Mute Swan: A native to Eurasia, the mute swan has long been cherished for its beauty and was imported to this region in the late 1800s to grace private ponds. In the early 1960s, five escaped into the wild, and their population has since mushroomed to more than 4,000 on the Bay.
The large birds, measuring up to 62 inches in length, have no natural predators and have been a menace to some native species, trampling nests and killing chicks of least terns and skimmers, both threatened species in Maryland. They are also major consumers of underwater grasses, which are important food for native waterfowl as well as habitat for fish and crabs. Flocks of mute swans have overgrazed grass beds in some areas, and it’s estimated the population as a whole consumes about 9 million pounds of underwater grasses annually.
Nutria: A native of South America, they were imported to Maryland’s Eastern Shore in the 1930s in a failed attempt to bolster the fur industry. The nutria ultimately were released into the wild, where they chew away at the roots of the plants that hold together the soil of low-lying marshes. Without the root mat cementing the marsh together, the land is literally washed away with the tides. Since the 1950s, nutria have helped to turn about 6 square miles of marshes in Blackwater National Wildlife Refuge into open water. Losses in other Dorchester County, MD areas are thought to measure in thousands of acres, and scientists say the rate of loss has accelerated as the nutria population has boomed.
Nutria reproduce rapidly — females mature at six months and can produce multiple litters, averaging five each, during the year — and they have no natural predators in the region. Their population is estimated to number in the tens of thousands on the Eastern Shore, and they have spread into Delaware and Virginia.
Phragmites: A tall perennial wetland grass that grows 3 to 13 feet tall, phragmites is sometimes referred to as a common reed. It spreads through creeping roots and can quickly create a dense mat of roots that crowds out other species. It is unusual among invasive species in that it is native to North America, although some have suggested the aggressive strain may have been introduced from Europe. Others say that human activity has hastened its spread because it quickly dominates disturbed soils.
Wetlands taken over by phragmites provide less food and habitat for wildlife, particularly waterfowl and migratory birds, and also threaten rare native plants. Phragmites can also alter the hydrology of a wetland and increase sediment trapping, causing them to dry out.
Purple Loosestrife: A wetland plant native to Europe, it was introduced to North America in the 1800s where, free of its natural enemies, it has spread rapidly. An individual plant can produce more than 1 million seeds, which are easily transported by water, wind, animals and humans.
It grows in dense thickets which crowd out native plants that wildlife use for food, nesting and hiding. In addition, its dense roots and leaves choke waterways, slowing flows and promoting the deposition of silt, which reduce the wetlands’ ability to filter water. They can require costly dredging when they become entrenched in drainage ditches.
Water Chestnut: An aquatic plant native to Asia, it was first reported in Massachusetts in 1859 and has since become an invasive plant known for its aggressive growth habits. One acre of water chestnut can produce enough seeds to cover 100 acres the next year.
With four hard, half-inch spines sharp enough to penetrate shoe leather and large enough to keep people off beaches, its seeds are major hazards to water recreation. Additionally, water chestnut can wipe out native Bay grasses from some areas; prevents nearly all water use where it occurs; creates breeding grounds for mosquitoes; and provides only marginal habitat for native fish and birds.
Zebra Mussel: A native of the Caspian Sea in Eastern Europe, the zebra mussel was introduced to North America around 1985 by ocean-going ships releasing ballast water in the Great Lakes. A single zebra mussel can produce up to 40,000 eggs a year. Adults can grow into dense colonies of up to 60,000 per square meter, clogging water pipes and fouling buoys, locks and other solid surfaces.
Each adult can filter a liter of water a day, so thoroughly clearing the water of phytoplankton that they are suspected of altering the food chain in some places. They also outcompete rare native mussels. Zebra mussels are easily transported from place to place by attaching themselves onto recreational boats or trailers, where they can survive out of water for several days.
The Watch List
These are species for which the Invasive Species Workgroup will seek funding to develop risk assessments, including population status, impacts in the Bay, and policy or management plans in each jurisdiction. The risk assessment will help to determine if a management plan is warranted.
Asiatic Clam: A native of Asia, this clam has now spread throughout the watershed. It fouls the solid surfaces it settles on, competes with native species, and can alter benthic substrates.
Suminoe Oyster: A native of Asia often known by its scientific name, Crassostrea ariakensis, it is proposed for aquaculture use in the Bay. Its impacts are unknown, but there are concerns about competition with native oysters.
Blue Catfish: A native of central North America, it is now in Virginia’s large tributaries as the result of an intentional stocking in 1985, and its numbers are rapidly expanding. The large, aggressive fish is a major predator of anadromous fish, such as shad, and may be hurting restoration efforts.
Green crab: A native of Europe, it is now spread along the East Coast where it is a predator of bivalves and other invertebrates, causing changes in invertebrate communities. It’s also a threat to the scallop fishery, and can carry a parasite that infects shorebirds.
Hydrilla: An underwater grass found in tidal freshwater areas around the Bay. It can form dense monocultures, outcompeting native grasses and impeding the recreational use of waterways. On the other hand, it can improve water quality and provides habitat for fish and waterfowl.
