Fallopia japonica (Japanese Knotweed) flowers - Monatiquot River, Braintree, MA. |
Table of Contents:
Keywords: invasive plants, introduced plants, non-native species, plant introductions, gardening, botanical history, horticulture
Since Europeans began the exploration and colonization of America, plant species have enjoyed a greatly increased rate of introduction into new habitats. While most of these species are considered "benign", and do not spread beyond their designated areas, others have found their way out of the boundaries set by humans to exist on their own in the natural landscape. It is interesting and educational to look at the wide variety of ways in which plant species have been introduced to new habitats. This paper will review the many vectors of plant introduction, both accidental and intentional. Keep in mind that many species, especially the most invasive ones, tend to have multiple types and instances of introduction, and this paper will not be able to mention them all. While examples will be taken from all over the globe, an attempt will be made to focus on species well-known in the New England area. Because this review is of species that have in some way succeeded without the aid of humans, it will not touch on most of the non-native agricultural crops that we depend upon for food.
Plant introductions can fall into one or more of the following categories:
Species are often placed in more than one category because classification is done on a local, regional, or global basis, depending on the country (or countries) in which the introduction has occurred. While it has proven difficult to predict which introduced species will become invasive, we do know that the proportion of the total introduced flora that is naturalized or invasive is small (Forman in prep.).
Species that are naturalized or invasive can cause many ecological and economic problems. Over 25 billion dollars is spent annually in the United States to combat invasive plants (Pimentel et al. 1999). While a significant portion of this is used against agricultural weeds, this still means that millions of dollars are spent each year to control species invading our waterways or encroaching on conservation land.
Besides the high economic costs of invasive plant species, there are also ecological costs, though often they are a part of long-term processes that are difficult to spot. Non-native plants can displace native plant species, often forming monocultures where no other plant can grow. If they are displacing native plants, chances are they are also disrupting the ecosystem, removing sources of food or habitat for the local fauna. In some cases non-native plants actually replace sources for food or habitat, and this integration with the native flora and fauna makes management decisions much more complicated. Invasive plants can alter the hydrology of the ecosystem, or the fire regime. As these species enter our landscape and prosper, it becomes more difficult to enjoy nature-related recreational activities (hiking, biking, boating), because we must deal with clogged waterways, blocked paths, and most importantly, a reduction in the natural native beauty of the landscape.
Though species categorized as invasive tend to attract most of the attention, there are several reasons why we should be concerned about all non-native plant introductions. An introduced plant species that is not invasive now may have the potential to become invasive in the future, even if it has been benign for centuries. It has been well documented in the literature that introduced species often have a "lag time" during which they do not spread. This lag can last for long periods, during which additional introductions may occur, increasing the risk of invasion.
There have been cases where evolution has had a quick and drastic effect on a species’ ability to invade a new habitat. Trifolium hirtum (rose clover) was introduced to rangelands in California. The population was so small that inbreeding was inevitable, and this "genetic bottleneck" caused the evolution of individuals that had tiny new hooks on their seed pods. Suddenly a species once confined to rangeland was able to spread to bare patches of soil, and the escaped populations became naturalized.
Hybridization is another potential consequence of non-native plant introductions. Non-native species can hybridize with closely-related natives, leading to a loss of genetic and species diversity. This has occurred with Spartina alterniflora (smooth cord grass) in Britain, where this American species hybridized with the native S. maritima (small cordgrass) to produce a hybrid, S. x townsendii. The hybrid led to the development of a tetraploid, S. anglica (common cordgrass), that then outcompeted its parents, invading large stretches of British wetlands. Non-native plant species may also hybridize with other non-natives, possibly leading to the evolution of a stronger, more vigorous hybrid.
Accidental introductions are those which were not deliberately brought about by humans. They can be divided into four main categories: The Sneaky Ones, The Hitchhikers, The Waste Aliens, and The Natural Accidents.
