Introduced Species

Introduced species, whether released deliberately or accidentally, are a major environmental problem worldwide. They may compete with native species for food, living space or other resources. In the case of species like the Burmese python, they represent entirely new predators on native species. This page deals with two of the many species that have invaded communities in the US over the last few decades.

Burmese python

Burmese pythons are large constrictors that are among several species imported into the US by the exotic pet industry. Young pythons are only a few feet long but they can reach 20 feet as adults. They become difficult for many owners to handle and some are released into the wild. Burmese pythons are established as a breeding population in Everglades National Park (ENP) in South Florida, and are known to eat a variety of birds, mammals and small alligators. Populations of raccoons, opossums, rabbits and even bobcats and white-tailed deer have declined in ENP, and the python is almost certainly to blame. At least two other constrictor species, the North African python and the Boa Constrictor, have bred successfully in the wild in ENP, but they are not as common as the Burmese python.

Florida Everglades credit: Susan Jewell, USFWS (public domain)
Florida Everglades credit: Catherine Puckett, USGS (public domain)












The number of pythons captured and removed from Everglades National Park increased sharply after 2000.

pythons-removedThe Burmese python was established as a breeding population in ENP around 2000, and has quickly grown in numbers. A drop in 2010 was the result of record cold temperatures in January of that year. As the snakes cannot tolerate freezing temperatures, it is unlikely that this species will spread beyond South Florida.

Zebra mussel

You don't have to be a giant constrictor to have a huge ecological impact as an introduced species.  The zebra mussel and the quagga mussel are two small bivalves that invaded rivers and lakes of the eastern and central United States. Native to the Black Sea and Caspian Sea regions of eastern Europe, zebra mussels appeared near Detroit in the Great Lakes in the late 1980s.

The species arrived in the ballast tanks of cargo ships from eastern Europe that arrived in the lakes to pick up grain and other exports. These ships crossed the Atlantic with empty cargo holds and ballast tanks full of water so that they were stable as they sailed. The ships filled their tanks in their home ports, and the water contains many small aquatic animals, often as larvae. The animals are released when the ships dump their ballast water as they fill their cargo holds in American and Canadian ports on the lakes.

Zebra Mussels Amy Benson, USGS (public domain)
Retrieval of zebra mussel-encrusted Vector Averaging Current Meter near Michigan City, IN. Lake Michigan, June 1999. Photo by M. McCormick.
Mussels growing on a current meter M. McCormick, NOAA (public domain)















Zebra mussels attach themselves in large numbers to any natural or manmade hard surface on lake or river bottoms. They overgrow and smother the shells of native mussel species, several of which are now in danger of extinction. Damage to manmade structures has a huge economic impact. There may be so many mussels that they clog water intake pipes of power plants and municipal water plants. Cost of repairs to these plants between 1989 and 2004 was more than $250 million.

Map by US Geological Survey (Non-indigenous Aquatic Species database), public domain

Zebra mussels spread quickly following their first discovery in 1988 in Lake St. Clair. They are now present throughout the Great Lakes, in the Mississippi from its source to its mouth, and in almost all rivers that join the Mississippi. The quagga mussel, a larger relative of the zebra mussel, is established in the Great Lakes, and in some rivers of the western United States. The species reached the western US mostly as individuals attached to the hulls of recreational boats.