Biology Reference
In-Depth Information
superior ability of knapweeds to access soil water beyond the reach of native plants
was suggested to allow greater photosynthetic-related advantages (Hill et al. 2006).
Elsewhere, experiments on soil biota have tested the collective benefits provided by
mycorrhizae and the absence of soil pathogens in the success of invasive species of
Centaurea
. Mycorrhizae fungi provided a competitive advantage to spotted knap-
weed (Marler et al 1999; Callaway et al. 2004), but in another study were not bene-
ficial to plants losing tissue due to simulated herbivory (Walling & Zabinski 2006).
However, Callaway et al. (2001) demonstrated that mycorrhizal interactions with
C. melitensis
allowed the plant to exhibit compensation to grazing damage.
Somewhat surprisingly, low to intermediate levels of herbivory can apparently
benefit knapweeds (Callaway et al. 1999; 2006; Thelen et al. 2005; Newingham and
Callaway 2006). Herbivores appear to stimulate the competitive ability of the plant,
perhaps by enhancing root exudates that stimulate mycorrhizal activity or the pro-
duction of allelopathic chemicals. Alternatively, Newingham et al. (2007) found
that spotted knapweed benefits from root herbivory by shifting overall nitrogen
allocation from the damaged root to aboveground stems and reproductive tissues.
The combined effects of allelopathy and its reported response to herbivory appear
to make spotted knapweed a superinvader. Accordingly, this species was given the
title, “The wicked weed of the West” (Alper 2004). As of 2007, most species of
Centaurea
and particularly spotted knapweed were widely perceived as invasive
plant species that lack sustainable control measures.
Early attempts to control spotted and diffuse knapweed added a human health
concern to land management issues. Two species of gall flies (
Urophora
species)
were released on spotted and diffuse knapweed in 1970 (Harris 1980). The insects
greatly increased in numbers, but failed to control plant densities. The gall flies
provided a substantial amount of biomass available to native predators, and the
weed presented an abundant but unused resource by native herbivores, thus greatly
altering native food webs (Pearson et al. 2000). Among the native predators that
consumed the gall flies were deer mice (
Peromyscus maniculatus
) that were carriers
of a virus known to cause the potentially fatal human disease, hantavirus (Pearson
and Callaway 2003, 2005, 2006; Ortega et al. 2004). A single broadcast herbicidal
application was shown to decrease these impacts on deer mice populations by reduc-
ing knapweed abundance and the food source for
Urophora
(Pearson and Fletcher
2008). However, broadcast applications of herbicides intended to temporarily
decrease spotted knapweed densities were also meanwhile shown to increase the
abundance of another unwanted invasive plant,
Bromus tectorum
, which replaced
spotted knapweed as the dominant invader (Y. Ortega, USFWS, personal communi-
cation to D.G.K). Alternatively at our site, some of the indirect negative food web
effects caused by
Urophora
abundance have already been reduced by the presence
and consumption of
Urophora
by another biological control insect,
Larinus minutus
(Seastedt et al. 2007). Because
Larinus
has a large overwintering biomass in the
soil, the species may be causing other unknown alterations in food webs.
Recently, a second set of studies has appeared to suggest that spotted knapweed
is not invincible in its introduced environment. The identified allelopathic agent for
spotted knapweed may not occur in high enough concentrations to have impacts on
