Sources of Invasive Species Release
Globalization has vastly increased long-distance travel and commerce, and highly altered waterways. These and other factors have increased by orders of magnitude the frequency with which non-native plants, animals and pathogens are introduced to new areas, sometimes with costly results. Below are many known sources of non-native species introductions that can lead to problems for US waters and watersheds.
Ballast water. Since 95% of all foreign goods by weight enter the US through its ports, the potential for invasive species impacts on coastal communities is immense. Ballast water from foreign ships is the single most important source of species introductions to US waters.
Vehicular transportation. Both private and commercial transportation are major factors in the movement and range expansion of non-native species throughout the US.
Escaped ornamental plantings, nurseries sales or disposals. Many invasive plant problems began as ornamental plantings for sale in nurseries and garden shops. Purple Loosestrife, for example, is sold as an ornamental plant but takes over native vegetation in wetlands, and can clog western streams preventing water withdrawal and recreational uses. Only some of the problem species are banned from sale.
Cross-basin water transfers/diversions/channels. From small channels to major intercoastal waterways, new connections between isolated water bodies have allowed the spread of many invasive species; Great Lakes invasions increased markedly after the opening of the St Lawrence Seaway.
Fishing bait releases. Discarding unused bait can introduce species that disrupt their new ecosystems and eliminate competing native species; examples include non-native crayfish, baitfish that overpopulate certain waters, and earthworms that are depleting the leaf litter layer in northern forests where no indigenous earthworms existed.
Boat hulls, fishing boots and other recreational introductions. Boats, fishing boots and equipment, and other recreational implements that are transported among several water bodies have been known to spread invasive species problems to new waters. Some Zebra Mussel and Milfoil introductions have occurred in this manner.
Illegal stockings. Although prohibited by law, people release fish into new waters and sometimes cause severe impacts; Yellowstone Lake's world-class Cutthroat Trout fishery is now jeopardized by an illegal release of Lake Trout, and Asian swamp eels are spreading through the Southeast after introduction as a food source by immigrants.
Domestic animals gone feral. The impact of feral house cats on birds and small mammals in natural areas is well documented; feral pigs, escaped from farms, have recently begun to do significant damage to soils and plants in the Smokey Mountains.
Pathogens spread by non-natives to vulnerable native species. Non-native species problems include pathogens carried by resistant non-natives to vulnerable native species. Whirling disease, which has decimated Rainbow Trout in many western rivers, was originally introduced when European Brown Trout, tolerant of whirling disease, were imported to US waters and hatcheries.
Disposal of solid waste or wastewater. Seeds, viable roots or other propagules of invasive plants may be easily spread to receiving waters through wastewater discharge, then spread by water flow to distant areas downstream.
Science/laboratory escapes, disposals or introductions. Accidental or intentional release of laboratory animals has introduced some non-native species.
Seafood packing and disposal. Because seafood is transported long distances, organisms in packing seaweed may reach new waters as an unintended by-product.
Pet/aquaria releases. Escapes or intentional release of unwanted pets can be a source of new non-native species in all parts of the country.
Aquaculture escapes. Non-native shrimp and oysters, and Atlantic salmon in the Pacific Northwest, are just a few examples of non-native mariculture species that have generated concern over disease or other impacts that might arise from their escape.
Biological control introductions. Ideally, introducing a second non-native species to control an invader results in diminished numbers of both species after control is accomplished. But some introduced controls have backfired because they attack non-target species; mongoose introduced in Hawaii to control rats have wiped out many native bird species.
Past government programs. The establishment of a new invader is sometimes an unanticipated outcome of a government program; Kudzu, for example, was originally introduced through a government-sponsored erosion control program.
Moving and depositing fill in wetlands. Seeds and viable parts of invasive plants contained in fill material may rapidly colonize the new substrate, then compete with native species in adjacent wetlands.
Land/water alterations that help spread invaders. Many invaders are adept at rapid pioneering where soil has been disturbed or water levels or routes have been changed, leaving a temporary gap in occupation by native flora and fauna.