Pathways to Invasion

May 3, 2013

How do invasive species enter North America?

We bring them in.

Our ancestors.The early colonists, brought pigs, which they let range free, and seeds to plant as crops. Others just hitched a ride: on their shoes, in fodder, on animals, on boat hulls, and stowed among ballast cobbles.

Our tax dollars at work. Since the nineteenth century, the U.S. government has introduced several nonnative species, either as food sources or erosion control, always with unforeseen consequences. Then your tax dollars go to work on remediation.

Our military. It is believed that the brown tree snake first got to Guam as a stowaway in the wheel wells of Air Force jets. Invaders travel in military transport, bags, and equipment.

Our love of exotic plants. The nursery industry has a lot to answer for. Imported plants have a way of spreading into the wild. And they have a way of arriving with stowaway insects, fungi, and diseases. Invaders can travel in potting soils and love turf.

Our love of pets. The pet industry also has a lot to answer for. Four hundred fish species and 124 plant species were found in aquarium pet stores of Washington State over the course of a single year. Exotic pets can get too big, or too unruly, for their owners, who release them into the wild. Feral house cats, and their domestic neighbors, kill more than 2 billion birds and 12 billion mammals in the United States each year, most of them native species (alas).

Our love of hunting, fishing, and meat.  Ponds and streams have been stocked with nonnative species for fishing since the 1880s.  Bait has been released in waterways far from its source, resulting in the spread of crayfish, crabs, and other creatures. Hunters have imported wild boar from Europe for their sporting pleasure. After escaping from farms, pigs have damaged soils and plants in the Smokey Mountains. Some hunters are transporting feral pigs across state lines to encourage their spread.  They don’t need your help.

Our love of cheap manufactured goods. The wooden crates in which these goods arrive from Asia can contain invasive insects. These goods arrive on container ships from around the world, which discharge their ballast water at ports as far inland as the Great Lakes. Ballast water contains aquatic invaders.

Our love of fire. Firewood bought from a big-box store often comes from Asia. Buy local and don’t move wood around, as it can carry pests.

Our love of exotic fruit. Shiploads of fruit from Central and South America have arrived with invasive stowaways, whether insects, reptiles, or fruit diseases.

Our love of seafood. Along with ballast water, moving oysters and seaweed around the world for aquaculture has spread dozens of invasive species. Seafood is often packed in seaweed before being transported; the live packing can carry small and juvenile invaders to new waters.

So keep in mind. We can’t consume our way out of every mess.  The best way to stop invaders is prevention.  No new species should be brought in unless they are shown to be noninvasive.  Pathways are diverse and dynamic: strict controls on ballast water and wood imports are essential in saving our coastlines and trees.


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