Understanding invasions: the rise and fall of diffuse knapweed (Centaurea diffusa) in North America - Invasive Plants: Ecological and Agricultural Aspects - page 109

Understanding invasions: the rise and fall of diffuse
knapweed ( Centaurea diffusa ) in North America
Timothy R. Seastedt 1 , Katharine N. Suding 2 and Katherine D. LeJeune 3
1 Institute of Arctic and Alpine Research, Department of Ecology and Evolutionary Biology,
University of Colorado Boulder, Boulder, CO 80309-0450, USA
2 Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92697-
2525, USA
3 Stratus Consulting, Boulder, Colorado 80303, USA
Introduction
Diffuse knapweed ( Centaurea diffusa Lam.) is an aster of Eurasian origin that
has colonized 1.4 million hectares of semiarid grasslands in western North
America over the last century [1, 2]. The species can be found from
Washington to Michigan and from British Columbia to New Mexico. This spe-
cies is a short-lived perennial that often produces a rosette in the first year and
in the second year will flower, set seed, and die. The senesced adult plant can
function as a tumbleweed to disperse seeds. The weed reduces grassland for-
age and has the potential to exhibit negative effects on other ecosystem serv-
ices of these rangelands [3-5].
Midway through the last century C. diffusa was sufficiently widespread to
implement biological control efforts. The first species of biological control
insects (gall flies, Urophora spp) were released in North America in 1970, fol-
lowed by the release of about 10 additional species of insects over the next two
decades [6]. By 2000, 13 insects had been released [7]. In spite of these efforts,
diffuse knapweed continued to spread, and through the 1990s, biological con-
trol efforts appeared unsuccessful. As summarized by Carpenter and Murray
[8] “At least nine biological control agents that attack Centaurea diffusa are
established in parts of the United States. Unfortunately, it appears that none of
these agents, alone or in combination, effectively controls diffuse knapweed
populations.”
While evidence was accumulating indicating that top-down controls were
not affecting dominance for this and related species of Centaurea , the impor-
tance of bottom-up controls (competitive and resource acquisition mecha-
nisms) were being documented. Gerlach and Rice [9] indicated that a relative
of diffuse knapweed, C. solstitialis , was successful as an invader due to its
abilities to persist within a community and exploit resource opportunities.
Research on diffuse knapweed at our site [5] indicated that the characteristics