Agriculture Reference
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hydroxyquinoline from root exudates of
C. diffusa,
and found that levels of 8-
hydroxyquinoline were three times higher in N. American soils than in native
soils. They concluded that native plants and soil biota might have acquired
resistance to 8-hydroxyquinoline while N. American plants and soil biota were
highly susceptible, thereby explaining
C. diffusa
's profound effects in North
America. Prati and Bossdorf [137] investigated the allelopathic activities of
native and invasive
Alliaria petiolata
against two congeneric species,
Geum
urbanum
from Europe and
G. laciniatum
from N. America. Allelopathic sup-
pression of
G. urbanum
germination by invasive
A. petiolata
in N. America
was found. The germination of
G. urbanum
therefore depended upon the ori-
gin of
A. petiolata.
Although EMH is a testable hypothesis, it is not yet conclusively shown to
play an important role in invasion in field situations. In many allelopathy stud-
ies, the significance of larger-scale ecosystem processes is unappreciated
[138]. Most allelopathy research is carried out using an autecological
approach. Inderjit and Weiner [139] proposed that allelopathy is better con-
ceptualized and investigated in terms of soil chemical ecology. In their view,
placing allelopathy in the context of soil ecology can reduce some of the con-
troversy surrounding the phenomenon. It is not ecologically correct to label a
species as an 'allelopathic' species, as allelopathy is conditional on species-
specific effects [140]. Allelopathy as a potential mechanism of plant invasion
needs further study. More recently, Callaway and Ridenour have proposed the
'allelopathic advantage against resident species (AARS) hypothesis' [141],
which is conceptually similar to the EMH hypothesis and therefore falls under
the more general EMH.
Neutral community dynamics (NCD) hypothesis
One hypothesis that should not be ignored, and indeed might be considered a
null hypothesis, is simply that there is no underlying proximate mechanism for
invasions, other than random chance. Or, at least, that the outcome of interac-
tions among invaders and resident species are so complex that they can be
modeled stochastically. There has been a recent spate of work examining
which ecological patterns neutral dynamics can adequately explain [142-144].
Largely inspired by the neutral theory of evolution, neutral dynamics of com-
munities simply describe changes in the composition and abundance of species
as the outcome of random birth-death processes, resulting in random walk tra-
jectories [143]. Neutral models may be valuable for developing null models of
expected number and relative abundance of NIS, which would have to be
rejected before other hypotheses are inferred. However, neutral models may
also hold value in their ability to explain some current patterns of invasions
more parsimoniously than other hypotheses.
One universal observation of NIS patterns is that most invaders (those that
successfully establish) remain relatively rare [35]. For example, Cadotte et al.
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