Submodule 2: Invasive Species Defined
How They Get Here
“Pathways” describes the modes by which invasive species move from one place to another. These pathways can be natural, caused, as when species are moved on ocean currents, or wind currents, or carried by an animal from one place to another. Alternatively, some pathways are human-induced, and can be intentional or non-intentional. Let’s look at some examples of how each of these pathways contributes to non-native species invasions in the US:
Natural pathways that allow for the movement of species have been existed since the dawn of life on Earth. Studies of historical records have demonstrated similarities between species found, for example, on opposite sides of a sea or ocean. The natural migration of animals or plants often explains this similarity. One example is the migration of mammoths across the Bering land bridge that connected Alaska to Russia until about 11,000 years ago. The unintentional introduction of invasive species by natural pathways is rare.
Human-induced pathways are pathways created or enhanced by human activity. They are the source of most species invasions and continue to increase in frequency and severity.
Intentional introductions – Many species purposefully imported into the US have became invasive. Some of these introduced species were brought into the country for food, pets, and ornamentals. Some were used to combat other invasive species. The majority of invasive vertebrates and plants in the US were deliberately introduced. Below are a few notable examples:
Tamarix spp. (commonly known as salt cedar): In the early 1800’s, landowners and managers imported salt cedar into the US, primarily from Asia. These plants have favorable characteristics making them useful as ornamentals and windbreaks, and to prevent erosion. Unfortunately, they also have characteristics that make them invasive and very detrimental to their new environment in the US. Salt cedar, which grows primarily in dry regions of the western US, uses large amounts of water and fouls the soil in which it grows with salt. It also provides little forage or habitat for native fauna.
Myocastor coypus (commonly known as nutria): In the 1930’s the nutria was imported into Louisiana from South America for fur production. The animal was somehow released into the wild and soon established feral populations throughout the Gulf Coast. Incentive payment programs for nutria pelts have brought numbers down to an estimated 300,000 animals from a high of almost 2 million in the mid-1970’s. Interest in nutria fur has been sporadic throughout since nutria were first introduced into the US.
Unintentional introductions – Most invasive insects, marine invertebrates, and microorganism were accidentally introduced into the US. Many of these organisms were “hitchhikers” or “stowaways” unwittingly transported in ballast water on ships, in shipping crates, by travelers, or mixed in with seed purchased from other parts of the world. Evidence shows that the sources of introductions change with social changes such as different trading partners and travel habits. Some arrived in the US through other pathways, and many of these pathways are unknown. Members of this group include:
Agrilus planipennis (commonly known as the Emerald ash borer): This insect was found infesting trees in Michigan and Ontario in 2002; it has since been found in other US states. It is thought that the borer was introduced into the area in packing crates and other packing materials several years before it was actually observed in the US. State and federal quarantines on the movement of ash trees have been imposed in the infested areas.
Dreissena polymorpha (commonly known as zebra mussel): These freshwater bivalve mollusks were first identified in North American in Lake St. Claire, the waterway connecting Lake Huron and Lake Erie located between Michigan and Ontario; this discovery was made in the mid-1980’s. The mussel originated in Asia and most likely made its way to Europe and then to North America in the ballast water of transatlantic vessels. Unfortunately, it is very adaptable to a wide range of environments. It has altered native environments by causing a decline in native species and decreasing water quality in addition to clogging many man-made water facilities.
Flavivirus (commonly known as West Nile virus): The mosquito-borne [West] Nile virus was first identified in the US during an outbreak in New York City in the summer of 1999. The virus, which infects birds, horses, and humans, was first found in a Ugandan woman in 1937, and it has been spreading north and west ever since. For most humans, West Nile fever is mild, sometimes with no symptoms. In very rare cases, inflammation of the brain (encephalitis) can occur which can be quite serious.
