Allelopathy as a mechanism for resisting invasion: the case of Polygonella myriophylla

Conclusions

Past research on allelopathy has been plagued by methodological criticisms [2, 5, 40], and one lingering result of this has been that allelopathy has often been viewed as the “hypothesis of last resort”. However, a body of recent work provides a new basis for supporting allelopathic mechanisms for some striking phenomena — the noteworthy examples include the obvious, devastating success of some invasive species (described above), as well as the equally impressive resistance to invasion by a natural community seen in the Florida scrub. Furthermore, the success of scrub vegetation in deterring invasion raises questions about the hypothesis that allelopathy will be most likely to occur as a mechanism that enables exotic invasions. The argument that allelopathy is likely to be of limited significance in natural communities is based on the assertion that over time, plants will likely have evolved tolerance to chemical defenses of their neighbors [19, 40]. However, the evolution of tolerance to chemical defenses should be no more significant among neighboring plants in terrestrial plant communities than it is for herbivores or pathogens of these plants. There are many examples of evolution of tolerance to toxins by specialist herbivores and pathogens, but for unadapted organisms in natural communities these defenses usually remain effective.

Bioassays with Centaurea diffusa imply an important role for allelopathy in natural community structure, a point which has been overlooked [33]. While activated carbon increased the growth of North American grasses at the expense of C. diffusa (supporting the hypothesis of allelopathic effects by C. diffusa in this exotic environment), the growth of Eurasian grasses from its native habitat was reduced by activated carbon, and C. diffusa biomass increased. These results are consistent with allelopathic inhibition of C. diffusa by grasses in its native habitat, and contrary to the hypothesis that allelopathic interactions will be most important for exotic species invading new habitats. Considered together with work on the Florida scrub and on non-exotic invaders such as crowberry (Empetrum hermaphroditum), these results require the admission that while allelopathy may be of importance to the success of certain exotic invaders, it may be an important factor, along with competition, herbivory, and abiotic factors, in structuring natural plant communities.