Many plant species have evolved methods of dispersal that have made them quite successful at exploiting their relationships with humans. The term zoochory, where plant are dispersed by animals, takes on a special twisted meaning when these animals are then dispersed by people, whether across continents or across oceans. Species such as Cenchrus ciliaris (buffelgrass),
 The notorious wetland invasive Lythrum salicaria (Purple Loosestrife) |
Lythrum salicaria (purple loosestrife), and Amsinckia menziesii (fiddleneck) are all invasive plants that have spread in part by the ability of their seeds to cling to the fur of mammals or the feathers of birds.
In addition, plant propagules may often be hidden in what may be considered the foulest of places: animal feces. The same seeds that cling to the coats of grazing mammals are often also found in their droppings. These seeds can pass through the intestinal system undigested, to be deposited in a convenient packet of fertilizer. One of the best examples of a species that spreads in this manner is Euphorbia esula (leafy spurge), an invasive plant that is toxic to many grazers, and costs the northwestern United States over $100 million a year due to control measures and agricultural losses (see link for E. esula). Having such a close relationship with grazing animals would normally be effective only for plants with local, short-range seed dispersal, but when these animals are shipped from state to state or even overseas, the potential range of these introduced plants is expanded far beyond what would be considered natural.
New plant introductions can also occur through the introduction of contaminated soil or seed. When plants are imported from other countries or even brought from one coast to another, there is a risk of spreading non-native species that are present as seeds or rhizomes (underground stems). A visit to the woody perennials section of your local nursery will reveal many smaller, herbaceous species growing in the soil surrounding the root ball of these plants. Similarly, the plants grown in pots or flats of soil at greenhouses often have accidentally-introduced species growing right alongside them. Greenhouses have their own set of weeds growing under benches and in other empty spaces.
Contaminated seed is a problem that has been plaguing farmers for centuries. The process of harvesting crops and culling them to get pure seed has inadvertently led to the selection for the seeds of companion weeds that are most similar in size and shape to the crops they grow with. When the peoples of the Mediterranean migrated to Europe, they brought with them grain crops like wheat and oats, as well as the common weeds of these crops. The same weeds were also introduced to America with the colonists. Similarly, when European colonists made their way to the western United States, they brought with them the staples they needed for long-term survival. As they planted their crops, they also accidentally introduced many weed species, forever altering the ecosystem of the West. More recently, the process has been repeated when Israel, eager to develop land for agriculture, imported seeds from the United States for cotton and corn crops, and unintentionally introduced many American species, including Aster subulatus (saltmarsh aster) and the weedy grass Cenchrus incertus (coast sandbur).
Other products that can be contaminated with the seeds of non-native plants include animal feed (also see below) such as grain, or hay used for forage for grazing animals. Contaminated hay has been linked to the spread of many invasive plants, such as Centaurea maculosa (spotted knapweed), as well as Senecio jacobaea (tansy ragwort), and Solanum viarum (tropical soda apple), both of which cause significant problems, because they can be toxic to livestock.
Waste aliens fall into two general categories: those spread through standard waste products and those spread in waste from the textile industry. The simplest example is that of refuse aliens. It is important to consider whether you are truly disposing of a plant if you place it in the trash and send it off to the local dump. One can imagine the potential consequences if the plants in yard waste (pulled weeds as well as clippings) are capable of taking root once they arrive at the dump. A plastic or paper bag is not an effective barrier against plants that are capable of resprouting from roots or stems.
One also cannot overlook the potential that the seeds of our fruits and vegetables that have been discarded in our trash or garbage may be left to germinate in, again, what is probably a nice packet of fertilizer. In Britain, naturalists have been tracking this category of refuse aliens for many decades. Examples include Cucurbita maxima (winter squash), Phaseolus coccineus (runner bean), and Physalis peruviana (Peruvian groundcherry), which is now naturalized in parts of Europe.
