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33, 51, 54-56]. Positive associations between diversity and invasion suggest
that the same basic factors which make a microsite able to support many
native species also make them able to support a diverse group of exotic spe-
cies [4].
The quantity of evidence currently available on the diversity-invasibility
hypothesis shows that diversity can regulate neighborhood invasibility in some
systems, but that the directionality of this relationship appears to vary. The
more important research themes now become the determination of the condi-
tions under which diversity regulates community invasibility and the develop-
ment of conceptual models that explain the variation in the direction of this
relationship. Neighborhood diversity appears to limit the establishment of
invading plant species in communities dominated by equilibrium dynamics,
particularly in those communities strongly structured by competitive interac-
tions such as in the nutrient limited sites studied by Tilman and colleagues [10,
45, 57-59]. Sites dominated by non-equilibrial dynamics, such as succession-
al sites or those with otherwise high species turnover, appear to have a positive
association between invasion and fine-scale diversity [19, 32, 35, 55].
However, this dichotomy may be artificial, as invasibility has been predicted
to respond unimodally along diversity gradients when both facilitative and
competitive interactions structure the community [23, 55]. The directional
responses seen in many systems may therefore represent the extremes of the
gradient, or situations where either positive or negative interactions dominate.
The variation in the diversity-invasibility relationship across systems may
also reflect the short-term nature of the majority of studies. Our ability to make
reliable predictions about ecological systems is often limited by the temporal
extent of available data [60]. The lack of predictive models in invasion biolo-
gy may therefore be linked with the lack of sufficient long-term data.
Consequence: invasion impacts on diversity
While exotic plant invasions alter plant community composition and structure,
the direct effects on diversity patterns are largely unquantified. Unfortunately,
the vast majority of studies are observational, with little ability to assess the
direct impacts of plant invasion on diversity [8]. It appears that exotic plant
invasions are associated with a decrease in the number of species in natural
communities [6, 61-64]. However, this pattern is clearly not ubiquitous across
all plant communities [65-68].
Differences among studies in the measure of invasion complicate attempts
at generalization. Studies that relate richness of exotics to native diversity
often find a positive correlation [66] while studies that assess invasion by cover
or dominance tend to find negative associations [61, 69, 70]. Within a single
community, it is possible to have both a positive association between native
and exotic species richness and a negative association between total richness
and exotic plant cover (Figs 1A and 1B).
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