The earth is covered with a countless number of different living species, animals, plants and microorganisms. Amazingly, humans have not yet seen and cataloged all of these varying species, the world is so large and diverse. All of these organisms have a place of origin, where they historically had been found living. Some species have the capability to move on their own and others rely on other forces, like the wind, rain or animals. As humans explore the world and travel far away from their homes, they take with them all sorts of other plants and animals, both intentionally and unintentionally. When these species get to a new location, they may blend in with the existing flora and fauna and reach a happy coexistence. However, some species like their new environment so much, they take over and crowd out the ones who lived there first. These bullies are known as invasive species.

What are Invasive Species?

Biologists use these categories to describe the ecological status of plant and animal populations to describe how they fit into a particular geographical region. Four categories serve to cover the concepts used to describe the status and the distribution of a particular species.

  • Native, Indigenous: species naturally ocurring or originating in a geographical region since prehistorical time;
  • Introduced, Alien, Exotic: deliberate or accidental release of a species into an area in which it has not occurred in historical times;
  • Invasive: the establishment of self-regenerating and spreading populations of a naturalized species in a free-living state in the wild, takes possesion and may affect injuriously;
  • Nuisance, Noxious, Weed: any plant, either native or introduced, with a harmful or destructive influence on existing natural communities, interfering with the objectives or requirements of people.

These categories apply to biological communities, which are always evolving or changing due to fluctuating environmental conditions. Some species may be considered invasive if they occur in Maine but have been transported between watersheds and their introduction has caused detrimental effects to existing populations.

Organisms that are moved to a new location from their native habitat are usually referred to as non-native or exotic to their new environment. A new environment can be the next country, state, watershed or waterbody. Non-native species can seriously degrade their new environment, reduce social and economic values and possibly cause human health issues. These species are called invasive species. Invasive species that live in freshwater, inland wetlands, coastal wetlands or marine waters are known as invasive aquatic species.

How are Invasive Species Spread?

In addition to being transported by birds and mammals through their droppings, invasive species are also spread by human activities. These activities include:

  1. Transporting species between water bodies via watercraft, trailers, and other equipment;

  2. Releasing invasive species into the wild from aquariums, water gardens, research and education projects, and illegal stocking;

  3. Discharging untreated biological waste from aquaculture, seafood or other processing facilities that introduce pathogens and other organisms to marine waters;

  4. Releasing ballast water containing invasive species into marine waters.

How do Invasive Species Impact Wetlands?

Invasive species thrive in new habitats because they generally lack predators and other natural controls, they have reproductive adaptations which allow them to disperse successfully, they can tolerate and adapt to a variety of environmental conditions and they establish self-sustaining populations. Invasive species can threaten the diversity or abundance of native species and the ecological stability of the whole habitat. Invasive species displace native species by outcompeting natives for breeding sites, prey and other needed resources. They disrupt food webs, degrade habitats and alter biodiversity. Wetland habitats are very productive and support a large number of threatened and endangered species, unfortunately, the habitat that these species rely on is being altered and destroyed by invasive species.

What Invasive Species are Found in Wetlands?

Terrestrial Invasive Species: Three common terrestrial invasive species found in wetlands are Purple Loosestrife, Glossy Buckthorn and Common Reed.

Purple Loosestrife (Lythrum salicaria), seen below, is commonly found in cattail marshes, sedge meadows and open bogs. opportunistic in areas of recent soil disturbance and is commonly seen in manmade stormwater retention ponds and in roadside ditches. An invasion of purple loosestrife leads to a loss of plant and wildlife diversity. The plant ranges in height from 2 to 6 feet, it has a four-angled stem and the sessile leaves are opposite or in whorls, narrow to narrowly oblong with heart shaped bases.

Photo credit: State of Kentucky, Division of Forestry



Glossy Buckthorn (Frangula alnus) is a deciduous shrub or small tree found in wet meadows, marshes, calcareous fens, sedge meadows, sphagnum bogs and tamarack swamps. It has glossy oval shaped leaves, which have several pairs of distinct veins curved toward the leaf tip. Twigs may be tipped with sharp, stout thorns. Natural community composition may be altered by an invasion of glossy buckthorn. It can cause habitat degredation, shade out rare species and cause declines in native species diversity.

Photo credit: Hugh H. Iltis, University of Wisconsin-Madison



Common Reed (Phragmites australis) is a tall perennial grass, ranging in height from 3 to 13 feet, with vertical stalks that have sheath-type leaves, 1/2 to 2 inches wide near the base and tapering to a point near the end. It thrives in sunny wetland habitats, along the borders of brackish and freshwater marshes and along riverbanks. Stands of common reed displace native species and eliminate diverse wetland plant communitites and provide little food or shelter for wildlife.

Photo credit: Steve Young, IPC NYS (Invasive Plant Council of New York State)



Aquatic Invasive Species: Three of Maine's common aquatic invasive species are Fanwort, Hydrilla, and Eurasian Milfoil. For the complete list of all eleven aquatic invasives in Maine, please visit the Maine State-listed Noxious Weeds.

Fanwort (Cabomba caroliniana) is a perennial, submerged aquatic plant native to South America and parts of southern North America. It produces small white flowers, leaves are opposite, and the stem can grow up to thirty feet. Fanwort is highly competitive and can quickly colonize ponds, lakes, and quiet streams. As an infestations grows, fanwort crowds out native plants, affect fish habitat, and impedes recreational activities.

Photo credit: Kerry Dressler @ University of Massachusetts Amherst © 1996



Hydrilla (Hydrilla verticillata (L.f.) Royale) is a perennial, submerged aquatic plant native to Asia. Its stems can grow very long and have whorls of 3-8 leaves at each node. Hydrilla can reproduce by seed or vegetatively, and only one whorl of leaves is needed to start a new infestation. It is highly aggressive and can grow in a range of pH values, salinity, and low light levels, which does not bode well for most of Maine's ponds, lakes, and rivers. Its ability to grow up to one inch per day allows hydrilla to outcompete native plant and destroy habitat.

Photo credit: Colette Jacono @ USGS Nonindigenous Aquatic Species



Eurasian milfoil (Myriophyllum spicatum) is a herbaceous perennial native to Europe, Asia, and northern Africa. It produces small yellow flowers, leaves occur in whorls, and reproduces mostly through rhizomes. Milfoil aggressively grows in lakes, ponds, slow-moving streams, reservoirs, and estuaries, and creates dense mats that hinder recreational activities, crowd out native plant communities, and degrade habitat.

Photo credit: Don Cameron @ MNAP, VLMP © 2007



What can be done about Invasive Species?

Action to combat the spread of invasive species is already occurring within Maine, throughout the Northeast and at the Federal level. Once an invasive species has established itself, it can be very difficult to eliminate the whole population. Techniques have been developed for treating invasive species in physical, biological and chemical ways. While aquatic pesticides are widely available at hardware stores and through mail order catalogs, their use is regulated in the State of Maine and a permit is required from the DEP.

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