Diffuse knapweed
Encyclopedia
Diffuse Knapweed also known as White Knapweed or Tumble Knapweed, is a member of the genus Centaurea
in the family Asteraceae
. It is native to Asia Minor
(Turkey
, Syria
), the Balkans
, (Bulgaria
, Greece
, Romania
), Ukraine
, and southern Russia
. Diffuse knapweed is an annual
or biennial plant
, generally growing to between 10 and 60 cm in height. It has a highly branched stem and a large taproot
, as well as a basal rosette of leaves
with smaller leaves alternating on the upright stems. Flower
s are usually white or pink and grow out of urn-shaped heads
carried at the tips of the many branches. Diffuse knapweed often assumes a short rosette form for one year, reaching maximum size, then rapidly growing and flowering during the second year. A single plant can produce approximately 18,000 seeds
.
in some parts of North America
, having established itself in many areas of the continent. C. diffusa was first identified from North America in 1907 when it was found in an alfalfa
field in Washington state. The seeds had presumably been transported in an impure alfalfa seed shipment coming from somewhere in the species native range. Now present in at least 19 states in the United States
, it has naturalized in all contiguous states west of the Rockies
and additionally in Connecticut
, Massachusetts
, and New Jersey
. Portions of western Canada
have also been colonized by this plant.
Areas in which diffuse knapweed has been established generally are plains rangelands or forest benchlands. Land that has been recently disturbed—by human or natural processes—is favored for the establishment of diffuse knapweed. It grows in semi-arid and arid environments and seems to favor light, dry, porous soil
s. Areas with large amounts of shade or high levels of water discourage diffuse knapweed growth.
Dispersion occurs in the following ways:
Wind is the primary means by which diffuse knapweed seeds are spread.
, as its thistles can damage the mouth and digestive tract of animals that attempt to feed on it. A study in 1973 concluded that ranches lost approximately US$
20/km² (8 cents per acre) of diffuse knapweed due to decreased grazing area. In an agricultural setting, it can greatly reduce crop yield and purity.
and Urophora quadrifasciata, the root boring beetle, Sphenoptera jugoslavica, and the weevil Larinus minutus
. Research based on simulation models have shown that for biocontrol agents to be effective, they must kill their host, otherwise plants can compensate by having increased seedling survival.
Some of the more commonly utilized biocontrol agents are:
s to control diffuse knapweed. The herbicide Tordon (picloram) is recognized as the most effective, but it is common to use multiple herbicides in order to reduce strain on local grasses. The herbicides 2,4-D, dicamba, and glyphosate are also effective for control. In order to be most effective, it must be applied before the knapweed plants have released their seeds, regardless of which herbicide is used. Ongoing research at the University of Colorado suggests that Tordon treatment does not contribute to long-term reductions of exotic species cover and may contribute to recruitment of other invasive species, such as redstem filaree and Japanese brome, which quickly take the place of herbicide-treated diffuse knapweed.
Diffuse knapweed is known to establish more easily and effectively in recently disturbed environments. Disturbed environments generally present low environmental stress because more resources are available than are being used. These available resources often allow the establishment of an invasion in an ecological community. The concentration of diffuse knapweed in such an area is often linked to the level of soil disturbance. Human disturbances often lead to less species diversity
in a community. In turn, less species diversity can lead to unused resources, which allow invasive species to more readily establish. Areas such as fallow land, ditches, rangelands, residential and industrial districts and roadsides are all disturbed habitats where diffuse knapweed frequently establishes. Additionally, the removal of foliage and other ground cover increases the likelihood that seeds will come in contact with the soil and germinate.
The largest impact of humans on diffuse knapweed is certainly due to our efforts in controlling and eradicating its invasive populations. The several methods outlined in the control section represent a small sample of literally hundreds of approaches being tried with varying levels of effectiveness. Besides reducing the spread of diffuse knapweed, we are also providing selective pressure against the individuals that cannot withstand a certain method of control. Selective pressure, given sufficient time, can cause the adaptation
or evolution
of invasive species such as diffuse knapweed. If an individual diffuse knapweed plant survives control efforts because of a trait it possesses, its progeny will make up a greater portion of the population than the plants that succumbed to the control.
