Agriculture Reference
In-Depth Information
age caused by the adult weevils. Second, perennial plants may be able to persist
over multiple years by producing few or even no flowers. Seed head weevils
could not persist in such areas and therefore the densities of weevils required to
control perennial
Centaurea
species may never be obtained. Finally, the allelo-
pathic agent identified for spotted knapweed is a carbon-based compound.
Hence, nitrogen allocation conflicts are not likely to be as important. This does
not preclude the possibility of other insect species with different life history
strategies to function as potential biological control agents. For example, there
exists some hope that the root-feeding weevil,
Cyphocleonus achates,
may
eventually function in this fashion at least in some regions for
C. maculosa
[34].
A subset of exotic plants exhibit a combination of traits such as enhanced
rates of alleopathic chemical production and beneficial plant-microbial inter-
actions formed when exotic plants enter new communities. These 'bottom-up'
effects appear to give this group the properties of 'super invaders'. Exotic con-
trol agents, some which may also exhibit more effective control characteristics
due to release from their own predators and pathogens, can greatly diminish
the dominance of these invaders. While high competitive ability can confer
high survivorship of mature plants, seeds, seedlings and juvenile plants appear
more vulnerable to top-down controls and also appear more susceptible to
competition-mediated resource limitations. The net outcome between the rel-
ative strengths of 'bottom-up' and 'top-down' benefits and constraints pro-
duces the patterns we observe for invasiveness and dominance. While
Centaurea diffusa
appears to no longer be an unmanageable invasive threat, we
do not know if the control mechanisms that reduce the dominance of diffuse
knapweed will work for other invasive
Centaurea
species.
Summary
The relative importance of mechanisms explaining the invasiveness and dom-
inance of alien plant species remains a subject of active debate. Diffuse knap-
weed has been identified as a strong competitor capable of using allelopathic
chemicals to achieve dominance in plant communities that have not co-
evolved with this species. Positive feedbacks with soil biota may further
enhance the competitive abilities of
Centaurea
species. The failure of classical
biological control after 30 years of effort was seen as negative evidence for the
enemy escape hypothesis as a mechanism explaining dominance. However,
control of this invasive species by insect herbivory now appears to have been
achieved in widely separated ecosystems in North America. While we do not
know if these same insects exerted this regulatory function in the native habi-
tat of diffuse knapweed, we do see top-down controls operating effectively in
the invaded ecosystems. Traits conferring strong competition such as enhanced
rates of allelopathic chemical production or those produced by new
plant-microbial interactions formed when exotic plants enter new communi-
ties can be negated by biological control mechanisms.
Search WWH ::
Custom Search