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].