How Invasive Species Work

Invasive species are nothing new. Nature has always transported plants and animals to where they weren't before. New Zealand, for example, brims with birds, plants and insects from Australia that floated on the winds that blew across the Tasman Sea. However, it wasn't until humans started introducing nonnative species into ecosystems that things got out of hand [sources: CISR, National Wildlife Federation].

In the past, oceans, rivers, mountain ranges, deserts and other geographic features provided barriers that limited the spread of species from one place to another. We humans, with our cars, our boats and our airplanes, have made these natural barriers meaningless. As a result, plant and animal species can move rather quickly from one area to another.

Scientists estimate there are 50,000 nonnative species in the United States [source: CISR]. Some spread by swimming in ballast water or attaching to the bottoms of boats. That's how the zebra mussel entered the Great Lakes from Europe [sources: National Wildlife Federation]. The mussels are so resilient and so invasive that they eat the food usually reserved for native species of fish. When they enter a body of water, they can clog water intake pipes and muscle out native species.

Invasive insects can move from area to area by boring into firewood, shipping pallets and other products. Sometimes humans release killer species into the environment on purpose. The ornamental plants we put in our gardens can sometimes escape into an ecosystem and take root where they weren't before. Hunters, fishermen and those in the pet trade often release nonnative animals for one reason or another. Just ask Floridians what they think of the nonnative Burmese python that is infesting the Everglades. Kudzu is another hallmark invasive species that humans let spread. It came to the United States from Japan during the 1876 Centennial Exposition. Americans used the leafy plant as an ornament, then as food for livestock. Today, kudzu is all over the place [sources: CISR, National Wildlife Federation].

Once an invasive species such as kudzu is in an ecosystem, fuhgeddaboudit. It's here to stay for the most part. David Pimentel, a Cornell professor emeritus who has studied invasive species at length, estimates that they cost the United States between $100 and $200 billion in damages each year [source: Keim]. How? Like something out of a 1950s black-and-white horror movie, invasive species destroy, replace or crowd out native species in an ecosystem. Once kudzu, for instance, anchors its roots, it can overwhelm native plants, creating a monoculture. Some invasive species can alter the chemistry of soil, which could then fan the flames of wildfires

In 2011, the U.S. Department of the Interior spent $100 million on detecting, managing and controlling the aliens. The Massachusetts Department of Conservation and Recreation earmarked more than $25,000 just for reading material related to zebra mussel prevention. Eurasian watermilfoil, which we mentioned grows like no one's business in Lake George, Lake Champlain and other lakes, has reduced Vermont's lakefront property values by 16 percent. Moreover, U.S. farmers lose $13 billion each year in crops from invasive insects [source: U.S. Fish & Wildlife Service].

In the 1970s, fish farmers in the southern United States imported Asian carp to clean up the mess left by their catfish. One day, the carp made a break for it and escaped into the Mississippi River. In the 21st century, the Asian carp swims in the waters of at least 23 states, destroying the biodiversity in many bodies of water. How do you kill the fish? The U.S. Geological Survey has developed a poison. Some fishermen, however, go Rambo on them, shooting the fish with arrows as they jump out of the water. Others, however, are catching the carp and eating them. They say it tastes similar to tilapia [source: Kraft].

Dealing with the Asian carp reflects the problems humans face when trying to control or eradicate an invasive species. You would think killing these pests would be an easy thing, considering that when a nonnative species enters a new ecosystem, only about 10 percent of the population will survive. Of those survivors, another 10 become pests [source: CISR]. That is, not all foreign species new to an area will become invasive. What's the problem?

At first, all infestation is localized. In fact, it's so local that humans don't even know its occurring. Eventually, the species breeds and gradually moves into other areas. However, the climate has to be right, and conditions have to be perfect for an invasive species to take over an area. That means the area has to lack natural enemies, including parasites, predators or pathogens. Some species, such as aphids, spread more easily than others because a male is not required for fertilization. By the time humans figure out an invasive species has taken over the area, it's much too late to do anything about it [source: CISR].

We can try, can't we? Of course. There are two main ways of controlling an invasive species. The first is something researchers call proactive management. That means excluding species by passing laws banning certain animals, plants, food and other products from coming into a country. Officials can catch many invasive species by stepping up border inspections. Proactive management also means destroying an invasive population during the early stages of infestation. In California, environmental officials successfully beat back an invasive weed that occurs naturally in tropical waters. When state officials spotted the plant, they shrouded it with tarpaulins and poured chlorine on top, killing the weed before it had a chance to spread [source: CISR].

The second way to control invasive species is through reactive management. That could mean doing nothing, or it could mean controlling the invaders by limiting their capacity to reproduce. This might mean planting crops that are resistant to a plant disease. It could also mean planting during a time of the year when a pest is not around. We could also use chemicals to kill the pests or introduce a natural predator, such as a parasite or pathogen, into the ecosystem [source: CISR].

What can you and I do? For one thing, go around your house and try to find any plants that don't belong there. Generally speaking, you can tell which plants are invasive if they crowd out the native vegetation. You can also go to this U.S. Department of Agriculture website and print out identification sheets. And please, don't flush baby alligators down the toilet or let a python wiggle out of the bedroom window. Grow only plants native to your area, and try not to move soil or wood great distances. We can keep our boats and fishing equipment clean, and notify custom officials anytime we travel overseas and decide to bring food, plants or animals into the country.

And please, no more kudzu or Eurasian watermilfoil.