Plants thought to be confined to a restricted area may also escape through another source: waste water. For example, dumping of aquarium water containing viable plant parts may have a much more dramatic consequence than the aquarist intended. Overflow of sewage systems, a frequent occurrence in many regions following significant rainfall, can lead to the introduction of aquarium plants into local bodies of water via storm drains. Invasive aquatic species such as Elodea canadensis (Canadian waterweed) and Myriophyllum spicatum (Eurasian water-milfoil) are examples of plants currently grown in hobbyists’ aquariums.
The best method for preventing introduction of refuse aliens into the natural environment is to practice composting. Whole plants or plant parts, especially fruits or vegetables with seeds, should not be disposed of in any manner that could lead to the species taking root in the wild. Proper composting ensures decay of plant material, and exposes this material to a high enough temperature to ensure that seeds or vegetation are no longer viable methods of reproduction.
The second category of waste aliens are those introduced and spread by industry. Some of these introductions are tied to long-abandoned industrial practices; unfortunately the damage has already been done. The wool industry has one of the strongest associations with non-native plant introductions, mainly due to the fact that wool waste, usually disposed of in waste places by factories, is full of the plant propagules that cling to the coats of sheep. Some of the most infamous invasive plants in the U.S. were associated at least in part with wool waste, including Centaurea maculosa (spotted knapweed) and Lythrum salicaria (purple loosestrife). The history of the use of wool waste, also known as "shoddy," as a fertilizer means that the associated weedy species were dispersed far away from the location of the buildings where the wool had been processed. Cotton waste was also applied in this manner, and probably aided the spread of Galinsoga parviflora (gallant soldier) .
The paper industry is also associated with the spread of introduced plant species. Specifically, the production of "esparto", a high-quality paper made with grass pulp, is associated with the introduction of many invasive American plant species into Europe. Species such as Amaranthus hybridus (slim amaranth) and Paspalum distichum (joint grass) are just two of the many seed contaminants found in esparto waste. However, it should be noted that these species have been introduced to new environments through many different vectors; there are no known invasive species introduced solely by esparto waste.
There are many other examples of textile waste contributing to the spread of non-native species, including the processing of cotton, the tanning of leather, and the production of coir-fibre (from coconuts) . The theme that ties all of these processes together is that when a textile industry is started in a new region using the tools and species of another region, it is inevitable that the weeds of the home region will also be along for the ride.
Flowers and fruits of Vincetoxicum nigrum (Cynanchum louiseae, Black Swallowwort), a common weed in suburban areas that is also spread along railways |
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On a smaller scale, boats, jet skis, and even canoes and kayaks can lead to the extension of range of fresh water aquatic species when plants that cling to these modes of transport are carried along to a new body of water. Often, the first known location of an aquatic plant in a body of water is at the harbor or dock where marine vehicles enter. As a result, some states are passing laws that require boat owners to remove all water and vegetation from their vehicles at a safe distance from water sources (Vermont, Minnesota, Maine).
Land vehicles are also common vectors for plant introductions, though on a more local scale. Seeds of many species stick to the tires of automobiles, off-road vehicles (ORVs), motorcycles and bicycles, often traveling long distances before they are re-released into the wild. Examples include Lepidium latifolium (perennial peppergrass) in North America, Matricaria matricarioides (pineapple weed) in Britain , and yes, Lythrum salicaria (purple loosestrife).
The railway system in the United States has been traced to the introduction of several plant species into new habitats. The methods are the same (mostly seeds sticking to the vehicles), but the range can be much larger because of the frequent long distance of railway travel. Euphorbia dentata (toothed spurge), native to the West Coast of the U.S., is an example of a species now being discovered along railways and subway tracks in New England. Other examples of invasive species whose spread was aided by railroads are Vincetoxicum nigrum (black swallowwort) and Galinsoga ciliata (hairy galinsoga).
