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noted above, the recent finding [24] that Eurasian soil microbes have a strong
inhibitory effect on
C. maculosa
growth, while North American microorgan-
isms are much less inhibitory suggests that the success of
C. maculosa
may
involve both the escape from certain enemies and pathogens as well as
allelopathy.
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 pro-
vides a new basis for supporting allelopathic mechanisms for some striking
phenomena - the noteworthy examples include the obvious, devastating suc-
cess of some invasive species (described above), as well as the equally impres-
sive resistance to invasion by a natural community seen in the Florida scrub.
Furthermore, the success of scrub vegetation in deterring invasion raises ques-
tions 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 spe-
cialist herbivores and pathogens, but for unadapted organisms in natural com-
munities 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 allelopath-
ic 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.
Acknowledgements
The authors thank Gary Ervin of Mississippi State University for his constructive criticism of this
manuscript.
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