Invaders TitleNYSM home


Purposeful to Accidental and All Stages in Between
Intentional introductions are often undertaken by people who believe that the introduced species will benefit them in some way. Game species like Ring-necked Pheasant, Mallard, or Chinook Salmon were imported for hunting or fishing. The Eurasian Mute Swan, House Sparrow, and European Starling were meant to beautify the landscape.

Accidental introductions are usually a consequence of non-native species taking advantage of human movements. Species can be introduced in the release of ship ballast water or through international movement of goods. They can hitchhike by attaching to clothing, recreational gear, or boats. Without knowledge or intention, people can move a species to a new area.

Whether introductions are purposeful or accidental, these movements can often be avoided. Take time and care to minimize the chances of moving invasive species to new locations. In this section, you will see species that represent both purposeful and accidental introductions.


New York State Department of Environmental Conservation

Enid Leff © California Academy of Sciences

Historical Introductions of Invasive Species in the Hudson River Valley

A different attitude existed in the past when it came to non-native species. Between 1850 and 1950, several non-native species were purposefully introduced in the Hudson River Valley. Private estates, hunting clubs, and even state agencies all took part. These new species were brought in for game hunting, sport fishing, and as ornamental animals.

Among the most successful was the Mallard. Native to the western and central United States, it is now common throughout the Northeast. Mallards were introduced as game to Dutchess, Albany, and Saratoga Counties between the 1910s and 1950s.

Other imported game species are the European Hare and the Carp. The European Hare was imported between 1893 and 1911 to estates in Columbia, Dutchess, and Rockland Counties. It was considered a serious orchard pest by the beginning of the 20th century. Today, it is rare in New York. The Carp was originally released as a sport fish, and Carp fishing competitions can still be found in the spring, up and down the Hudson River.

Ornamental introductions include the Goldfish and the Eurasian Mute Swan. Goldfish were released into the Hudson River by a private individual who initially imported them for ornamental ponds. The Eurasian Mute Swan was introduced to adorn ponds on large estates from the late 1800s to the 1920s. They are currently found along the Atlantic coast and in the Great Lakes region. Mute swans compete with native waterfowl, disturb basking turtles, and consume large quantities of aquatic vegetation.

Mile-a-Minute Weed

Mile-a-minute Weed

Persicaria perfolia
Native to Asia, it was accidentally introduced in Pennsylvania in the 1930s with imported nursery stock. It is a fast-growing annual capable of smothering other plants in its path. Like so many invasive species, it has spread rapidly from its point of introduction. In New York, it is locally abundant on Long Island and in the lower Hudson River Valley.

The use of natural predators of this weed is the latest and perhaps most promising control method available. United States researchers teamed with Chinese scientists to identify a weevil that feeds exclusively on this vine. “No choice experiments” were conducted. The weevils were given closely related plants to feed on. The results showed little foraging and no egg laying. This suggests the weevil specializes on Mile-a-minute Weed and may not pose a risk to non-target plants.
Eurasian Watermilfoil
Larry Eichler, Darrin Fresh Water Institute, Rensselaer Polytehnic Institute

Eurasian Watermilfoil

Myriophyllum spicatum
Native to Europe and Asia, it was first documented in the eastern United States in the 1940s. The original source of introduction is unknown. It is probably the most invasive aquatic plant in the state. It has been found in nearly every county.

New plants can easily grow from fragments transported by boat activity, waterfowl, and bait buckets. This accounts for its rapid spread. It can form dense mats that crowd out native plant species. It grows to the water surface and forms a canopy, cutting off sunlight to the native plants beneath it.
Viburnum Leaf Beetle
E. Richard Hoebeke
Cornell University,

Viburnum Leaf Beetle

Pyrrhalta viburni
Viburnum Leaf Beetle is native to Europe. It was first detected in New York in 1996 along Lake Ontario. It likely arrived in North America on imported nursery stock. The beetle feeds on native and cultivated Viburnum species. These shrubs often flower, produce fruits, and have brightly colored fall foliage.

Viburnum Leaf Beetle can defoliate shrubs. Heavy infestations for several years may kill the shrub. Native Arrow-wood Viburnum (Viburnum dentatum) is particularly susceptible to the beetle. Attacked plants typically die within two to three years.

Researchers at Natural Areas Consultants and Cornell University monitored Arrow-wood Viburnum populations. What did they find? Cover and reproductive effort decreased the longer the Viburnum Leaf Beetle attacked. As viburnum cover declined, cover of other plant species increased. This was particularly true of non-native grasses. Therefore, the Viburnum Leaf Beetle may indirectly help the expansion of non-native plant species. The loss of Viburnum fruit could negatively affect migrant and resident songbirds, as well as small mammals.

Mosses consist of a leafy plant on which a different appearing leafless plant is attached. The leafless plant produces spores that give rise to leafy plants. The leafy plant can break into pieces, and each one has the potential to give rise to a new plant.

Lawn Moss

Pseudoscleropodium purum
Lawn Moss is native to central and western Europe. It is unclear when it was introduced. It may have arrived in soil associated with nursery stock, or in the late 1800s as packing material. Once established locally, it reproduces by the dispersal of plant fragments.

