The relationship between community diversity and exotic plants: cause or consequence of invasion? - Invasive Plants: Ecological and Agricultural Aspects - page 85

tive interactions in the exotic species literature [23]. Under this hypothesis,
early seedling establishment and performance of the exotic species would be
expected to vary with neighborhood diversity.
In the impact hypothesis, exotic species invade a neighborhood and subse-
quently interact with species residing within the community, resulting in
altered diversity. This is typically thought to occur via the invader competi-
tively displacing species currently in the area [8, 64, 77], or by preventing the
establishment of other species [11, 22]. Species displacement would result in
a reduction in neighborhood diversity if individual exotic species, on average,
displaced more than one resident species. While positive interactions between
invaders and other species have rarely been documented, the invasion of a spe-
cies that facilitates the growth or establishment of other species may directly
increase neighborhood species richness.
Both invasibility and impacts processes may generate similar changes in
neighborhood diversity in association with exotic plant species, but would dif-
fer mechanistically. The diversity/invasion relationship of different exotic spe-
cies may be explained by different mechanisms, or both may simultaneously
operate to determine the relationship of an invader to community structure.
The diversity/invasion pattern exhibited by the plant community will be the net
effect of these two independent processes. It is also possible that species that
exhibit no associations with neighborhood diversity may actually have coun-
teracting invasibility and impact relationships.
Finally, as null a hypothesis, there may be no mechanistic relationship
between exotic species and diversity. Diversity and invasion may both respond
to similar extrinsic factors that generate associations without direct interaction.
For example, microsite conditions that generate spatial patterning in diversity
may also favor the establishment of an exotic plant species. However, this
would probably lead to fine-scale variation in dominance of individual exotic
species with variation in microsite conditions. Since many exotic species tend
to be problematic across many community types and at regional scales, this
alternative seems unlikely. Variation in local seed input may also generate pos-
itive associations between exotic and native species, even when higher diver-
sity results in lower invasibility [19]. Because most plant communities are
seed-limited [10, 90, 91], and exotic plant species tend to be extremely vagile
[44, 92, 93], exotic species may be the first plants to invade a disturbed area,
resulting in low diversity with high invasion. In this situation, the relationship
between invasion and diversity would disappear as the less vagile native spe-
cies invade [94].
The variation in the direction and strength of the relationship between exot-
ic plant invasion and diversity in the ecological literature may partly result
from the lack of a useful conceptual framework that separates out the invasion
process from subsequent species interactions. Most observational and experi-
mental studies artificially integrate both mechanisms into a single assessment
of invasion, therefore obscuring the species interactions underlying the com-
munity dynamics associated with the invasion.