The role of the novel weapons and enemy release hypotheses on the invasiveness of knapweeds, Centaurea spp., in North America

Abstract

Knapweeds, Centaurea spp., are among the most successful grassland invaders in North America. Despite similar life histories, only certain members of this genus have become problem species, while others remain as benign, naturalized exotics. Two ideas which have been proposed to explain the success of exotic plants are the novel weapons (allelopathy) and enemy release hypotheses. In this study, both hypotheses were examined by comparing highly invasive knapweeds to their noninvasive kin. Leachates collected naturally from the soils of both highly invasive and non-invasive knapweeds had no noticeable allelopathic effects on indicator species.Though allelopathy has been reported in invasive knapweeds, it is likely the result of specific environmental factors. In terms of herbivory, non-invasives were fed on more by a generalist feeder, and incurred significantly more herbivory than invasives in a common garden. This indicates that invasive knapweeds may possess novel defenses which deter local herbivores.iii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.