, ERH and superior resource competition. However, the most importance must be attributed to the ERH because diffuse knapweed, while a very effective invasive species in its novel environment, is non-invasive and doesn’t establish monocultures in its native range. It is the differences, biotic
and abiotic, between its novel and native surroundings that cause it to be invasive.
To demonstrate that the ERH applies to diffuse knapweed, it is essential to show that the absence of natural enemies has a significant positive effect on its success. One way to show this is to observe the effect of introducing some of diffuse knapweed’s natural enemies into its novel environment. If diffuse knapweed, which generally thrives in its invaded environment, is significantly inhibited through the introduction of natural enemies, it can be concluded that diffuse knapweed is more competitive in the absence of its natural enemies. A recent effort at biocontrol of diffuse knapweed in Idaho’s Camas County
effectively reduced 80 square kilometres (19,768.4 acre) of knapweed to minimal levels through the release of the lesser knapweed flower weevil and the knapweed root weevil. Since both of the insects released are natural competitors of diffuse knapweed, and since this and other similar efforts at biocontrol
have been successful, there is significant evidence that diffuse knapweed benefits from the absence of its natural enemies.
Another aspect of diffuse knapweed’s success relies on the effect of its allelopathic chemicals in its novel environment. Although there is still debate concerning the effectiveness of allelopathic chemicals in the field, the evidence of allelopathic effects demonstrated in a laboratory setting and its propensity to establish monocultures support the importance of allelopathy to diffuse knapweed’s success.
Curiously, diffuse knapweed’s allelopathic chemicals were shown to have a deleterious effect on the North American competitors but were beneficial to its native competitors. While diffuse knapweed’s native competitors are able to compete more effectively in the presence of allelopathic chemicals, the novel competitor’s fitness is decreased. This situation provides an example of the effectiveness of the allelopathy mechanism benefiting from the ERH. The increased effectiveness of allelopathic chemicals cause diffuse knapweed to experience less competitive pressure. As a result, diffuse knapweed is able to establish more predominantly in this new area.
Another connection between allelopathy and the ERH is the fact that concentrations of allelopathic chemicals were found to increase when diffuse knapweed was planted in North American soil as opposed to Eurasia
n soil. This effect is probably due to the absence of unfavorable soil conditions or soil microorganisms that exist in its native environment. As a result, the allelopathic chemicals will be able to reach higher concentrations, spread farther and therefore be more effective. By effecting more neighboring plants, the favorable changes in soil condition contribute to the success of diffuse knapweed.
Besides the advantages that diffuse knapweed gains from the ERH and allelopathy, it also possesses several characteristically invasive traits. One factor leading to the superior resource competition of diffuse knapweed is its ability to exist in drought conditions. This advantage allows diffuse knapweed to devote its resources to competition while its neighbors are conserving resources to survive. The high number of seeds produced by diffuse knapweed is also a common trait of invasive plants. A higher density of knapweed will not only increase the concentration of allelopathic chemicals in the soil but will also restrict the nutrients available to native plants. Unfortunately, very little research has been conducted to determine the relative competitive ability between diffuse knapweed and its novel competitors. However, tests conducted on the effect of diffuse knapweed on North American grasses in the absence on allelopathic chemicals demonstrated that the fitness of these grasses declined in the presence of diffuse knapweed. Regrettably, we cannot decide if diffuse knapweed is, for general purposes, a better competitor from this data alone. Comparisons of the deleterious effects between these and other pairs of competitors to arrive at a conclusion.
Diffuse knapweed is successful in its novel range primarily because the organisms and conditions that prevent it from becoming invasive in its native environment are absent. It follows that the introduction of species from its native habitat would be an effective method of control. However, the introduction of a non-native organism has the potential to result in another invasive species outbreak. Therefore, any method of biological control must be preceded by analysis of possible effects.
