The transmogrification of plant invaders: biogeographic differences in allelopathic effects and native evolutionary responses
 
Ragan M. Callaway1,* and Jorge M. Vivanco2
1 The Division of Biological Sciences, The University of Montana, Missoula, MT 59812, USA
2 Department of Horticulture and Landscape Architecture, Colorado State University, Fort Collins, CO 80523, USA
*email: ray.callaway@mso.umt.edu
 

Many exotic plant species undergo astounding increases in dominance when introduced to new communities by humans. This is primarily attributed to escape from specialist consumers. However, strong allelopathic effects (a form of plant-plant communication) by some powerful invaders and much stronger allelopathic effects in invaded than in native ranges suggest a new theory for invasive success – the novel weapons hypothesis. We discuss the evidence for allelopathic effects of Centaurea maculosa and C. diffusa, evidence for belowground cross-talk, and propose that some invaders transmogrify because they possess novel biochemical weapons that function as unusually powerful allelopathic agents or as mediators of new plant-soil microbial interactions. Novel biochemical weapons possessed by some plant invaders may provide an advantage based on differences in historical coevolutionary trajectories. Furthermore, exotic plant invasions often cause high mortality in native populations and therefore have the potential to be a selective force on natives. We found that individuals of some North American species that survived Centaurea maculosa invasion have higher tolerances to the European invader than individuals from communities that had not experienced invasion. These results provide initial evidence that native plants species may evolve to tolerate the effects of an exotic invader, and in particular an invader’s novel allelochemistry.

 
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