In research article, 
Effects of Interspecific Competition on Early Life History of the Invasive Thistles Carduus nutans and C. acanthoides, D.C. Ruggiero and K. Shea report that the emergence and survival of exotic thistle species, 
Carduus nutans and 
C. acanthoides seems to be significantly higher on bare ground than when there are other plants present to compete with them. They recommend conservation practices which will reduce the amount of bare ground in order to limit  invasion by these two exotic thistle species.
To read the complete article go to 
Northeastern Naturalist 18(2):197-206. 2011.
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    | Musk thistle, Carduus nutans photo by Steve Dewey, Utah   State University, Bugwood.org
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