Healthy native ecosystems are dynamic and ever-changing, but their changes occur within a range of natural variability (see the Agents of Watershed Change module). When is the `balance of nature' tipped too far? Some kinds of non-native plants and animals can cause havoc when, accidentally or intentionally, they are released outside their normal range into a new region (see the Sources of Invasive Species Release). The Gypsy Moth, Nutria, Zebra Mussel, Hydrilla, Sea Lamprey and Kudzu are examples of non-natives that have caused massive economic and ecological losses in new locations because the natural controls of their native ecosystems were not there.
Not all non-native species become pests, or even survive, in new locations (see the Ten Percent Rule). But when they do, they often displace a whole suite of native species to become dominant. They then take on new labels: invasive exotics, or non-native nuisance species, or simply, invasive species (see Definition). Their impacts are insidious (see Traits of Invasive Species) because they often invade the open space areas we have preserved for native flora and fauna, as well as farmlands, forests and suburbs. How big is the problem? Consider the following:
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Damages from invasive species, including only those damages that can be expressed in monetary terms, have been estimated as high as $ 138 billion per year. These damages affect agriculture, rangeland, forests, people's homes and yards, human and animal health, food supplies, fishing and boating, outdoor recreation, and many other areas;
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Invasive species are thought to have been involved in 70% of this century's extinctions of native aquatic species, and 42% of current endangered species are impacted significantly by invasive species;
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In January 2003 the Director of the US Fish and Wildlife Service called invasive species "the biggest environmental threat to this country... it's something everyone needs to take very, very seriously."
It is increasingly important that watershed managers become aware of invasive species in their watersheds, in both the aquatic and terrestrial environments. Aquatic invaders are clearly of concern to a water resources manager, but invasive species in the watershed can have significant effects on water quality and aquatic ecosystem health due to the ways they affect bank stability and the volume and pollution levels in runoff (see Terrestrial Invasive Species that affect Water Quality). Below are recommended types of watershed management-related invasive species information needs, paired with links to the Aquatic Nuisance Species Task Force Web site and other web sites:
[The links below are to federal government sites including those of EPA and other agencies.]
General Information:
What are invasive species Exit and their impacts?
A nationwide database Exit with names and ranges of invasive species that may be in your watershed
Gateway to Invasive Species Information Exit
Government Efforts at Control
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.
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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.
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Vehicular transportation. Both private and commercial transportation are major factors in the movement and range expansion of non-native species throughout the US.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Science/laboratory escapes, disposals or introductions. Accidental or intentional release of laboratory animals has introduced some non-native species.
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Seafood packing and disposal. Because seafood is transported long distances, organisms in packing seaweed may reach new waters as an unintended by-product.
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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.
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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.
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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.
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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.
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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.
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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.
Ten Percent Rule
The "Ten Percent Rule" is a general rule of thumb that says of all non-native species that are released into new ecosystems, about 10% survive at all, and of these survivors, about 10% (or 1% of the original number of species released) become invasive. Nevertheless, there are still hundreds of invasive species established in the US.
Working Definitions (based on Executive Order 13112)
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Non-indigenous (non-native) species: with respect to a particular ecosystem, any species that is not found in that ecosystem. Species introduced or spread from one region of the US to another outside their normal range are non-indigenous, as are species introduced from other continents.
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Invasive species: a species whose presence in the environment causes economic or environmental harm or harm to human health. Native species or non-native species may show invasive traits, although this is rare for native species and relatively common for non-native species. EO 13112 actually uses the term invasive species to mean invasive non-native species, as does this paper.
Traits of Invasive Species
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capable of doing significant harm to ecosystems, economy or public health;
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capable of spreading without apparent natural controls (natural predators, disease);
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population levels that are unchecked;
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causing major change faster than native ecosystems can accommodate;
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changing major ecological processes (nutrient cycling, hydrology, fire regime, energy);
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destabilizing environmental (physical or community) structure;
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forming undesirable monotypic stands of vegetation that replace diverse communities;
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reducing biodiversity/integrity, causing extirpations and extinctions;
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reducing or eliminating a natural product, ecological service or other valued attribute.
Example Terrestrial Invasive Species that Affect Water Quality
Invasive species effects on water resources can be direct, as in the case of many aquatic nuisance species, or indirect, as in terrestrial species that change water tables, runoff dynamics, fire frequency, and other watershed attributes that in turn can alter water body condition. Beyond the dramatic estimate of $138 billion for yearly economic impacts and control costs, impacts to ecosystems and their beneficial services are estimated to be several times more than this total. Example watershed impacts include:
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Cheatgrass now covers millions of acres of the Inter-mountain West, where it increases the occurrence of fires from 60-110 year frequency of small fires to a 3-5 year frequency over much larger areas. This increases fire-related impacts on water bodies from heavily burned watersheds; also cheatgrass reduces runoff in western watersheds due to its heavy use of early season moisture.
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The Hemlock Woolly Adelgid is spreading throughout the Appalachians where in most areas it is 100% fatal to the Eastern Hemlock, a key species providing riparian forest cover and bank stability for headwaters streams. Deforested streams can then develop temperature and sediment problems and lose their coldwater aquatic communities.
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Pepperweed invades wetland perimeters, riparian areas, salt marshes and other habitats in the West; it has interfered with regeneration of cottonwoods and willows, two key native species in western stream corridors.
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Invasive species might cause indirect social/behavioral effects that could result in significant water quality impacts as well. For example, fear of non-native pests may prompt more pesticide and herbicide use and over-application, potentially increasing the amount of these chemicals entering water bodies through runoff.