Centaurea
Centaurea is a genus of between 350 and 600 species of herbaceous thistle-like flowering plants in the family Asteraceae. Members of the genus are found only north of the equator, mostly in the Eastern Hemisphere; the Middle East and surrounding regions are particularly species-rich...
in the family Asteraceae
Asteraceae
The Asteraceae or Compositae , is an exceedingly large and widespread family of vascular plants. The group has more than 22,750 currently accepted species, spread across 1620 genera and 12 subfamilies...
. It is native to Asia Minor
Asia Minor
Asia Minor is a geographical location at the westernmost protrusion of Asia, also called Anatolia, and corresponds to the western two thirds of the Asian part of Turkey...
(Turkey
Turkey
Turkey , known officially as the Republic of Turkey , is a Eurasian country located in Western Asia and in East Thrace in Southeastern Europe...
, Syria
Syria
Syria , officially the Syrian Arab Republic , is a country in Western Asia, bordering Lebanon and the Mediterranean Sea to the West, Turkey to the north, Iraq to the east, Jordan to the south, and Israel to the southwest....
), the Balkans
Balkans
The Balkans is a geopolitical and cultural region of southeastern Europe...
, (Bulgaria
Bulgaria
Bulgaria , officially the Republic of Bulgaria , is a parliamentary democracy within a unitary constitutional republic in Southeast Europe. The country borders Romania to the north, Serbia and Macedonia to the west, Greece and Turkey to the south, as well as the Black Sea to the east...
, Greece
Greece
Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....
, Romania
Romania
Romania is a country located at the crossroads of Central and Southeastern Europe, on the Lower Danube, within and outside the Carpathian arch, bordering on the Black Sea...
), Ukraine
Ukraine
Ukraine is a country in Eastern Europe. It has an area of 603,628 km², making it the second largest contiguous country on the European continent, after Russia...
, and southern Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
. Diffuse knapweed is an annual
Annual plant
An annual plant is a plant that usually germinates, flowers, and dies in a year or season. True annuals will only live longer than a year if they are prevented from setting seed...
or biennial plant
Biennial plant
A biennial plant is a flowering plant that takes two years to complete its biological lifecycle. In the first year the plant grows leaves, stems, and roots , then it enters a period of dormancy over the colder months. Usually the stem remains very short and the leaves are low to the ground, forming...
, generally growing to between 10 and 60 cm in height. It has a highly branched stem and a large taproot
Taproot
A taproot is an enlarged, somewhat straight to tapering plant root that grows vertically downward. It forms a center from which other roots sprout laterally.Plants with taproots are difficult to transplant...
, as well as a basal rosette of leaves
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
with smaller leaves alternating on the upright stems. Flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...
s are usually white or pink and grow out of urn-shaped heads
Head (botany)
The capitulum is considered the most derived form of inflorescence. Flower heads found outside Asteraceae show lesser degrees of specialization....
carried at the tips of the many branches. Diffuse knapweed often assumes a short rosette form for one year, reaching maximum size, then rapidly growing and flowering during the second year. A single plant can produce approximately 18,000 seeds
SEEDS
SEEDS is a voluntary organisation registered under the Societies Act of India....
.
An invasive species
Diffuse knapweed is considered an invasive speciesInvasive species
"Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions....
in some parts of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, having established itself in many areas of the continent. C. diffusa was first identified from North America in 1907 when it was found in an alfalfa
Alfalfa
Alfalfa is a flowering plant in the pea family Fabaceae cultivated as an important forage crop in the US, Canada, Argentina, France, Australia, the Middle East, South Africa, and many other countries. It is known as lucerne in the UK, France, Australia, South Africa and New Zealand, and known as...
field in Washington state. The seeds had presumably been transported in an impure alfalfa seed shipment coming from somewhere in the species native range. Now present in at least 19 states in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, it has naturalized in all contiguous states west of the Rockies
Rocky Mountains
The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...
and additionally in Connecticut
Connecticut
Connecticut is a state in the New England region of the northeastern United States. It is bordered by Rhode Island to the east, Massachusetts to the north, and the state of New York to the west and the south .Connecticut is named for the Connecticut River, the major U.S. river that approximately...
