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Purple Loosestrife Essay, Research Paper

Purple Loosestrife

The scene is breathtakingly beautiful, a thick brush of purple flowers blankets

Canada’s wetlands. This blanket silences the expected sounds of the wetland

environment, birds chirping, ducks splashing, insects buzzing and animals

thriving. This unnatural silence is disturbing, the favourite flowers that used

to litter this landscape are no longer visible, the water that used to ripple

continuously is perfectly still. The wetland is dead, except for this

overpowering, hardy purple flower that has choked out all other vegetation and

species. Purple loosestrife now controls this landscape.

Purple loosestrife is an exotic species that was introduced to North America

from Europe during the early 1800’s. Europeans sailing to North America would

fill their ships ballast with wet sand taken from shores of Europe, a habitat

where purple loosestrife thrived. Upon arrival in North America the ballast

would be dumped overboard on the shoreline. By 1830 the plant was well

established along the New England seaboard. Purple loosestrife seeds were also

found in sheep and livestock feed that was imported from Europe during this

period. This new organism was introduced to a new habitat free from traditional

parasites, predators and competitors, purple loosestrife thrived in the

environmental conditions and by 1880 was rapidly spreading north and west

through the canal and marine routes. Purple loosestrife stands also increased

due to the importation of seeds and root stalks by horticulturists. It was

introduced to many communities as an herb, an ornamental garden flower and as a

desirable honey plant.

One of the earliest reported studies of purple loosestrife being a problem in

Canada was documented by Mr. Louis – Marie, in 1944. He stated that purple

loosestrife was invading the St. Lawrence flood plain pastures between Montreal

and Quebec. At that time Louis – Marie conducted a study to find suitable

control methods for purple loosestrife. His results indicated that repeated

mowing, continuous grazing, deep discing and harrowing were effective in keeping

the spread of purple loosestrife controlled on agriculture land. Since the

1940’s purple loosestrife infestations have increased greatly and the plant is

now a major problem threatening many wetland ecosystems across North America.

Figure 1 – Purple loosestrife flowers.

(Parker 1993)

Lythrum Salicaria, commonly known as purple loosestrife belongs to the

Lythraceae family, which consists of 25 genera and 550 species worldwide. The

genus Lythrum consists of thirty – five species, two of which are located in

North America, Lythrum Purish which is native to the continent and the invasive

purple loosestrife. Through cross breeding, purple loosestrife is quickly

overtaking Lythrum Purish and causing a decrease in native species. “The

generic name comes from the Greek luthrum, blood, possibly in reference to the

colour of the flowers or to one of it’s herbal uses, as an astringent to stop

the flow of blood.” (Canadian Wildlife Federation 1993, 38) Purple loosestrife,

an aggressive, competitive, invasive weed often grows to the height of a human

and when it is mature can be 1.5 metres in width. The stalk of the plant is

square and woody and may grow to 50 centimeters in diameter. The perennial

rootstock can give rise to 50 stems annually which produce smooth edged leaves

on oppositesides of the stalk. Purple loosestrife flowers are long pink and

purple spikes which bloom from June to September (Figure 1). One purple

loosestrife plant alone is solid and hardy but when this plant invades an area

it creates a “dense, impermeable stands which

Figure 2 – Purple loosestrife growing in a typical habitat.

(Parker 1993)

are unsuitable as cover, food or resting sites for a wide range of native

wetland animals…” (Michigan Department of Natural Resources 1994). Due to the

lack of predators which feed upon purple loosestrife, this dominant plant has an

advantage when competing against most other native wetland species for food

sunlight and space. These advantages allow purple loosestrife to create dense,

monotypic stands which reduce the size and diversity of native plant populations.

Purple loosestrife can also grow on a range of substrates and under nutrient

deficit conditions. It has the ability to regenerate quickly after cutting or

damage and can withstand flooding once adult plants have been established. There

are no native species that are as hardy as purple loosestrife, therefore without

competition and predators the wetland ecosystem cannot control the spread of

purple loosestrife.

