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to competitive interactions, all of these interactions function at relatively fine
scales.
Scale emerges as a critical variable when assessing apparent conflicting
relationships of diversity and invasion published in the literature. At regional
scales, the relationship between exotic and native species richness is often a
positive one [4, 31-34], while at fine scales within sites, the same communi-
ties may show a negative correlation between exotics plants and native species
richness [32, 33, 35]. Diversity at regional scales is less determined by species
interactions and more the result of changes in abiotic factors such as distur-
bance rate or productivity [36, 37] or variation in historical, evolutionary and
biogeographical influences [38]. Native and exotic species appear to respond
to this regional variation in abiotic conditions in a similar manner [4, 19, 34,
39, 40], resulting in the positive spatial associations at coarser scales.
Overall, community-level controls on invasion must function through local
processes of interaction with the resident community. Likewise, the majority
of impacts on the community would be expected to be manifested at fine scales
that match the scale of species interactions. These local dynamics are nested
within the regional context that determines the pool of native and exotic spe-
cies and the availability of species to colonize a particular site (i.e., propagule
pressure). Throughout the remainder of this paper, we will focus only on fine-
scale causes and consequences of plant invasion.
Cause: diversity and the regulation of community invasibility
Interactions between invading species, the existing plant community and
microhabitat conditions influence the ability of an exotic plant species to
invade a site. Within plant communities, areas with lower diversity are often
thought to be more susceptible to invasion than relatively species-rich areas [9,
39, 41-44]. This is because species-poor microsites may have more available
resources, or vacant niches, allowing a specific exotic plant species to become
established [9, 10, 45]. Experimental evidence suggests that higher diversity
areas use resources more completely, leaving few colonizable niches [10, 12,
19, 45]. Local resource availability has been found to be critical in determin-
ing neighborhood invasibility in many studies [12, 46-49], and may represent
a general explanation for fine-scale invasibility patterns [48, 50]. Alternatively,
it may not be the diversity, but the composition of the neighborhood that reg-
ulates invasibility [11, 22, 51, 52]. Higher diversity areas would be more like-
ly to contain competitively dominant species through sampling from a limited
species pool [10, 11, 53] and are thus more likely to resist invasion. This is
commonly referred to as an ecological sampling effect [53].
The diversity-invasibility hypothesis has been tested in herbaceous com-
munities with contrasting results. Neighborhoods of higher diversity were
more resistant to invasion in some studies [10-12, 19, 52] while other stud-
ies found higher diversity plots to have equal or greater invasibilities [4, 32,
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