, Massachusetts
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...
, and New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...
. Portions of western Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
have also been colonized by this plant.
Areas in which diffuse knapweed has been established generally are plains rangelands or forest benchlands. Land that has been recently disturbed—by human or natural processes—is favored for the establishment of diffuse knapweed. It grows in semi-arid and arid environments and seems to favor light, dry, porous soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
s. Areas with large amounts of shade or high levels of water discourage diffuse knapweed growth.
Dispersion occurs in the following ways:
- Agriculture – alfalfaAlfalfaAlfalfa is a flowering plant in the pea family Fabaceae cultivated as an important forage crop in the US, Canada, Argentina, France, Australia, the Middle East, South Africa, and many other countries. It is known as lucerne in the UK, France, Australia, South Africa and New Zealand, and known as...
contaminated with diffuse knapweed seed can promote the spread of diffuse knapweed; - Wildlife – wild animals eating the seeds or transporting the seeds on fur;
- Wind – seeds blown out of their capsules held on the plant are distributed over a short range, but when dried out the plant may become a tumbleweedTumbleweedA tumbleweed is the above-ground part of a plant that, once mature and dry, disengages from the root and tumbles away in the wind. Usually, the tumbleweed is the entire plant apart from the roots, but in a few species it is a flower cluster. The tumbleweed habit is most common in steppe and desert...
, rolling for great distances and releasing seeds along the way; - Water – waterways carry seeds in their flow for long distances before depositing them onto a shore where they germinate.
Wind is the primary means by which diffuse knapweed seeds are spread.
Effects
By 1998 diffuse knapweed had naturalized over 26640 square kilometres (10,285.8 sq mi) in the western US, and was increasing its range at a rate of 18% annually. Diffuse knapweed can establish itself in grassland, scrubland and riparian environments. It has little value as feed for livestockLivestock
Livestock refers to one or more domesticated animals raised in an agricultural setting to produce commodities such as food, fiber and labor. The term "livestock" as used in this article does not include poultry or farmed fish; however the inclusion of these, especially poultry, within the meaning...
, as its thistles can damage the mouth and digestive tract of animals that attempt to feed on it. A study in 1973 concluded that ranches lost approximately US$
United States dollar
The United States dollar , also referred to as the American dollar, is the official currency of the United States of America. It is divided into 100 smaller units called cents or pennies....
20/km² (8 cents per acre) of diffuse knapweed due to decreased grazing area. In an agricultural setting, it can greatly reduce crop yield and purity.
Control
Effective control of diffuse knapweed requires a fusion of well-executed land management, biological control, physical control, chemical control and reestablishment of the native species. Any method of control must ensure that the root is removed or the plant will grow back. Additionally, native plant growth in areas where diffuse knapweed has been removed should be encouraged to prevent reestablishment.Biological control
Biological control involves the introduction of organisms, usually natural competitors of the invasive species, into the invaded environment in order to control the invasive species. Since 1970, 12 insects have been released to control diffuse knapweed. Of these 12, 10 have become established, and 4 are widely established (Urophora affinisUrophora affinis
Urophora affinis is a species of tephritid or fruit flies in the genus Urophora of the family Tephritidae. It has been released in the United States and Canada as a biocontrol agent to control spotted knapweed. U. affinis became established in Montana in 1973.-Life History:U. affinis is...
and Urophora quadrifasciata, the root boring beetle, Sphenoptera jugoslavica, and the weevil Larinus minutus
Larinus minutus
Larinus minutus is a species of true weevil known as the lesser knapweed flower weevil. It is used as an agent of biological pest control against noxious knapweeds, especially diffuse knapweed and spotted knapweed....
