Biodiversity wanes in New York

Throttling invasive species by TJ Sinnott

Bulwark for the Great Lakes and Hudson River by P Gerrity

Stopping ballast water "stowaways" by D Pughiuc

Biological pollutants in the Great Lakes by EL Mills, KT Holeck

Water quality signatures and the zebra mussel invasion by DA Matthews, SW Effler

Zebra mussel population dynamics: Implications for water quality modeling by CL Lange, DR Opdyke, JC Powers

Bad seeds: an introduction to invasive plants by AD Halpern, CA Boesse, AE Altor

You can help stop the plant invasion

President's message by D Ellis

Executive director's message by P Cerro-Reehil

People and places

NYWEA calendar

Sponsors at 73d Annual Meeting

Spring 2001 — Vol. 31, No. 1

Bad seeds: an introduction to invasive plants

 
The term invasive is used to describe a plant species that aggressively competes with and eventually dominates local plant communities. This article elucidates the serious problems invasive plants pose and provides examples of aquatic invasive species inhabiting New York.

Typically, an invasive plant is a non-indigenous species (also known as exotic or non-native) that has been successfully introduced to a new more favorable environment having better resources or lacking its native predators. Most non-indigenous plant species are not considered invasive, and a native plant can be invasive when its growth becomes aggressive, particularly in disturbed habitats. The biodiversity in an infested area is often reduced as the invasive plant species replaces the native plant species.

Once established, the spread of Eurasian watermilfoil is difficult to control. (Photo by Donald J. Leopold).

Invasive plants share similar characteristics that make them highly successful. Typically, they have a relatively short life cycle and reach reproductive maturity early. They allocate many of their resources to sexual and/or vegetative reproduction and have effective means of seed dispersal. In addition, invasive plants can acclimate to a variety of environmental conditions and respond rapidly when resources become available, often growing at a much higher rate than do native species.

Many non-indigenous species are introduced to new regions through deliberate, unintentional, and accidental anthropogenic activity. Non-indigenous plants are sometimes deliberately introduced for aesthetic reasons and with the intention of increasing biodiversity of an area. Potentially invasive plants are also unintentionally introduced to new regions. Plants may escape the confines of ornamental and garden use, and aquarium plants are released into natural aquatic habitats. Seeds and plant fragments can be carried in ship ballast and in cargo.

Eurasian watermilfoil

Eurasian watermilfoil, Myriophyllum spicatum, is a submerged aquatic plant with whorled Y-shaped filamentous leaves. Documented in the Chesapeake Bay in the 1940s, its introduction probably occurred during the late 1800s through shipping ballast, and the plant is now widespread in North America.

Eurasian watermilfoil begins its growth cycle at spring thaw—well before many native plants—because it over-winters underneath the ice. This invasive plant grows aggressively in the summer and has been reported to grow between 5-7 cm daily. Once the main stem reaches the water's surface, the plant expands horizontally through the development of shoots that radiate from the main stem, forming a canopy. This growth pattern maximizes its exposure to sunlight, thereby inhibiting the growth of surrounding submersed plants, including numerous native species of watermilfoil. The resulting Eurasian watermilfoil beds, dense enough to support wading birds, displace the native flora, thereby affecting the animal community dependent on native plants for food and shelter. The thick beds impede boat navigation and provide favorable breeding grounds for mosquitoes, which is of particular concern in light of the threat of the West Nile virus in New York. It reproduces vegetatively: plant fragments grow rapidly into new plants. The wave action of water, or contact with waterfowl or boats, is sufficient to create fragments that can colonize new areas.

Frogs-bit

Frogs-bit Hydrocharis morsus-ranae, is a floating-leaf freshwater plant. Native to Europe, it was brought from Zurich, Switzerland for horticultural use at the Central Experimental Farm in Ottawa, Canada in 1932. Since then it has spread throughout parts of southeastern Ontario, western Québec, and in areas along the St. Lawrence River in upstate New York.

Mats of frogs-bit impair submerged plants beneath them by reducing sunlight, impede navigation, and make wading through them challenging (Photo by Alison D. Halpern).

Frogs-bit superficially resembles miniature water lilies with its quarter-sized, floating, cordate leaves; however, unlike the water lily, its long, fibrous roots float freely beneath the plant instead of anchoring the plant in the substrate. The entanglement of roots allows frogs-bit to form dense floating mats in ponds and quiet areas of streams.

The small, white flowers of frogs-bit are either male or female. While it is not uncommon to see mats of frogs-bit blooming profusely, it is the vegetative reproduction that makes frogs-bit problematic. In the fall, the plant forms buds, called turions, which sink to the substrate below where they over-winter. In spring, they float to the surface and develop into new plants. It has been estimated that a single frogs-bit plant can produce over 100 turions annually. Plantlets also develop on the stems during the growing season.

Use of the water chestnut as a water-garden ornamental contributed to its spread throughout New England, New York, and the Great Lakes (Photo by Donald J. Leopold).

Water chestnut

Trapa natans, is a floating aquatic plant native of Eurasia and Africa that was first introduced to Concord, Massachusetts during the mid-1800s. Like frogs-bit, water chestnut has floating leaves, which form thick, impenetrable mats that also restrict navigation and may provide a breeding habitat for mosquitoes. However, water chestnut spreads primarily by means of seed dispersal, producing durable, two-seeded fruits, often called nuts that sink to the substrate, where they over-winter. Occasionally, the fruit may not dislodge from a fragment of the plant and be dispersed through the water current.

(Photo by Donald J. Leopold).