Purple loosestrife is now found world wide in wet, marshy places, coastal areas,

ditches and stream banks. (See Figure 2) It is prevalent in most of Europe and

Asia, the former USSR, the Middle East, North Africa, Tasmania, Australia and

North America. It has not been found in cold Arctic regions. In North America

purple loosestrife is located between the Canadian territories and north of the

35th parallel with the exception of Montana. The most serious infestations are

found in the wetlands of Southern Quebec and Ontario and along the Red and

Assiniboine Rivers in Manitoba. Second to Manitoba, British Columbia has the

next largest purple loosestrife infestation, weed populations are reported from

Vancouver Island to the lower Fraser and Okanogan Rivers south of Penticton. In

Saskatchewan and Alberta. small, isolated stands of purple loosestrife are

reported and the Atlantic Provinces are quickly being invaded. Currently areas

that are sensitive to the new invasions are the salt and freshwater marshes in

the Maritimes. (See Figure 3)

Figure 4 – Purple loosestrife seedling.

(Parker 1993)

Regardless of the purple loosestrife location, one of the main reasons for the

rapid infestations is due to the plants prolific seed production and

reproduction cycle. “It has been estimated that a mature plant can produce 2.7

million seeds per growing season” (DeClerck-Float 1992,15). Purple loosestrife

seeds are small and easily transported by water or by mud that attaches to the

feet of birds or off road vehicles. The seeds remain dormant over the winter and

germinate in late spring or early summer. They are capable of germinating in

either the mud or when submerged under water providing the water temperature is

between 15 – 20 oC and there are adequate light levels (See Figure 4). Through

experiments performed by S.R.A. Shamsi and F.H. Whitehead, it has been

determined that “prolonged seed dormancy may be possible, since seeds stored for

three years in a refrigerator at 3 – 4 oC were still useable and had a

germination rate of 80% ” (DeClerck-Float 1992,15). The production of purple

loosestrife seeds and their exclusive characteristics allow the plants to

develop large seed banks at a site which is a factor that makes purple

loosestrife so difficult to control. The plant has the ability to reproduce from

the seed bank. Purple loosestrife can also spread vegetatively by adventitious

shoots and roots from clipped or tramped plants. Any part of the plant that

falls to the ground, even from a wheelbarrow, can develop into a plant. This

shows the plant’s desire to live no matter what obstacles it faces, again making

it very difficult to control. Purple loosestrife plants have three style lengths

(short, mid, long) and three strengths (short, mid, long). Pollination occurs

between plants with the same style and stamen length. Purple loosestrife flowers

have of one style length and the two sets of stamens are different lengths,

therefore a plant is technically self – incompatible. “However, Ottenbriet

(1991), found that the self – incompatibility system is not strict, as mid -

styled plants showed a highdegree of self fertility with themselves and other

mid – styled plants.” (DeClerck – Float 1992, 16) This proves, it is not safe to

plant self – incompatible purple loosestrife, there is a risk of pollination

which will lead to further distribution of the plant. This misconception is a

problem because nurseries are selling self – incompatible plants as garden

flowers which reproduce with themselves or with other species from the

loosestrife family creating more invasive stands.

Purple loosestrife’s hardy, competitive and reproductive characteristics

classifies it as a large environmental concern. The plant is threatening wet

lands, decreasing water foul population, clogging irrigation systems and

becoming a threat to the fisheries. “Mosquin and Whiting (1992) regard purple

loosestrife to be one of the five invasive alien plants that have had a major

impact on natural ecosystems in Canada.” (Canadian Wildlife Federation 1993,41)

Canadian wetlands are rapidly being over taken by purple loosestrife, large

stands of the plant displace native species that can’t compete against this

exotic species. The loss of native flora and fauna means the loss of habitat and

food for wetland animals, this destroys the well balanced, wetland ecosystem.

Across the Maritimes, prarie sloughs are becoming increasingly infested with

purple loosestrife thus destroying the breeding ground of many North American

waterfowl. This additional stress compiled with urbanization and pollution could

cause theextinction of North America’s waterfowl population. The invasion of

purple loosestrife across the Maritimes is causing extra labor for farmers as

well as an increased cost because the plants are clogging the irrigation systems.

