Common Invasive Pathways

Live Animal Trade

The live animal trade is immense, and a common source of invasive wildlife, and even disease.  From 2000 through 2006 some 1.48 billion live animals –an average of 600,000 per day– were legally imported into the United States.  For perspective, the United States had 69.6 million international visitors in 2013, just 1/20th of the number of important animals.

Burmese Python

Nearly 10,000 Burmese pythons were imported annually between 1996 and 2006. The Burmese python was probably both intentionally released and escaped into the wild in Florida. In the 15 years since it was first known to have become established in the wild, its population has exploded in southern Florida to an estimated 100,000 snakes and the cost of control is estimated to be about $720,000 annually. The species has devastated native populations of raccoon, opossum, fox, rabbits, and even deer. Only after they were established in the wild was their importation prohibited, but there is no known way to control or eradicate them in the wild.


In 2003 about 800 rodents of 9 different species were imported from Africa for the live animal trade. Some of these were infected with monkeypox which was then transmitted to prairie dogs at the facility of an animal vendor.  Put on the market, these prairie dogs transmitted monkeypox to about 50 people in 5 states. With the help of the Center for Disease Control, this outbreak was brought under control. Not observed in this outbreak, it is known that monkeypox can be transmited person-to-person.

Wood Packing Materials

Wooden pallets and wooden crates have been widely used for the international transport of goods.  Unfortunately, absent proper treatment or the use of alternative materials, wood-boring insects such as the Asian longhorned beetle (Anoplophora glabripennis) and emerald ash borer (Agrilus planipennis have found their way to the United States.

Emerald Ash Borer

The emerald ash borer is deadly to ash trees, usually killing them within 4 years after infestation. It is believed to have arrived accidentally in 2002 and has spread throughout the Great Lakes region. To inhibit its further spread, citizens are strongly discouraged from transporting firewood. Ash seeds are an important food source for many birds and mammals and the hollow trunks provide homes for wildlife.  Ash trees are used by the forest products industry. Baseball great Derek Jeter of the New York Yankees uses bats made of ash. Just remember: Burn it where you buy it!

Asian Longhorned Beetle. Photo by Donald Duerr, USDA.
Asian longhorned beetle

The Asian long-horned beetle has been found in southwestern Ohio, central Massachusetts, and the central New Jersey/western Long Island area.   At least 72,000 hardwood trees have been destroyed in an effort to stop its spread.  If established, it would likely bring large losses to the forest products, maple syrup, nursery, and tourist industries. Learn more >>

Nursery and Aquarium Trade

The intentional importation of plants and animals for commercial purposes poses many risks.  Nursery stock can carry plant diseases, while the aquarium trade has facilitated the spread of several aquatic plants.


This freshwater plant was intentionally brought into the United States for use in the aquarium trade. A few fragments of the plant escaped, and now the plant is well established.  Hydrilla has been found primarily in the southeastern states. It covers the entire surface of waterways, affecting their entire ecology by choking out other plants, reducing sunlight in the water, and secondarily affecting fish and wildlife.

Sudden oak death

Sudden oak death has killed more than 100,000 oak trees in California and southern Oregon, and infected virtually all woody species in the forests of the affected region. Numerous oaks and shrubs native to the eastern United States could be killed if the pathogen is introduced there. While it is not known either where it originated or how it entered the country, suspicion is focused on imports of woody plants.

Ballast Water

Ballast water is water taken into a ship’s hold to increase maneuverability and stability while in transit.  It is often then discharged, hundreds if not thousands of miles from where the water was taken up.  Ballast water has transported many organisms from their natural ranges to other areas throughout the world.  Although some standards have been developed to reduce the transport of species via ballast water, it continues to be a problem in many areas.

Zebra mussel

This freshwater mussel was first found in the Great Lakes in 1988 and rapidly spread throughout the Great Lakes Basin and then into the Mississippi River Drainage.   Widely believed to have been brought in by ballast water, it can rapidly block intake and discharge pipes due to its massive numbers and growth.  The effect on aquatic ecosystems is profound.  These filter feeders greatly reduce the phytoplankton and zooplankton communities, with a likely impact on populations of fish that depend on plankton, as well as fish further up the food chain.

Veined rapa whelk

Veined rapa whelks (Rapana venosa) are large, predatory marine snails native to the Sea of Japan. Likely introduced to the lower Chesapeake Bay via ballast water transport of larvae from the eastern Mediterranean or Black Sea, rapa whelks eat a variety of mollusks, including oysters, clams, and mussels. They were first detected off Hampton Roads, Virginia in 1998. Since rapa whelks have been blamed for destroying shellfish stocks in other habitats that they have invaded, there is concern about this predator’s likely impact on the clam and oyster populations in the lower Chesapeake.