Seed "hitchhikers" will even hitch rides on the most ubiquitous form of transport: the human. Technology does lend a hand, providing humans with the capability to travel across oceans in less than a day. Not counting mandatory disinfection to prevent the introduction of foot-and-mouth disease, how many of us can remember the last time we checked our clothing, especially our shoes, to make sure we were not accidentally introducing a non-native plant when we entered or exited a foreign country? Walking outside at any time of the year always carries a risk, however small, of picking up plant propagules in the cracks of shoes, the cuffs of pants, etc. Travelers who visit agricultural regions, national parks, or any natural land should take care to avoid becoming vectors themselves, even if they are not legally required to do so. Unfortunately, even legislation may not be enough for prevention. On Prince Edward Island, after the institution of a strict policy regarding inspection of both people and their transportation (see Weed Control Act - .pdf file), new invasive plants were still discovered.
Lest you are starting to feel guilty, there are actually examples of plant introductions that can occur without the aid of humans. These methods of introduction are rare, and there is little evidence for specific introductions, none of them invasive species.
On a regional scale, species can expand beyond their natural range when seeds travel in the wind or along waterways. Waterways also contribute to the spread of a species by spreading vegetative plant parts. On a global scale, natural disturbances caused by hurricanes and other weather events can cause birds to be blown off-course by the wind. If they land in a new habitat, any seeds expelled in their droppings could germinate and establish new non-native plant populations.
The natural flow of currents has also been linked to plant introductions. Probably the best known example of this is the "sea bean". Sea Bean is actually a generic term used to describe the seeds of species that travel on ocean currents and wash up on the shores of beaches far from the parent plant. There are several dozen species that bear sea beans, and they range from tropical vines (Caesalpinia bonduc, grey nickernut) to temperate forest trees (Juglans nigra, black walnut).
Although much attention is paid to accidental introductions, the majority of introduced species have been introduced intentionally. Species with several vectors of introduction are more likely than not to have had at least one purposeful introduction by humans. Therefore, the following review will include several of the most common invasive plant species. The focus will be on species that have a tendency to escape from cultivation or from the boundaries imposed on them by humans: The Ornamentals, The Consumables, The Animal-Related Introductions, The Plant Products, and The Necessities.
When Europeans first began arriving in America, gardening was not the same as the hobby it tends to be today. Gardening was a means of survival for homeowners, and only useful plant species were cultivated. The first introductions of truly ornamental plants were restricted to large botanical gardens and the estates of the rich, where professional and amateur botanists eagerly sought curiosities from abroad . Robinia pseudoacacia (black locust), native to the eastern U.S. but currently invasive in most of North America, was first introduced to Europe in 1700 by the botanist Jean Robin, who planted it in Paris at the estate of the King of France. A few decades later, his nephew planted it at the Jardin des Plantes, a Parisian botanical garden . Black locust is now naturalized or invasive in many regions of Europe.
Black locust is just one of the thousands of "botanical curiosities" introduced to Europe from America. Galinsoga parviflora (gallant soldier), an herbaceous annual that is native to South America, was first introduced to Great Britain via the Kew Botanical Gardens in the 1700s. It was known as a local "garden escape" in and around Kew until World War II, when the disturbance caused by bombings altered the dispersal pattern of the seed from short-range to long-range . Gallant soldier is now considered a weed in Great Britain, most of Europe, and in Alaska.
As botanists from Europe settled in America, they established their own gardens and nurseries. Men such as John Bartram, Bernard M’Mahon, and the Prince family set up trading networks with their counterparts across America and in Europe so that they would be sure to receive newly discovered species. Some examples of the European species and their many cultivars that botanists in America received are Aesculus hippocastanum (horse chestnut), and Digitalis purpurea (purple foxglove).
Fruits of Trapa natans (European Water Chestnut) line the trail at Great Meadows National Wildlife Refuge, Concord, MA |
Botanical gardens continue to keep collections of non-native species on display, both in Europe (see Ventnor Botanic Garden) and in the United States (see New York Botanical Garden), for either botanical or cultural interests. These collections often contain species that have escaped into the wild or have the potential to do so. In addition, there is also the possibility that the insect- and wind-pollinated species in botanical gardens could be contributing pollen to wild congeners and creating problems relative to hybridization.