Japanese Shore Crab: A native of the shores of Russia, Korea China and Japan, it is now found in Maryland’s coastal bays. It preys on bivalves and can cause changes in invertebrate communities, impacting commercial species such as scallops. It has enormous reproduction potential.
Rapa whelk: A native of Asia, it was introduced to Virginia’s portion of the Bay in the mid-1990s, probably through ballast water release. It preys on clams, oysters and mussels and may pose a threat to the hard clam fishery. Its numbers have been growing rapidly.
These are forest and field nonnative invasive species that impact the watershed but may be managed by current state and local programs. The workgroup will document current management infrastructure for each species and and determine if any interstate agreement is needed to facilitate further action.
Asian Long-Horn Beetle: A native of China, it infects and kills hardwood trees, favoring maple. birch, poplar, willow, elm and ash trees. If it becomes established in North America, the beetle could cause annual damages of $138 billion. It has yet to be documented in the watershed.
Gypsy Moth: A native of Europe and Asia, it is found in forests across the watershed, where it defoliates oaks and aspens, killing up to 20 percent of the trees in the forest.
Japanese Honeysuckle: A native of Asia, this climbing woody vine is now found in fields and open areas across the watershed where it forms dense canopies that kill small trees and shrubs.
Japanese Knotweed: A native of Asia, this perennial shrub is found in riparian areas and roadsides across the watershed, where it crowds out native vegetation and can withstand floods, making it a particular threat along streams.
Japanese Stiltgrass: A grass native to Asia, it’s now found in Maryland and Virginia parks. It’s a dense, mat-forming annual grass that can displace native plants and dominate riparian areas, wetlands and damp fields. It is a major threat to rare, upland habitats supporting rare plant species.
Mile-a-Minute Weed: A native of Europe and Asia, this vine inhabits streambanks and areas with wet soils where it smothers and kills native plants.
Morrow’s Honeysuckle: A native to Europe and Asia, this shrub grows to heights of up to 16 feet in fields and forest edges, rapidly forming a dense shrub layer that displaces native plants.
Multiflora Rose: A native of Japan, Korea and China, this thorny shrub is found in pastures, fields and riparian areas throughout the watershed, where it can form impenetrable thickets, displacing native plants.
Oriental Bittersweet: A native of Asia, this perennial vine and shrub inhabits woodlands and forest edges throughout the watershed, displacing native plants and threatening all vegetation levels of the forest.
Tree-of-Heaven: A native of China, this rapidly growing deciduous tree (made famous in “A Tree Grows in Brooklyn”) is found along roadsides and forest areas, where it is a prolific seed producer, allowing the formation of dense stands that force out native plants.
Woolly Adelgid: A native of Europe, this wingless insect infects fir trees in wet woodlands and riparian areas throughout the Northeastern United States, including all of the Bay watershed, usually killing the infected trees. It is a particular concern in hemlock stands.
The workgroup will gather information on the management, status and monitoring of each of these species to assess the need for further regional action.
Asian Swamp Eel: A native of Asia that has turned up in Hawaii, Florida and Georgia, it inhabits wetlands and waterways, can breathe air and move over land. It has the potential to spread rapidly, and its populations can increase without detection. It has no known predators.
Brazilian Elodea: A native to South America, this underwater plant is found in slow moving, slightly acidic waterways in the watershed. It can form monolithic stands, displacing native plants.
Cabomba: A native to the southeastern United States, this rooted aquatic plant grows very densely in the wetlands and waterways where it has been introduced, displacing native plants. It is a popular aquarium plant.
Chinese Mitten Crab: A native to Germany that has been introduced to the California coast, it inhabits rivers and estuaries and can carry the Oriental lung fluke which can infect mammals and humans.
Eurasian River Ruffe: A native of Eastern Europe that has been introduced to the Great Lakes, this member of the perch family inhabits deep waters, where it outcompetes native fish such as yellow perch and the spot-tail shiner.
Eurasian Watermilfoil: A native to Eurasia and Africa, this aquatic plant has been introduced in the watershed where it can inhabit lakes, rivers and estuaries and forms large floating mats on the surface, displacing native species.
Flathead Catfish: A native to the Central United States, which has been found in the lower Schuylkill River in Pennsylvania (outside the Bay watershed), it can inhabit rivers and lakes, where it depletes native fish populations.
Giant Salvinia: A native of Brazil, this free-floating aquatic fern has been found in the Southeastern United States, including Southeastern Virginia. It inhabits lakes and ponds where it has the potential to completely cover water surfaces, displacing native species and impairing the use of the waterway.
Grass Carp: A native of Asia that has been introduced in the southern United States, this fish inhabits lakes and ponds, where it feeds heavily on submerged aquatic vegetation, removing vegetation needed by native species.
Quagga Mussel: A native to Europe and relative of the zebra mussel, it inhabits deep water substrates, fouls surfaces and competes with native species.
Round Goby: A native of the Black and Caspian seas, this bottom-dwelling fish has been introduced to the Great Lakes where its voracious appetite and prolific spawning makes it a threat to native species.