What are invasive species?

Invasive species are organisms that have been introduced into an area where they didn’t evolve, and that usually have no natural enemies in areas where they have been introduced (after Westbrooks, 1998).

What can I do about invasive species?

• Be an informed consumer. When you purchase plants for your yard and garden, buy only plants that you know are not invasive or likely to become so. See the “Alternatives” section of the web site for specific planting ideas.
• If possible, buy native plants and encourage local nurseries to offer a wide variety of native plants and non-invasive non-native species. Our native plants are adapted to the local soils and climate, and in addition, have co-evolved with the native wildlife. By planting native species you are helping to maintain biological diversity right here in Rhode Island.

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If a particular plant is invasive, does this mean that all cultivars and hybrids of that plant are also invasive?

This is a difficult question to answer, and a topic which needs immediate research. Evidence thus far suggests that if a cultivar produces fruit, that fruit produces plants which act like the original species. For example, if a cultivar of Japanese Barberry, such as ‘Compacta,’ produces fruit, its fruit will grow into the normal-sized, invasive Japanese Barberry.
The criteria of invasive species for both Connecticut and Massachusetts state: “[the term] Species…includes all subspecies, varieties, forms, and cultivars unless otherwise noted.”
The Rhode Island Invasive Species Council would like to see more research into the fruit-producing capability of widely used landscape plants and their cultivars, and research to develop cultivars that are truly sterile.

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I am a good gardener and keep my landscape tidy. If I am vigilant, what is the harm in planting invasive plants in my yard?

Even the best gardener cannot control the wind, rain, and birds. The fruit of invasive species can be spread by runoff water during heavy rains, by the wind, and by birds eating the fruit and then depositing seeds in their droppings far from your yard. The safest route is to avoid planting a species known to be invasive.

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What are some common invasive species in Rhode Island?

Invasive non-native animals include the Norway Rat, Rock Dove, House Sparrow, House Finch, Mute Swan, Monk Parakeet, Japanese Shore Crab, and Green Crab. Invasive plants include Purple Loosestrife, Multiflora Rose, Autumn Olive, Glossy Buckthorn, Tree of Heaven, Garlic Mustard, Japanese Honeysuckle, and many others. Click here to see more complete lists of invasives species in Rhode Island.

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How long does it take for a species to become invasive?

Recent research calculated that it make take anywhere from 8 to 388 years for a species to become invasive, with an average of 147 years. How quickly a species becomes invasive depends on many factors, including:

• the lifespan of the introduced species (a short-lived insect, for example, may be able to become invasive far more rapidly than a tree that requires many years to reach maturity);
• the stability of the environment. Many invasive species are rapid growers that thrive in disturbed and/or polluted habitats. If the environment changes in ways favorable to an invasive species, that will enhance its spread.
• the ability of the species to jump spatial gaps will speed its spread.

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What are some traits that allow species to become invasive?

There are a variety of adaptations that allow species to become invasive. No one invasive species possesses all of these traits, but each usually has a combination of several of them:

• Rapid growth. Kudzu (Pueraria lobata), for example, can grow up to a foot a day, or 100 feet in a season, smothering other plants around it. Small wonder it is called “The plant that ate the South”! Mile-a-minute Weed (Polygonum perfoliatum), a relatively new invasive species from eastern Asia, is also a rampant grower, and has just recently been found in Rhode Island.
• A short juvenile period, and thus the ability to reach maturity quickly.
• The ability to produce large quantities of fruit, seeds, or offspring, which are then easily spread.
• Having the means to reproduce successfully in the new home. For example, an introduced plant that needs a specific pollinator which is not found in the new home will be unlikely to be able to produce fruit and spread widely. The most successful invasive plants are usually wind-pollinated or pollinated by the insects already present in the new home.
• The ability to reproduce and spread vegetatively, via suckers, stolons, and rhizomes, or rooting at the tips of canes. Multiflora Rose, for example, roots at the tips of the canes and thus creates new plants; Common Reed (Phragmites australis) can send out long stolons to colonize new areas.
• The ability to jump spatial gaps in the landscape. The most successful invaders can move from the original site of infestation to new sites at a distance from the original locale. Many invasive plants, for example, have fruits that are moved by wind or water, or are eaten by birds and spread in their droppings. Multiflora Rose (Rosa multiflora), Japanese Barberry (Berberis thunbergii), Asiatic Bittersweet (Celastrus orbiculatus), Autumn Olive (Elaeagnus umbellata), and Burning Bush (Euonymus alatus) are examples of invasive species whose seeds are spread in bird droppings, often deposited far from the site where the fruit was eaten.
• A phenology that differs from the local flora and fauna. Phenology is the timing of breaking dormancy, flowering, setting fruit, and going dormant. Many invasive plant species break dormancy earlier than the native species and thus can begin to remove nutrients in the soil before the native plants have access to them.

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How do species become invasive?

