Invasive plants: the process within wetland ecosystems - Invasive Plants: Ecological and Agricultural Aspects - page 103

sesquiterpenoid and glucosinolate compounds function both as anti-herbivore
defenses as well as allelopathic agents in suppressing growth of competitive
plants (e.g., [99-103]. Sulfur-containing compounds (trithianes) are known
among aquatic plants and macroalgae (Characeae) that have both antimicrobial
and anti-herbivore properties [98, 104]. Some elegant studies have isolated
specific polyphenolic and lignan compounds from specific aquatic plants that
confer anti-herbivory properties against the crayfish [105, 106]. Glucosinolate
compounds are produced by watercress ( Nasturtium officinale ) that functions
in both allelopathic and anti-herbivory properties [107-109].
Such studies support the hypotheses of Lodge [110] and Newman [111] that
aquatic and wetland macrophytes may be well defended chemically against
herbivory. Evidence suggests that phenolic compounds are more likely respon-
sible for herbivory deterrence than are alkaloids. Plant damage, as via herbi-
vore feeding activities, can result in immediate induction of phenolic produc-
tion and suppression of feeding by a variety of invertebrate herbivores.
Less toxic chemical defenses are more widespread among plant species
than are more narrowly distributed highly toxic compounds. Herbivores can
adapt to novel, more toxic chemical defenses of plants by becoming special-
ists. Alternatively, herbivores can become generalists but at the cost of reduced
feeding success on any particular plant species [112]. The limited literature on
phytochemical defense mechanisms among aquatic plant and wetland com-
munities provide support for a co-evolutionary model. Herbivory responses are
sufficiently distinct, however, to indicate diffuse co-evolution.
Microorganisms may select for plant defense compounds. As plant species
diversify, novel defense chemicals will become widespread. The tendency then
would be for herbivores to adapt to it and eventually develop mechanisms to
disable such compounds.
Conclusions
Scientific understanding of the competitive capacities of invasive species is
critical to effecting prevention, detection and rapid management responses to
invasive species introductions. Many of the changes being imposed upon
indigenous communities by invasive species offer direct evidence for the
capacities of these advantaged plants to compete with native species. In some
cases, anthropogenic alterations of environmental parameters, such as changes
in atmospheric concentrations of CO 2 and related climatic and hydrological
changes, have both direct and indirect effects on facilitation of biological inva-
sions [113-117]. Truly effective management skills emerge, however, when
the underlying physiological and genetic control mechanisms are understood.
In many cases, the inadvertent invasions of exotic species into natural com-
munities provide useful experiments - their rigorous analyses and interpreta-
tions can effectively augment controlled experimental analyses at specific bio-
chemical and genetic levels that are so essential to gain needed understanding.