By the 19th century, gardening was becoming less of a necessity and more of a hobby in both America and Europe. The search for new "material" for English gardens was the social trend, with new species from North, Central, and South America often featured in magazine articles (Leighton 1987). Species such as Yucca filamentosa (Adam’s needle), Impatiens capensis (jewelweed), and Thermopsis montana (mountain goldbanner) now naturalized throughout Europe, were introduced during the 1800's.
In America, the popularity of ornamental gardens also increased greatly during the 19th century. Garden species introduced during this period included Fallopia japonica (Japanese knotweed) , Lonicera japonica (Japanese honeysuckle), and yes, even Lythrum salicaria (purple loosestrife). The same nurseries that had been set up along the Atlantic coast of the U.S. to serve Europe also began to make money selling plants to Americans who were eager to set up their own gardens. Most gardeners, professional and amateur alike, did not realize that the same species marketed as having quick growth, proliferous flowering, or excellent coverage for unsightly areas were later going to be the ones on which we would be spending millions of dollars in an attempt to control them.
Eichhornia crassipes (Water Hyacinth), introduced from S. America as an ornamental pond plant, is now a serious invader in many parts of the world. |
Houseplants are members of a category of ornamental plants that is not usually considered to have the potential to escape cultivation. Tropical species meant to be grown indoors, not considered viable in temperate regions, still have the potential to become naturalized. Tradescantia fluminensis (small-leaf spiderwort), a South American species often cultivated as a houseplant, is commonly seen as a weed growing in greenhouses. It is now naturalized in Southern Europe as well as in Great Britain, and is categorized as an invasive species in Florida (see link above).
In addition, houseplants are often planted in containers outside in warmer climates, or placed outside for the summer months in more temperate zones. Ignored or abandoned plantings have the potential to spread either vegetatively or by seed. There is an additional danger of new individuals arising from discarded cuttings of houseplants, which has purportedly occurred in the Pacific Islands with the Tropical American native Dieffenbachia maculata (dumbcane). If global warming trends continue, it is expected that tropical species such as this will eventually be able to routinely survive the harsher winters of the northern temperate zones.
Finally, the use of ornamental species in dried flower arrangements can lead to the spread of non-native species if the dried parts include viable seeds. Bottlebrush grass, Elymus hystrix, is an American species sometimes used in flower arrangements in Europe, where it is occasionally found growing in the wild. Celastrus orbiculatus (Oriental bittersweet), an aggressive Asian vine that is potentially wiping out the related American native C. scandens (American bittersweet), is used in floral arrangements because of its attractive dried fruits. Seeds from these fruits can be spread when the waste from arrangements of Oriental bittersweet is discarded.
Instances of accidental plant introductions through refuse material have already been addressed. However, there are also instances where the cultivation of species for food or spices can lead to garden or agricultural escapes. Abandoned garden patches, orchards, and agricultural fields can continue to harbor non-native species grown for food; any of these casual plants has the potential to become naturalized. Persea americana, the avocado, is a Central American species now considered invasive in Micronesia, and is a casual escape in Europe as well.
There are dozens of other examples of how our food choices have come back to haunt us. Alliaria petiolata (garlic mustard), is a well-known invasive species introduced to America more than 150 years ago as a pot herb. The sunflower (Helianthus annuus), grown for agricultural uses as well as in family gardens for its ornamental beauty and seed, has become a noxious weed in America, and is also very invasive abroad. The sunflower is often cultivated specifically for its massive seed heads, and it is easy to imagine how growing fields of sunflowers would lead to some seeds being scattered and then germinating in the wild. In the same genus, H. tuberosus (Jerusalem artichoke), introduced to Europe in the 1600's, was once widely grown as a food plant for its starchy tubers. While it is still in cultivation, it has also become one of the most invasive plant species on the European continent.
Many of the herbs we cultivate for use in cooking are from the mint family (Lamiaceae). These species, which are known for the ease of growing new individuals from cuttings, are also prone to aggressive vegetative spread on their own. Mentha spicata (spearmint), M. x piperita (peppermint), and Thymus serpyllum (creeping thyme) are all species in the spreading mint category; all are now invasive in the United States.