What makes this fruit so memorable to those who come into contact with it are the four sharp spines that protect the fruit, and which are sharp enough to puncture skin. This hazard makes areas infested by the water chestnut unappealing to swimmers, boaters, and fisherman alike. The nuts are a high source of protein, edible to muskrats and squirrels. Nonetheless, water chestnut is an inferior food source compared to the native plants it out-competes.

Water chestnut thrives in eutrophic calm water; therefore, it is extremely successful in disturbed aquatic habitats, such as those that have been affected by agricultural run-off of crop fertilizer and manure, sewage treatment, and soil disturbance.

A Eurasian perennial that is a familiar sight along roadside ditches (Photo by Donald J. Leopold).

Purple loosestrife

Purple loosestrife, Lythrum salicaria, was accidentally and deliberately introduced to North America in 1814. Its seeds may have been carried in ship ballast or on the coats of imported animals. Immigrants brought the plant for its medicinal value. Recently, purple loosestrife has been planted for honey production and ornamental purposes. While loosestrife populations are especially concentrated in the Northeast, purple loosestrife occurs in every U.S. State except Florida. It is also found in Ontario, Québec, the Maritime Provinces, and British Columbia. It is regarded as such a serious problem that it is illegal to sell this plant in some states.

(Photo by Donald J. Leopold).

Its ability to produce 120,000 seeds per flower head and almost three million seeds per plant — achieved by a long flowering season — makes purple loosestrife a formidable invasive, indeed! The small seeds have an 80% survival rate after 2 years of dormancy, establishing a large long-lived seed bank. Seeds are also dispersed by birds and waterways. Additionally, the plant forms adventitious roots and shoots from damaged or buried stems.

Because North American predators are few and because of its highly efficient means of reproduction, purple loosestrife has colonized disturbed wetland areas quickly. It out-competes even robust species such as cattail, and reduces the quality of wetland habitats for wildlife dependent on cattail for food and shelter, such as muskrats, marsh wrens, and swamp sparrows. Foraging by deer is insufficient to damage the loosestrife and actually promotes the growth of additional stems.

Feathery brown plumes and heights up to 4 meters make the common reed easily recognizable (Photo by Donald J. Leopold).

Common reed—phragmites

Phragmites australis, also known as common reed, is a member of the grass family that occurs in wetland habitats throughout North America and, like purple loosestrife, is often seen in roadside ditches. Although phragmites is indigenous to North America, the invasive nature of this species may be the result of an aggressive Eurasian strain. Under natural circumstances, phragmites does not pose a problem. However, it spreads aggressively in habitats disturbed from anthropogenic activities, such as pollution, dredging, increased sedimentation, and the manipulation of hydrologic regimes.

Phragmites dominates many habitats because it can withstand a wide range of conditions. Reproducing primarily through the growth of perennial rhizomes, this grass can attain a stem density of up to 200 stems/m². Dense stands form thick mats of rhizomes and roots that block sunlight, thereby inhibiting the germination of native seeds and consequently affecting the wildlife dependent upon the native plant community. Wind and birds disseminate phragmites seeds, but their survival rate is low in part because the seeds are unable to germinate in water over 5 cm deep. As with many invasive plants, there are no native herbivores to control phragmites.

(Photo by Donald J. Leopold).

Controlling invasive plants

Invasive plants can be controlled by a variety of methods, although cost and effectiveness vary. Hand-pulling is the least expensive procedure, but it is only feasible during the early stages of an infestation. Mechanical methods, including mowing and discing, are used for more established invasive plant populations. Timing is critical for the successful removal of prolific seed producers, such as purple loosestrife, and should occur before the plants set seed. Aquatic invasives can also be removed by hand-pulling or such mechanical methods as Rotovating and diver-operated suction harvesting. Any plants capable of vegetative reproduction must be completely removed; otherwise, plant fragments or rootstocks may develop into new plants, counterproductive to the goal.

Herbicides are also used to combat invasive plants, although foliar spraying may not be suitable in areas where native plants cohabit. Amitrole, dalapon, and Rodeo (glyphosate) have been successful against invasives such as phragmites, especially when a prescribed burning follows the chemical treatments. Aquatic plants such as Eurasian watermilfoil can be controlled with Aquathol-K or the systemic herbicide triethylamine salt triclopyr, although the success level depends on the herbicide concentration and the timing of the application.

The careful use of biological controls may also help to combat established invasive plants, including the introduction of both native and non-indigenous herbivores. Studies indicate that the herbivory by a native weevil, Euhrychiopsis lecontei, can detrimentally affect the growth of Eurasian watermilfoil. Moreover, the weevil does not appear to harm native plants. The risk in using biological controls such as herbivores is that once the target is eliminated, they may rely on native plants as their primary food source.

Invasive species are regarded as one of the greatest threats to natural environments and biodiversity. The replacement of a diverse plant community by a monotypic stand of an invasive species negatively affects the patterns of that habitat. The plants drastically alter food sources, nesting habitats, shelter, and hiding areas for numerous species of birds, mammals, amphibians, and myriad invertebrates.

The economic effect must be measured by changes in recreation, tourism, commercial fishing, and importantly, the high cost associated with wetland and aquatic conservation and restoration. To realize the goal of protecting and maintaining healthy native habitats and free-flowing water systems, invasive species must be addressed through research, education, and implementation. Vigilance is also necessary in the management of current infestations and the prevention of further introductions of potentially invasive species.
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Alison D. Halpern is a graduate student studying plant ecology, Cynthia A. Boesse completed her master's degree in plant ecology, Anne E. Altor is an undergraduate—all with the SUNY College of Environmental Science & Forestry, Department of Environmental and Forest Biology. Alison D. Halpern is corresponding author, phone 315-457-5832.

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