organisms like rats that easily adapt to various environments.

The best way to limit damage from invasive species? Prevent them from getting a foot in the door. "A really good analogy is to think about invasive species [as you would] wildfires," Rice says. "You can control a small fire when all you have to do is step on it, but once it's a large blaze, the logistics are much more complicated."

The first line of defense, he says, is to take appropriate biosecurity measures whenever possible. For example, many of the sub-Antarctic islands have implemented simple procedures, such as requiring visitors to dip their shoes in biocide (a chemical that kills microbes and plants) before disembarking onto the islands to prevent seeds or new pathogens from being introduced.

But it is impossible to prevent all invasions, which is why Rice stresses the importance of a network of experts who can immediately identify new threats and nip them in the bud. The Invasive Plant Atlas of New England project is doing just that, using trained volunteers to inventory habitats and document the arrival and spread of invasive species.

Once intrusive populations become established, it can be almost impossible to get rid of them. Often, as is the case with rats, feral pigs and many plant species, they have extraordinarily high reproductive rates.

Rice says that efforts to control invasive species should be tailored to each situation. Called adaptive management, this practice takes into consideration the characteristics of each site, goals, the manpower required and financial resources.

Economics almost always figures into decisions about managing invasive species, given limited conservation budgets and the tremendous damages inflicted by invaders on ecosystems. According to The Nature Conservancy, invasive species are estimated to cause $120 billion in damages each year in the U.S. alone. The worldwide toll is hundreds of billions of dollars greater, not to mention the human health and environmental costs.

"If I'm a preserve manager and I decide to wipe out every nonnative species, I will drive myself crazy and break my budget," Rice says. "I have to look at the site and ask myself, 'What is important? What am I trying to protect?' Then I ask myself which invasive species are causing damage to that purpose and how to deal with those on a local level."

For instance, officials would not waste time or resources trying to remove wild grass from a stopover for migrating birds unless it was interfering with their efforts to, say, drink from ponds there.

Management decisions to eradicate populations of introduced species, especially mammals, can be fraught with emotion and regret. In 2005, for instance, The Nature Conservancy killed 5,000 wild pigs on Santa Cruz Island to save the isle's endangered foxes and plants.

According to The Conservancy, the action was taken only after extensive consideration of other options. In the end, officials concluded that transporting pigs to the mainland carried too great a risk of disease.

"When we find ourselves in a situation [in which] we have to kill some organisms because we introduced them and they're damaging native organisms, ultimately this is a tragedy," Rice says. "We should take responsibility for that tragedy and we should channel that energy into avoiding introducing such organisms in the future."

Still, he says, although problems associated with invasive species may be pervasive and complicated, they are not insurmountable.

"You can define successes on a regional level—they're not measured only by whether the organism has been eradicated," he says. "Keep your eye on what you're there to protect and not what you're there to control. The goal is protecting native biodiversity. The goal is not to kill nonnative species."

View slideshow of invasive species.