These are perhaps generated by lawn mowers. When a commercial machine is used on multiple lawns over a wide area, it may contribute to this plant’s spread. It has the potential to be a strong competitor with lawn grasses. Lawn Moss has been found in New York state mostly in cemeteries and also in residential lawns in a few places. Research on this species is being conducted by a scientist at the New York State Museum.

In the Northeast, where this moss is widely naturalized, the plant has not produced spores. Male and female plants have been noted at a number of sites but never together. This suggests that Lawn Moss has become established recently. Not enough time has lapsed to allow male and female plants to come together by dispersal. Should that happen, thereby allowing spore production, the distribution of Lawn Moss may undergo an explosive expansion, becoming aggressively invasive.

Don’t Release Bait!

You finish a day of angling on a local pond. You have not used up all the bait you purchased that morning. You dump the remaining fish, crayfish, or mudpuppies into the lake. You have just perpetrated a “bait bucket introduction.” This is a general term that refers to releasing bait into the wild, but it also includes releasing aquarium fish or pet snakes or birds. The purposeful release of organisms into the wild is illegal. Although well-meaning, the release of bait or pets often results in their rapid death by shock, starvation, or predation. Released bait or pets can also carry diseases or parasites into the environment.

At times, the release can also result in the eventual development of a self-sustaining population. Oftentimes the bait sold in bait shops is not harvested from the local area; most is trucked in from the southern United States. These non-native species are able to expand their range and population sizes by repeated introductions. A good example is the spread of Rudd, a popular bait minnow brought in from out-of-state suppliers. In the past decade, individuals have been reported in lakes across the state.
The House Sparrow and European Starling are native to Europe, Asia, and North Africa. Both were intentionally introduced into New York, but in two very different ways.
If we could go back 160 years, neither one of these now common birds would be seen anywhere in North America. Today, there are over 100 million of each. They are found in every county in New York state. How did they get here and what makes them successful invaders?

House Sparrow
Terry Spivey, USDA Forest Service,

House Sparrow

Passer domesticus
The House Sparrow was desired as a songbird. Over the course of twenty-five years, beginning in 1851, the House Sparrow was introduced multiple times across the United States. Its first introduction was in Brooklyn, New York. This was followed by releases in several other states, including Hawaii. There are now nearly 150 million of them. Why are they so successful? They are prolific breeders, and unlike many birds, they eat a wide range of foods.

European Starling
George W. Robinson
© California Academy of Sciences

European Starling

Sturnus vulgaris
European Starlings were introduced in the United States in 1890. In a single introduction event, 100 starlings were released in Central Park. This was in an effort to establish in the New World all the birds mentioned in Shakespeare’s works. In 100 years, the North American starling population has grown to an estimated 200 million birds!

This is the single most “successful” avian introduction to this continent. Why are they so successful? European Starlings are habitat generalists. They are able to exploit a large variety of habitats and nest sites. They are also aggressive and compete with native species for food. European Starlings displace Eastern Bluebirds, Tree Swallows, and Purple Martins from nesting
Northern Snakehead
Susan Trammell, USGS

Northern Snakehead

Channa argus
The Northern Snakehead is native to eastern Asia. It was first discovered in Maine in 1976 and was probably released from aquaria. In New York, Northern Snakehead has been found in at least one lake. We do not know if this population is reproducing.

All snakeheads are predators. Unlike native fish-eating predators, snakeheads are air-breathers. In fact, if unable to surface, they can drown. The ability to breathe air allows them to move for short distances on land and invade neighboring ponds. They can do this when, for example, their home pond dries up.

The human element is key for this species. If people continually introduce the snakehead into different lakes, the species will become a nuisance. This aggressive predator will likely negatively affect populations of native fish, amphibians, and large invertebrates.
 S. Pothoven, NOAA, Great Lakes  Environmental Research Laboratory   Swede Midge
Susan Ellis,

Swede Midge

Contarinia nasturiti
This insect is native to Europe. It is a recent invader to New York state, first detected in Niagara County in 2004. A pest species on brassicas (members of the cabbage family), adults emerge from the soil in the spring. After mating, females lay clusters of eggs on the growing point of crucifer plants. These are such plants as broccoli, Asian greens, cabbage, cauliflower, and brussels sprouts. After hatching, larvae feed heartily in protected areas of the plant. Mature larvae “jump” to the ground and burrow for pupation.

The Swede Midge’s original means of introduction is unknown. Surveys conducted from 2005 to 2007 detected the Swede Midge in twenty-two of New York’s sixty-four counties. This is a dramatic increase from its first detection just three years earlier.
Red-eared Slider

Red-eared Slider Shell
Both Red-eared Slider photos by Ariana Breisch

Red-eared Slider

Trachemys scripta elegans
The Red-eared Slider is native from Illinois south to northern Mexico and from western Georgia to eastern New Mexico. A prominent red spot on each side of its head gives this turtle its name. Millions were sold as pets in department stores, a trend that increased sharply after World War II.

Because of its popularity in the pet trade, it was inevitable that some of these turtles would become established outside of their native range. Some releases were accidental, like when individuals escaped from ornamental ponds or backyard aquaria, but many were deliberate. People grew tired of caring for them, especially when they realized they could grow up to one foot in shell length and live for forty years! Many people released them into local ponds, without thinking of the implications.

Red-eared Sliders are now widespread in New York. They can damage native vegetation and compete with native turtles for basking sites, food, and nesting areas. In southeastern New York, they outnumber native turtles in some ponds.