. Research based on simulation models have shown that for biocontrol agents to be effective, they must kill their host, otherwise plants can compensate by having increased seedling survival.
Some of the more commonly utilized biocontrol agents are:
- Lesser knapweed flower weevil (Larinus minutusLarinus minutusLarinus minutus is a species of true weevil known as the lesser knapweed flower weevil. It is used as an agent of biological pest control against noxious knapweeds, especially diffuse knapweed and spotted knapweed....
). Individuals of this species lay their eggs on the seed heads of both diffuse and spotted knapweed. When the larvae emerge from the eggEgg (biology)An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
s, they feed upon the seeds of their host plant. As the females of this species can create from 28 to 130 eggs and each larva can consume an entire seed head, an adequate population of Larinus minutus can devastate entire stands of knapweed. The adult weevils feed upon the stems, branches, leaves and undeveloped flower buds. It is native to Greece and is now found in Montana, Washington, Idaho and Oregon. - Knapweed root weevil (Cyphocleonus achatesCyphocleonus achatesCyphocleonus achates is a species of true weevil known as the knapweed root weevil. It is native to southern Europe and the Mediterranean and is used as an agent of biological pest control against noxious knapweeds, especially spotted knapweed .-Life History:The adult weevil is dark gray and white...
). Knapweed root weevils lay approximately 50 to 70 eggs on either diffuse or spotted knapweed. As the name suggests, the larvae produced burrow into the root where they metamorphose into adult form. At this point, they will tunnel through the root to the surface where they will feed on the leaves of knapweed plants. It is native to Austria, Greece, Hungary and Romania and has been introduced to Idaho, Montana, Washington and OregonOregonOregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
. - Yellow-winged knapweed root mothAgapeta zoeganaAgapeta zoegana is a species of moth known as the sulphur knapweed moth and the yellow-winged knapweed root moth. It is used as an agent of biological pest control against noxious knapweeds, particularly spotted knapweed and diffuse knapweed .The adult moth is bright yellow with areas of brown on...
(Agapeta zoeganaAgapeta zoeganaAgapeta zoegana is a species of moth known as the sulphur knapweed moth and the yellow-winged knapweed root moth. It is used as an agent of biological pest control against noxious knapweeds, particularly spotted knapweed and diffuse knapweed .The adult moth is bright yellow with areas of brown on...
). This mothMothA moth is an insect closely related to the butterfly, both being of the order Lepidoptera. Moths form the majority of this order; there are thought to be 150,000 to 250,000 different species of moth , with thousands of species yet to be described...
with root-mining larvaLarvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e attacks diffuse and spotted knapweed in parts of the United States where it was introduced to control these plants. - Two species of Tephritid flies Urophora affinisUrophora affinisUrophora affinis is a species of tephritid or fruit flies in the genus Urophora of the family Tephritidae. It has been released in the United States and Canada as a biocontrol agent to control spotted knapweed. U. affinis became established in Montana in 1973.-Life History:U. affinis is...
and Urophora quadrifasciata. Females lay their eggs on flower buds and larvae form galls on the flower head.
Physical control
Physical control of diffuse knapweed primarily comprises cutting, digging or burning to remove the plants.- Cutting - While cutting the aboveground portion of diffuse knapweed will greatly decrease the spread of seeds, it does not remove the root. With only its root still intact, diffuse knapweed can survive and continue to grow. For a program of cutting to be effective, it must be long-term so that the effect of reduced seed spreading can be realized.
- Digging - this removes both the portion above ground and the root of diffuse knapweed and has shown to be very effective; if the plant is properly disposed of, it can neither regrow nor spread its seeds. The largest problem with digging knapweed is that it is extremely labor intensive. Additionally, the recently vacated soil should be planted with a native species to avoid knapweed reintroducing itself in the disturbed soil.
- Burning - setting fire to a crowd of knapweed, if the fire is sufficiently severe, can successfully destroy the above ground and belowground sections of diffuse knapweed. However, precautions must be taken to first ensure that the fire is properly contained and that a new plant community is established to prevent the reintroduction of diffuse knapweed.
