The relationship between community diversity and exotic plants: cause or consequence of invasion? - Invasive Plants: Ecological and Agricultural Aspects - page 83

Negative associations between exotic plant cover and community diversity
have been used to suggest that invading exotic species do not merely fill vacant
niches in natural communities [10], or replace native species one-for-one, but
that they displace species disproportionately from the community, lowering
diversity. Field studies of invasive exotic plants often cite this hypothesis,
either as a correlation or as anecdotal information. However, this direct inter-
action has rarely been experimentally tested.
The competitive displacement of native plant species is often used as justi-
fication for the eradication of individual exotic species, although the perceived
relationship with diversity may not exist [13, 67, 68, 71]. Many, if not most
exotic species have only trivial impacts on community structure, becoming
minor components of the plant community that increase regional species diver-
sity [5, 72]. Problematic exotic species, those which become widespread and
locally dominant, are generally found to be competitively superior to native
species in two-species competition experiments [73-75]. However, it is not
known whether exotic species are, on average, competitively superior to native
plant species, which would be necessary to result in lowered diversity across a
community. Increases in richness or species performance following invasive
species removal suggest that competitive displacement may result from some
invasions [76-83].
An excellent example of exotic plant invasion impacts is that of Lonicera
maackii . This species, a bird-dispersed shrub native to Asia, has become wide-
spread throughout the eastern United States [84]. It has become a problematic
invader of deciduous forests, particularly second growth and disturbed forests
[85-87] and often dominates the forest understory in heavily invaded sites.
Observational data show that this invader is associated with declines in tree
seedling abundance and in the abundance and diversity of the herbaceous
understory [63, 83, 87]. The mechanisms behind this association have been
tested experimentally, showing that L. maackii directly reduces the growth and
fecundity of herbaceous annual and perennial understory plants [77, 78, 83]
and competes with tree seedlings [88, 89]. Indirect impacts of L. maackii
include protection of tree seedlings from deer browse [88] and increased tree
seed predation by rodents (S. J. Meiners, unpublished data). Taken together,
this suite of studies documents both patterns indicative of invasion impacts and
documents direct and indirect interactions that generate these patterns. This is
one of the few species invasions whose community-level impacts have been
characterized mechanistically. However, beyond this species' connection to
relatively open forest stands and gaps [85, 87], community-level controls on
invasion success are unknown.
Cause versus consequence: invasibility versus impacts
Clearly, the mechanism(s) that result in the often-observed relationships
between exotic plant invasion and community diversity are unclear in current