The usual way is for an organism to be moved, by design or accident, to a part of the globe where it has not evolved. Once in a new home, invasive species are able to spread beyond the point where they are introduced and become naturalized (able to grow and spread without human cultivation or aid). Invasive species tend to lack the predators, herbivores, and diseases that helped keep them in balance in their original home. Once naturalized, species that become invasive can out-compete native species, sometimes causing local extinctions of native species. It is estimated that over 40% of the plants and animals on the US Endangered Species List are at risk because of invasive species.

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Do all species that are moved to new areas become invasive?

No. It is estimated that only about 1% of all introduced species become invasive. The vast majority of introduced species cannot thrive without the aid of cultivation or human intervention, and are not able to naturalize.

Here in Rhode Island, this difference in timing is easily seen in early spring; if you look carefully, you will note that virtually every tree and shrub leafing out in late March and early April is a non-native species (look for Autumn Olive, Japanese Barberry, and Morrow Honeysuckle, for example). In the fall, the non-natives tend to remain green longer than the native species (good examples of this are Norway Maple [Acer platanoides] and English Oak [Quercus robur]).

• The ability to thrive in a broad range of environmental conditions, often including disturbed and/or polluted habitats. Among animals, the Norway Rat (Rattus norvegicus) and European Starling (Sturnus vulgaris) can thrive even in urban habitats. Plants that do well in disturbed habitats include Tree of Heaven (Ailanthus altissima), Autumn Olive, Multiflora Rose, and Japanese Knotweed (Polygonum cuspidatum).
• The ability to inhibit the growth of competitors. Some invasive species create a dense shade and/or such a dense root mass that it is difficult for most other plants to grow beneath them. Norway Maple, for example, suckers heavily and has very shallow roots; both characteristics keep down competing growth from other plants. Japanese Knotweed also creates a dense root mass that prevents competition. English Ivy (Hedera helix) produces such a dense shade that little can grow beneath it.
• Other plants, such as Canada Thistle (Cirsium arvense), create chemicals (allelopaths) that inhibit the growth of other plants. Canada Thistle is known to inhibit the growth of wheat, flax, pigweed, and green foxtail; this “chemical warfare,” along with its sharp, prickly leaves and stems, make it a very serious agricultural pest, responsible for tens of thousands of dollars of damage to livestock and crop plants each year. Many invasive aquatic species (such as Eurasian Water-milfoil), also create a dense shade that prevents sunlight from reaching other plants in a pond or stream.
• The possession of traits that make them less attractive to herbivores or predators. Prickles, thorns, unpalatable taste, and similar traits help protect species from being eaten.

Pest free–Will Grow Anywhere–Rapid Growth–Great for Wildlife are how a number of plant species that have since become invasive have been advertised in the nursery trade, advertisements that reflect some of the most common traits of non-native invasive plant species, as described above.

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What are some of the ways organisms have been introduced into new parts of the world?

Organisms have been moved around for many purposes, including:

• Agriculture: People have traded in seeds and plants for thousands of years. Plants such as corn, tomatoes, and potatoes came to the Northern Hemisphere from South America; Old World crops such as wheat, barley, rye, and oats were brought to the New World. Such trade continues to the present day.
• Transportation: Whenever people move around, seeds, spores, and small animals and plants of all sorts move with them. Ballast water, sheep’s wool, packing materials such as crates and excelsior, boat trailers, and car tires are among the myriad ways that organisms can “hitchhike” from one place to another. The large red seaweed Grateloupia turuturu, and the Asian Shore Crab, Hemigrapsus sanguinea, which now grow throughout Narragansett Bay, came to Rhode Island via ballast water. The Asian Longhorned Beetle (Anoplophora glabripennis) arrived in the US in wooden packing material, and is destroying hardwood trees in Chicago, IL and Brooklyn, NY.
• Nursery trade: People have moved plants for ornament almost as long as they have cultivated plants for food, medicine, and fiber. A number of plants introduced by the nursery trade–such as Japanese Honeysuckle, Multiflora Rose, and Autumn Olive–have become invasive. In addition, whenever plants are moved, other organisms, such as insects, earthworms, bacteria, fungi, and viruses may also be inadvertently introduced. Dutch Elm Disease and the Chestnut Blight are examples of diseases that have entered North America via nursery stock from overseas.
• Aquarium and pet trades: Many new species have been introduced when people have released exotic pets in the wild or dumped aquarium plants into local water bodies. Fanwort (Cabomba caroliniana) is an example of an aquarium plant that has become invasive in Rhode Island.
• Construction: Soil brought to a construction site may contain nvasive plant seeds and new animals (such as earthworms). Hay bales used in highway construction and wetland buffers also contain seeds; if the hay has been grown far from the construction site (as is often the case in Rhode Island, where most of the hay used in highway projects comes from Canada), it is likely to bring non-native seeds with it. Nodding Thistle (Carduus nutans) probably came to Rhode Island in the late 1960s, in highway construction. Garlic Mustard (Alliaria petiolata) has also moved into new sites in the state in “clean fill.” Construction of roads over wetlands may also result in enhanced habitat for non-natives. Common Reed (Phragmites australis) often moves into salt marshes when road construction lowers the salinity of the salt marsh.
• Aquaculture: Fish and shellfish cultivated for commercial purposes may sometimes escape and have ecological effects. The European Oyster (Ostrea edulis), for example, was released intentionally in Maine, and now grows in Rhode Island waters.

