Biology Reference
In-Depth Information
7.
Invasion Resistance of
Native Communities
“Current hypotheses or generalizations about traits that distinguish both suc-
cessful invaders and vulnerable communities all concern some extraordinary
attributes or circumstances of the species or communities. And all are based
on retrospective explanations for past invasions.”
—M
ACK ET AL
. (2000)
Diffuse knapweed (
Centaurea diffusa
) is an annual or short-lived perennial
Eurasian forb that has become one of the most serious rangeland weeds
in western North America. In native areas of the Republic of Georgia, it
grows in steppe communities with bunch grasses closely related to those
of the shrub steppe communities it invades in Washington, Oregon, Idaho,
and other western states and Canadian provinces. Diffuse knapweed is a
highly competitive invader, reducing the abundance of many native range
plants. In the Republic of Georgia, however, its abundance is small and its
impact minor. In eastern Washington, its annual economic damage is esti-
mated at several million dollars.
Laboratory competition experiments between diffuse knapweed and
three pairs of closely related species of bunch grasses, each pair consisting
of one North American species and one Eurasian species, revealed some
surprising results (Callaway and Aschehoug 2000). Diffuse knapweed
exerted a much greater inhibition of growth on North American grasses
than on those with which it coexists in Eurasia. When the experiments
were repeated with activated carbon, which absorbs many organic sub-
89