Chemical Control
Chemical control involves the use of herbicideHerbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...
s to control diffuse knapweed. The herbicide Tordon (picloram) is recognized as the most effective, but it is common to use multiple herbicides in order to reduce strain on local grasses. The herbicides 2,4-D, dicamba, and glyphosate are also effective for control. In order to be most effective, it must be applied before the knapweed plants have released their seeds, regardless of which herbicide is used. Ongoing research at the University of Colorado suggests that Tordon treatment does not contribute to long-term reductions of exotic species cover and may contribute to recruitment of other invasive species, such as redstem filaree and Japanese brome, which quickly take the place of herbicide-treated diffuse knapweed.
Human influence on invasion
One of the first influences humans had on diffuse knapweed was to inadvertently introduce it to North America. Diffuse knapweed has reached a level of coverage in that area that dwarfs its native range.Diffuse knapweed is known to establish more easily and effectively in recently disturbed environments. Disturbed environments generally present low environmental stress because more resources are available than are being used. These available resources often allow the establishment of an invasion in an ecological community. The concentration of diffuse knapweed in such an area is often linked to the level of soil disturbance. Human disturbances often lead to less species diversity
Species diversity
Species diversity is an index that incorporates the number of species in an area and also their relative abundance. It is a more comprehensive value than species richness....
in a community. In turn, less species diversity can lead to unused resources, which allow invasive species to more readily establish. Areas such as fallow land, ditches, rangelands, residential and industrial districts and roadsides are all disturbed habitats where diffuse knapweed frequently establishes. Additionally, the removal of foliage and other ground cover increases the likelihood that seeds will come in contact with the soil and germinate.
The largest impact of humans on diffuse knapweed is certainly due to our efforts in controlling and eradicating its invasive populations. The several methods outlined in the control section represent a small sample of literally hundreds of approaches being tried with varying levels of effectiveness. Besides reducing the spread of diffuse knapweed, we are also providing selective pressure against the individuals that cannot withstand a certain method of control. Selective pressure, given sufficient time, can cause the adaptation
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
or evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
of invasive species such as diffuse knapweed. If an individual diffuse knapweed plant survives control efforts because of a trait it possesses, its progeny will make up a greater portion of the population than the plants that succumbed to the control.
Toward an integrated control strategy
To successfully control diffuse knapweed, an understanding of the mechanism that allows it to be invasive must be developed. Isolating the reason for its invasiveness would allow control methods designed to specifically target the effectiveness of that mechanism to be developed. Additionally, precautions designed to minimize the invisibility of at-risk environments could be carried out.Summary
The success of diffuse knapweed must be attributed to a combination of several mechanisms. Its invasiveness is due to a mix of allelopathyAllelopathy
Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial or detrimental effects on the target organisms...
, ERH and superior resource competition. However, the most importance must be attributed to the ERH because diffuse knapweed, while a very effective invasive species in its novel environment, is non-invasive and doesn’t establish monocultures in its native range. It is the differences, biotic
Biotic component
Biotic components are the living things that shape an ecosystem. A biotic factor is any living component that affects another organism, including animals that consume the organism in question, and the living food that the organism consumes. Each biotic factor needs energy to do work and food for...
and abiotic, between its novel and native surroundings that cause it to be invasive.