In B.C. purple loosestrife is invading the salt water shores and is becoming a

threat to the fisheries. The overpowering stands of purple loosestrife are

increasing costs and frustrations for many industries across Canada.

On the contrary, bee keepers and horticulturists have found economic uses for

purple loosestrife. Bee keepers favour purple loosestrife because the plant

forms dense stands and large quantities of pollen in July and August. Purple

loosestrife is one of the few plants producing large amounts of nectar during

the late summer. The downfall to purple loosestrife honey is that it is ill

tasting and greenish, although this can be diluted by the good nectar from other

flowers. Canadian bee keepers do not want purple loosestrife to spread for fear

of losing the nectar from the good flowers but they also don’t want to lose the

large quantities of nectar obtained from purple loosestrife. Horticulturists

favour purple loosestrife as a garden perennial in the prarie provinces. It is

favoured because it’s both showy and hardy and able to withstand the fluctuating

climate. Horticulturists are finally realizing that the pros of purple

loosestrife as a garden perennial are far outweighed by the cons of purple

loosestrife as an exotic invader.

The most pressing question with regards to purple loosestrife right now is, how

can we control it? Studies have been conducted since 1941 with the aim of

finding effective control processes – one has still not been found. To gain

control over purple loosestrife and to reduce it’s impact on the environment

three goals that must be attained: 1) Eliminate the species from highly

significant sites where a low infestation is present. 2) Eliminate the species

in geographical areas where it is just beginning to establish itself. 3) Contain

the plant in large sites in order to slow down it’s spread. By achieving these

goals the impact of purple loosestrife across Canada will be stabilized until an

effective biological control agent is found. (Canadian Wildlife Federation 1993,


There are three forms of control used on plant species, cultural control,

chemical control and biological control. Cultural control involves manual labor

such as mowing, cultivating, inundation, hand pulling, shearing, fire and

flooding. Each method is moderately successful depending on the specific

situation. Mowing, cultivating and inundation are not suitable control

mechanisms for purple loosestrife in many natural areas because by destroying

the exotic plant you also kill the struggling native species. In private areas

which are overrun with purple loosestrife these methods will reduce the spread

of seeds but will not kill the plants and therefore they will return the

following year. Hand pulling and shearing are only suitable for very limited

infestations due to their labor intensive nature. For these methods to be

effective all roots, stems, leaves and flowers must be removed and destroyed.

Fire has proven to be an ineffective method of control because the purple

loosestrife root crown iswell protected below the surface, the hot fire that is

necessary to kill the crown cannot be created. Flooding as a method of control

has proven redundant against mature plants. Adult purple loosestrife plants can

survive in water levels of 90cm. Flooding does however affect immature plants

but the water levels must be extremely high and it appears to take several years

to have an appreciable affect in the reduction. Unfortunately flooding will

also have a serious effect on native flora and fauna. Cultural control is both

labor intensive and not very productive.

Chemical controls for purple loosestrife have been tested in both Canada and the

USA but no herbicides have been accepted for use in Canada. In the USA, Rodeo,

See 2 and 4-D have been registered for use but there is limited benefit compared

to the high cost and temporary effectiveness. Canada has been testing Triclopyr

amine, which is a broad leaf herbicide, that can be used for control of purple

loosestrife. Researchers feel that it is an effective and safe product that can

be used to keep purple loosestrife in check. The largest problem when using

chemical controls is insuring that the effects of the herbicide will not

negatively effect the native species as well as purple loosestrife.

The final method of purple loosestrife control and the most promising for the

future, is biological control. This involves the introduction and management of

selected natural enemies of purple loosestrife. It is a slow process and is not

always efficient depending on the circumstances. The results are often long term

and the infested sites must be monitored for several years. Biological control

agents affect weed population indirectly by increasing the stress on the weeds

which may reduce their ability to complete with the native plants. Biological

control of purple loosestrife was initially investigated by the International

Institute of Biological Control (IIBC) in Europe. The USA contracted the

institute to conduct a study of possible biological control agents that could be

used to control purple loosestrife. (Canadian Wildlife Federation 1993, 42) As a

result of this study three insects were approved for release in the USA in June

of 1992 and at this time the insects were also released into field trials in

Canada. These three insects are Hylobius Transversouittatus, Galerucella

Calmariensis and Galerucella Pusilla.