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Why are people so concerned about invasive species?

• The primary reason is the loss of BIODIVERSITY. The world’s ecosystems are comprised of millions of animals, plants, protists, fungi, bacteria, and viruses, interacting with the physical environment. Each ecosystem has its own combination of organisms. Invasive non-native species can cause the extinction of native species, thus altering ecosystems permanently. Ecologists consider habitat loss/destruction and the presence of invasive species to be the two greatest threats to biological diversity around the globe.
• At the human level only, this loss of biological diversity can mean that basic ecological processes are altered, affecting such vital human activities as food production, the maintenance of good water quality, and health care. Lowered biodiversity also decreases our ability to “mine” species for new food crops, medicines and pharmaceuticals, and materials used in industry (such as lubricants, paper products, fibers, etc.).
• Invasive species are also very expensive, costing the United States an estimated $138 billion in economic and environmental losses each year, affecting food crops, golf courses, the growing of turf and ornamentals, industrial sites, forestry, aquatic sites, recreational areas, municipal water supplies, etc. See Pimentel et al. 1999 for an overview. Here are some examples:

1. Water Hyacinth (Eichhornia crassipes), used for aquaria and backyard ponds, has naturalized in the South, where it clogs waterways and impedes recreational and commercial fishing and boating. It costs the state of Florida more than $3 million a year to try to control its spread. (Water Hyacinth has been found in Rhode Island waters but so far does not appear to have survived over the winter.)
2. The Zebra Mussel (Dreissena polymorpha) was first discovered in the Great Lakes in 1988, where it entered from ships’ ballast water. This small Eurasian mollusk is notorious for clogging water intake pipes and costing public utilities millions of dollars in damage. Not yet reported in Rhode Island, they reached Connecticut in 1998.
3. European Water Milfoil (Myriophyllum spicatum) is a serious threat to water bodies in the Northeast, killing fish by causing low-oxygen situations, clogging water intakes, fouling lakeside beaches with decaying mats of vegetation, and seriously impeding recreational boating and swimming. The state of Vermont spends from $300 to $10,000 per acre to control it, as its presence in a water body can have a big effect on real estate values. Aquatic species such as this are easily moved from one waterbody to another by seeds and plant fragments clinging to boats and trailers that haven’t been carefully cleaned.
4. The State of Vermont has also spent about $2.5 million trying to keep Water Chestnut (Trapa natans) under control. This aquatic plant can also take over freshwater ponds and streams, and produces a spiny fruit that can puncture tires! As of summer 2005 it is spreading in Massachusetts and Connecticut.
5. Cheatgrass (Bromus tectorum), an annual, highly flammable grass that has spread across North America, increases the frequency and intensity of fires in the western United States. It is responsible for millions of dollars of damage annually to pastureland and agricultural facilities.

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Where can I go for more information?

1) Links

See the LINKS section of the Rhode Island Invasive Species Council web site.

2) Local organizations with information

• Rhode Island Natural History Survey: RINHS coordinates the Rhode Island Invasive Species Council and can provide a great deal of information about invasive species in Rhode Island.(401) 874-5822
• Rhode Island Wild Plant Society: RIWPS has 2 annual plant sales that feature native plants. (401) 453-3777 |office@riwps.org
• URI Watershed Watch: Watershed Watch volunteers monitor local water bodies, including looking for invasive aquatic species. (401) 874-2905 | URIWW@etal.uri.edu

3) Literature
Elton, C. S. 1958. The Ecology of Invasions by Animals and Plants. Chicago, IL: University of Chicago Press.
Marinelli, J. (ed.) 1996. Invasive Plants: Weeds of the GlobalGarden. Brooklyn NY: Brooklyn Botanic Garden Handbook # 149.
McKnight, B. N. 1993. Biological Pollution: The Control and Impact of Invasive Species. Indianapolis, IN: Indiana Academy of Science.
Mooney, H. A., and R. J. Hobbs. 2000. Invasive Species in a Changing World. Washington D.C.: Island Press. Shonbrun, S. B. (ed.). 1998.
Invaders. Conservation Notes of the New England Wild Flower Society, Vol. 2(3). Westbrooks, R. 1998.
Invasive Plants: Changing the Landscape of America: Fact Book. Washington, D. C.: Federal Interagency Committee for the Management of Noxious and Exotic Weeds.
The journal of the Natural Areas Association publishes many articles about the ecology and control of invasive species.

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The Rhode Island Invasive Species Council is an outreach program of the Rhode Island Natural History Survey, The Rhode Island Agricultural Experiment Station, and The University of Rhode Island Cooperative Extension