Understanding invasions: the rise and fall of diffuse knapweed (Centaurea diffusa) in North America - Invasive Plants: Ecological and Agricultural Aspects - page 115

part of the reduction in plant densities, because densities decline when seed
production is still moderately high [24]. Since root feeders have been observed
to cause mortality in immature plants [31], these species in particular likely do
assist in the speed of the decline in knapweed densities. Myers and Bazely [31]
indicate that seed predators should only be effective when host plants are poor
competitors and have low rosette survival. In Colorado, seedling survival
appears to be a vulnerable stage for this plant, and soil nutrient availability and
plant competition influence survival ([5, 10] and unpublished results). Thus,
similar to findings of McEvoy et al. [27] and research summarized by Muller-
Scharer and Schroeder [6], we believe that resource competition, generated by
low soil resource availability or through plant competition, mediates the exot-
ic plant response to herbivorous insects. Accordingly, we predict that in spite
of large reductions of knapweed in grassland areas, knapweed will remain
common in areas of soil disturbance with high nutrients and little plant com-
petition. To date, our observations match this prediction.
The relationship between biotic controls and 'super invaders'
Our findings do not negate the studies that show Centaurea species to possess
allelopathic chemicals, to maximize mutualistic associations with mycor-
rhizae, or to benefit from positive feedbacks with soil biota. Further,
Centaurea species may have escaped pathogens found in soils of their native
habitat, thereby conferring additional advantage. These mechanisms, however,
operate most strongly at high population densities. These factors are relative-
ly ineffective at maintaining dominance if propagule pressure (seed produc-
tion) and survivorship of juvenile plants are insufficient to maintain high den-
sities of the invader. Our data suggest that the combination of herbivore stress
to flowering plants and direct seed consumption by weevil larvae in undis-
turbed vegetation can reduce the abundance of this species to relatively low
densities. Therefore, top-down controls (i.e., the components of classical bio-
logical control) are capable of negating or overriding 'novel weapons' or other
mechanisms that confer competitive advantage to this species. Demonstration
of top-down controls in North America does not prove that these insects con-
trolled knapweed in their native lands (e.g., [32, 33]), but these results do show
that this negative feedback is sufficient to deter dominance in the invaded areas
studied here.
The Centaurea species that have invaded North America possess numerous
traits that have interacted with the invaded communities in ways that have
enhanced the dominance of these species. However, allelopathy may be the
equivalent of “bringing a knife to a gun fight” as a mechanism for maintaining
dominance. The ability to acquire and preempt limiting resources is a require-
ment for plant dominance. When that ability is precluded by the absence of
propagules or the absence of tissues necessary to obtain those resources in an
efficient manner, then dominance is unobtainable. Knapweed's ability to grow