Editing Centaurea diffusa (section)

===Summary===
The success of diffuse knapweed must be attributed to a combination of several mechanisms. Its invasiveness is due to a mix of [[allelopathy]], the enemy release hypothesis (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 component|biotic]] and [[abiotic]], between its novel and native surroundings that cause it to be invasive.{{Citation needed|date=July 2007}}

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, Idaho|Camas County]] effectively reduced {{convert|80|km2|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.{{citation needed|date=October 2022}}

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.{{citation needed|date=October 2022}}

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.{{citation needed|date=October 2022}}

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.{{citation needed|date=October 2022}}

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.{{citation needed|date=October 2022}}

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.{{citation needed|date=October 2022}}