Under the category of "consumables", we must also consider plants used for medicinal purposes. In the 19th century, when botanical gardens were amassing their various plant curiosities, there were also physic gardens set up throughout Europe to collect and grow any species with putative medicinal properties . Many American species were collected, from the benign Polygala senega (Senega snakeroot) and Panax quinquefolius (American ginseng), to the invasive Grindelia squarrosa (curlycup gumweed) and Datura stramonium (jimsonweed). Invasive plants introduced to America for medicinal purposes include many species in the Mint family, such as Mentha pulegium (pennyroyal) and the species introduced as herbs. The periwinkles (Vinca major, V. minor) were first introduced to the U.S. for medicinal purposes, but now have more widespread use as ground cover.
Xanthium strumarium (Rough Cockleburr), a common contaminant of wild bird seed |
Grazing animals are not the only creatures that need food. In order to fuel honey production, beekeepers seek out and plant species which produce large numbers of flowers. Species such as Lythrum salicaria (purple loosestrife) and Fallopia japonica (Japanese knotweed) may be important players in the game of honey production. American plants used as bee forage species that are now naturalized in Europe include Phacelia tanacetifolia (lacy phacelia).
Perhaps the most common animal-related products that cause new plant introductions are bird seeds. Commercial bird seed marketed towards pet owners contains many non-native species. As bird owners know, large amounts of seed are thrown away intact simply because the bird will not eat it. Naturalists in Britain tracked "bird seed aliens" for the past several decades. They found several dozen species that either grew unaided in garbage dumps, or would germinate under laboratory conditions. Many bird seed aliens easily germinate if exposed to water, even while still in the bird cage. Some of the better-known bird seed aliens include Panicum miliaceum (proso millet), Setaria italica (foxtail millet), and Helianthus annuus (sunflower), all common seed contaminants themselves. One common bird seed component that has recently come under increased scrutiny is Cannabis sativa (hemp); although the seed is supposed to be sterilized before it is included in mixtures to be sold in America, this species is still a noxious weed in many states.Bird seed for pet birds such as parakeets, cockatiels and canaries is subject to much higher quality control than wild bird seed. Often the waste products (weeds seeds, other plant parts, foreign objects) removed from pet bird seed is added to the cheaper, wild bird seed mixtures. Because wild bird food is more likely to be scattered outside and exposed to natural areas, the risk of non-native plant establishment becomes stronger. Companion weeds to low-growing species like millets are more difficult to separate from the harvesting process and are more likely to be a seed contaminant in pet bird food . Examples of conspicuous species which often end up in wild bird seed are Xanthium strumarium (rough cockleburr) and X. spinosum (spiny cockleburr, also known as "shepherd’s plague"), two very weedy species that are causing problems all over America and Europe.
Seed mixtures for other small animals (rabbits, hamsters, mice, etc.) have similar problems. In addition, the food fed to large pond fish such as koi can include mixed seed similar to that of wild bird food. In Israel, studies have shown that American species such as Amaranthus rudis (tall amaranth) and Sida acuta (common wireweed), which are characterized as invasive in many regions, are frequently found adjacent to fish ponds .
Certain plant species have had a long hunter-sustained association with birds and other animals. While the sport of hunting does allow for the protection of vast amount of land from development, the management of this land can have unintended consequences.
Phytolacca americana (American Pokeweed), planted as game cover in Europe, is now naturalized there |
Hundreds of plants fall into the category of those we process, rather than those we consume whole. Cultivating these species has led to several of them becoming naturalized. A primary example is the need for timber. One of the main reasons why European explorers were sent to America many centuries ago was to bring back fast-growing forest trees to replenish depleted forests in France and Britain. Some of the species sent back as seeds or saplings were Prunus serotina (black cherry) and Fraxinus pennsylvanica (green ash), two species now known for their colonizing ability, and consequently their invasiveness, in America. Black cherry is now invasive in many parts of Europe. Many American species in the pine family (Pinaceae) have become naturalized in Europe, having spread from the forests where they are being grown for timber. The list includes Picea sitchensis (Sitka spruce), Pinus muricata (bishop pine), and P. radiata (Monterey pine), which is invasive in America as well.