The Hylobius Transversouittatus is a root feeding weevil that is a parasite of

purple loosestrife. The climate in Europe, which is native to this insect is

very similar to the Canadian climate thus making it easy for the weevil to adapt.

The H. Transversouittatus larvae mine the roots and change the vascular system

which reduces seed production and germination. The adult weevils emerge in May

or June and begin laying their eggs in the roots. The females continue laying

their eggs until September thus covering 2/3 of the growing season. Over a

period of time the effect of the weevil will drastically reduce the purple

loosestrife stand. “The damage caused by the feeding of seven larvae per plant

was found to reduce seed germination by 50%.” (DeClerk – Float 1992, 10)

Similar to purple loosestrife, the H. Transversouittatus is easily adapted and

can withstand prolonged periods of flooding. The larvae do not feed off the

roots when the water levels are high, they go into diapause until the roots dry

out then they resume feeding. This weevil has only one natural enemy, the

Mymarid egg but this enemy is not parasitic and has little impact on the

population. H. Transversouittatus has been tested and results show that the

insect will not have an impact on native species growing in Canada but will have

a large impact on purple loosestrife. Feeding by the insects in high densities

causes defoliation in mature plants, kills seedlings and destroying or

preventing the formation of flower spikes. H. Transversouittatus appears to be a

very likely candidate as a biological control agent for purple loosestrife but

several years of trials will be necessary to determine it’s effectiveness. It

could take up to ten years to show it’s full potential.

Galerucella Calmariensis and Galerucella Pusilla can be classified together

because they are both leaf feeding beetles that have similar life histories,

occupy the same habitat and affect purple loosestrife in the same manner. These

two species are often found together in Northern Europe with one of the species

dominating destruction of the stand. G. Calmariensis extends farther north than

G. Pusilla and will be better suited for Canada’s northern sites of purple

loosestrife. Both species are parasites which have good host finding

capabilities. Females will move from one host to the next, once a certain level

of feeding damage has been reached, this guarantees the spread of the attack in

large purple loosestrife stands. After being put through the same tests as H.

Transversouitatus, Galerucella Calmariensis and Galerucella Pusilla were found

to be extremely host specific and do not pose a threat to native species in

Canada. In Europe these beetles are more commonly found than H.

Transversouitatus.All three of these insects appear to be very promising in

their control over purple loosestrife stands but, as mentioned earlier, it could

take a few years to notice any progress. The idea of introducing another species

to Canada’s wetland ecosystem is not approved by all due to the purple

loosestrife infestation incident. Many believe that tampering with nature is

what has caused the problems in the first place and hopefully by letting nature

run it’s course all will turn out for the best . Unfortunately this viewpoint

can not be supported for long. Canada is at a point right now that without the

biological control agents, purple loosestrfie will destroy a lot of wetland and

farmland. With biological control we can only hope that the ecosystems can be

brought back under control.

Purple loosestrife is a very serious problem. It’s rapid invasion is threatening

wetlands, waterfowl and fisheries as well as the diversity of Canada’s flora and

fauna. If this plant is not brought under control quickly then the result of

this exotic species being brought to Canada could be disastrous. The use of

cultural and chemical control has not been effective so we now rely on the

success of biological control to stop the spread of this hardy invasive plant

and to replenish the diversity of Canada’s wetland ecosystem. As a country we

must do everything we can to reduce the spread and growth of purple loosestrife.

As a concerned Canadian you can report any local purple loosestrife stands,

spread your knowledge about the problem, strongly discourage the plantings of

any new plants or the selling of the weed in nurseries and join the Ontario

Federation of Anglers and Hunters. By doing this you are donating money and

support the tests that are being conducted. We must work together to remove the

purple blanket that silences our wetlands.


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