Understanding invasions: the rise and fall of diffuse knapweed (Centaurea diffusa) in North America - Invasive Plants: Ecological and Agricultural Aspects - page 110

of persistence and opportunism also described the invasiveness and dominance
of C. diffusa as well. Suding et al. [10] demonstrated that rosettes of diffuse
knapweed were strong competitors under ambient nutrient conditions, but
were less competitive under lower nutrient conditions that may have charac-
terized North American grasslands until recently. Elsewhere, the role of soil
biota - collectively the benefits provided by mycorrhizae and the absence of
soil pathogens - has also been indicated in the success of invasive species of
Centaurea . Mycorrhizae fungi provide a competitive advantage to invasive
spotted knapweed C. maculosa [11, 12]. Callaway et al. [13] demonstrated that
mycorrhizal interactions allowed C. melitensis to exhibit compensation to
grazing damage. Those findings added to the work showing that overcompen-
sation to root herbivory occurred in C. maculosa [14].
Allelopathy was also identified as a competitive mechanism used by
Centaurea species. Spotted knapweed was found to possess an allelopathic
agent, (-)-catechin [15]. Another allelopathic chemical, 8-hydroxyquinoline,
was subsequently identified for diffuse knapweed [16], supporting the con-
tention of Hierro and Callaway [17] that C. diffusa used allelopathy to achieve
a competitive advantage. This particular chemical contains nitrogen, an ele-
ment that is generally more available in many areas of North America due to
increased atmospheric nitrogen deposition and chronic fire suppression [18].
Callaway and Ridenour [19] suggested that the relatively high production of
allelopathic compounds could explain the dominance of invasive Centaurea
species.
While specific findings were being reported for Centaurea spp, Klironomos
[20] demonstrated that invasive plant species with strong dominance (high
abundance in their respective communities) often exhibit positive feedbacks
with soil biota. Mitchell and Power [21] found that those invasive species that
exhibited dominance and were identified as noxious and invasive tended to be
those that had escaped their native fungal pathogens and viruses. These same
species had yet to accumulate an equivalent number of pathogens in their
introduced environments. Not surprisingly, Callaway et al. [22] subsequently
showed that C. diffusa growth exhibited positive feedbacks in soils of invaded
communities. Collectively, these results argue that the combination of traits -
persistence and opportunism, allelopathy, and potential positive feedbacks
from soil biota - allow for Centaurea species, including diffuse knapweed, to
function as something we might call “super-invaders” [23]. Such plants appear
to be superior competitors and capable of dominance across a broad range of
ecological conditions.
By 2003, however, evidence suggested that certain insects were having a
strong influence on diffuse knapweed densities in Colorado [24]. Subsequent
reports from Montana [25] and British Columbia [26] indicated that this
response was widespread. The common factor in this reduction was the addi-
tion, to the existing suite of herbivores, of a weevil that consumed both seeds
and maturing plants. Differences in nutrient availability and plant competition
have the potential to mediate weed responses to herbivory (e.g., [27, 28]).