The remaining examples in the "plant products" category have a wide variety of uses. Mimulus moschatus (musk flower), benign in its native America but invasive in Europe, was originally imported there to make perfume from the flowers . One of the main reasons that the species Opuntia ficus-indica (prickly-pear cactus) spread so rapidly in Europe after it was introduced in the 16th century, was that it had been planted in large populations in order to breed the scale insect which was the source for the dye known as Cochineal or Carmine Red . Other invasive plants introduced for their uses in dye-making include Phytolacca americana (American pokeweed), which has berries that produce a purplish color, and Caesalpinia spinosa (tara), which has seeds that are used in the tanning process.
We have found many uses for non-native plants throughout the world besides consuming them. Again, widespread planting of different species has led to several problems of invasion.
Planting projects, whether a local grassroots effort or sponsored by the state or federal government, have introduced many invasive species to America. Trees chosen to be planted along streets and in parks as ornamentals, or to provide shelter or protection against wind, are often selected based on their tolerance of pollution and disturbance. In both urban or suburban areas, such hardiness is precisely the quality that allows the non-native ornamental species to outcompete native species as they spread into neighboring green spaces. For example, Ailanthus altissima (tree of heaven), lauded for its ability to grow quickly just about anywhere, is now an incredibly common site in waste places, along highways and railroads across America. Introduced to Philadelphia in 1784 , this Asian native has now spread to less disturbed areas, including old-growth forests. Acer platanoides (Norway maple), introduced to America as a street tree, has also made its way into forests, displacing native species. On the west coast of the U.S., Eucalyptus species such as E. globulus (Tasmanian blue gum), planted across California for ornament and as windbreaks, can cause serious problems by causing American birds, which tend to have shorter beaks than their Australian counterparts, to suffocate when they are seeking nectar from a eucalyptus flower.
Fallopia japonica (Japanese Knotweed) fits right in with the Staghorn Sumac along the Monatiquot River, Braintree, MA. |
Finally, there is a list of species, all recommended at one time or another for erosion control, that number among the most invasive plants in America. Fallopia japonica (Japanese knotweed), Elaeagnus angustifolia (Russian olive), E. umbellata (autumn olive), and Pueraria lobata (kudzu) have all been planted in areas to prevent soil loss and degradation of the landscape. As recently as the early 1990's, Elaeagnus species, native to Asia, were recommended for site restoration by the U.S. Army Corps of Engineers. This is surprising because it was already known that efficient dispersal of the fruits by birds and the plants' continuous resprouting ability had enabled Elaeagnus species to become one of the most numerous invaders in the United States .
If one were to compare only the number of non-native plant species introduced to a region with the number of those species that are currently invasive, it would be easy to shrug off the potential dangers of blindly introducing species and declare the risk slight. In truth, only a very small percentage of non-native plants ever become invasive, but it is necessary to consider the significant damage that can be done by even one of those species. A purple loosestrife or a kudzu can cost millions of dollars in control efforts, and can have devastating ecological effects on native animals and vegetation.
To prevent further damage, non-native plants that are not yet considered invasive should be evaluated for invasive potential. High-risk species should be banned from further introductions. Educational programs should be implemented to encourage grassroots eradication efforts. Economic incentives could be used to encourage the horticultural industry to focus more on the cultivation and sale of native species, and to ease the transition from growing invasives that provide a significant income source. Most important, educational programs touting the benefits of planting native would be useful in convincing gardeners and landscapers who currently purchase non-native species. Rather than making the invasive species issue into a lose/lose situation, let us instead consider the benefits of taking back our distinctive, local heritage, and nurturing appreciation of our native flora.
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