To demonstrate that the ERH applies to diffuse knapweed, it is essential to show that the absence of natural enemies has a significant positive effect on its success. One way to show this is to observe the effect of introducing some of diffuse knapweed’s natural enemies into its novel environment. If diffuse knapweed, which generally thrives in its invaded environment, is significantly inhibited through the introduction of natural enemies, it can be concluded that diffuse knapweed is more competitive in the absence of its natural enemies. A recent effort at biocontrol of diffuse knapweed in Idaho’s Camas County
Camas County, Idaho
Camas County is a county in the southern portion of the U.S. state of Idaho. As of the 2000 Census, the rural county had a population of 991 . The county seat is Fairfield....
effectively reduced 80 square kilometres (19,768.4 acre) of knapweed to minimal levels through the release of the lesser knapweed flower weevil and the knapweed root weevil. Since both of the insects released are natural competitors of diffuse knapweed, and since this and other similar efforts at biocontrol
BioControl
BioControl is a peer-reviewed scientific journal published by Springer Science+Business Media covering all aspects of basic and applied research in biological control of invertebrate, vertebrate, and weed pests, and plant diseases. The journal was established in 1956 as Entomophaga and published by...
have been successful, there is significant evidence that diffuse knapweed benefits from the absence of its natural enemies.
Another aspect of diffuse knapweed’s success relies on the effect of its allelopathic chemicals in its novel environment. Although there is still debate concerning the effectiveness of allelopathic chemicals in the field, the evidence of allelopathic effects demonstrated in a laboratory setting and its propensity to establish monocultures support the importance of allelopathy to diffuse knapweed’s success.
Curiously, diffuse knapweed’s allelopathic chemicals were shown to have a deleterious effect on the North American competitors but were beneficial to its native competitors. While diffuse knapweed’s native competitors are able to compete more effectively in the presence of allelopathic chemicals, the novel competitor’s fitness is decreased. This situation provides an example of the effectiveness of the allelopathy mechanism benefiting from the ERH. The increased effectiveness of allelopathic chemicals cause diffuse knapweed to experience less competitive pressure. As a result, diffuse knapweed is able to establish more predominantly in this new area.
Another connection between allelopathy and the ERH is the fact that concentrations of allelopathic chemicals were found to increase when diffuse knapweed was planted in North American soil as opposed to Eurasia
Eurasia
Eurasia is a continent or supercontinent comprising the traditional continents of Europe and Asia ; covering about 52,990,000 km2 or about 10.6% of the Earth's surface located primarily in the eastern and northern hemispheres...
n soil. This effect is probably due to the absence of unfavorable soil conditions or soil microorganisms that exist in its native environment. As a result, the allelopathic chemicals will be able to reach higher concentrations, spread farther and therefore be more effective. By effecting more neighboring plants, the favorable changes in soil condition contribute to the success of diffuse knapweed.
Besides the advantages that diffuse knapweed gains from the ERH and allelopathy, it also possesses several characteristically invasive traits. One factor leading to the superior resource competition of diffuse knapweed is its ability to exist in drought conditions. This advantage allows diffuse knapweed to devote its resources to competition while its neighbors are conserving resources to survive. The high number of seeds produced by diffuse knapweed is also a common trait of invasive plants. A higher density of knapweed will not only increase the concentration of allelopathic chemicals in the soil but will also restrict the nutrients available to native plants. Unfortunately, very little research has been conducted to determine the relative competitive ability between diffuse knapweed and its novel competitors. However, tests conducted on the effect of diffuse knapweed on North American grasses in the absence on allelopathic chemicals demonstrated that the fitness of these grasses declined in the presence of diffuse knapweed. Regrettably, we cannot decide if diffuse knapweed is, for general purposes, a better competitor from this data alone. Comparisons of the deleterious effects between these and other pairs of competitors to arrive at a conclusion.
Diffuse knapweed is successful in its novel range primarily because the organisms and conditions that prevent it from becoming invasive in its native environment are absent. It follows that the introduction of species from its native habitat would be an effective method of control. However, the introduction of a non-native organism has the potential to result in another invasive species outbreak. Therefore, any method of biological control must be preceded by analysis of possible effects.
External Links
- Species Profile- Diffuse Knapweed (Centaurea diffusa), National Invasive Species Information Center, United States National Agricultural LibraryUnited States National Agricultural LibraryThe United States National Agricultural Library is one of the world's largest agricultural research libraries, and serves as a National Library of the United States and as the library of the United States Department of Agriculture...
. Lists general information and resources for Diffuse Knapweed.