The relationship between community diversity and exotic plants: cause or consequence of invasion? - Invasive Plants: Ecological and Agricultural Aspects - page 80

Scale and the invasion of plant communities
Before directly addressing the relationship between diversity and invasion, it
is first necessary to deal with the issue of scale. A hierarchical perspective of
biological invasions reveals three nested scales which interact to determine
invasions and their impacts in communities [15]. The coarsest scale, the
regional scale, determines the species pool of invaders and residents, setting
the potential range of species interactions. The intermediate scale, that of the
landscape, determines which species within that larger regional pool will be
able to colonize a given habitat based on their presence within the landscape,
their vagility and physiological tolerances. The finest scale of interest is that of
the neighborhood; the scale at which species interacts. Interactions lead to dif-
ferential performance of the species, resulting in the realized composition of
the neighborhood. At this fine scale, individuals may interact to influence inva-
sion success or to generate the impacts of an invasion. Therefore, the most
appropriate scale for studying the diversity/invasion relationship should match
the scale at which organisms interact within a system.
The scale of interaction varies widely with the type of system being studied
and with the specific interaction involved. Within experimental microcosms or
modeling studies of community invasibility, all species within the community
interact, or at least have the potential to interact [16, 17]. This is in marked
contrast to the condition in terrestrial plant communities, where interactions
occur at neighborhood scales [18, 19]. Typically, only plants with canopy or
root overlap have the potential to interact. For example, two herbaceous plant
species may compete strongly when in close proximity, but would have no
effect on each other when separated by even a few meters. For this reason, the
total number of plant species in an entire community should have no bearing
on the overall invasibility of that community. Rather, fine-scale, within-com-
munity patterns of diversity may determine neighborhood invasibility. What
constitutes a neighborhood in a particular system should parallel the scale at
which organisms interact, probably from 10 cm 2 or less for small plants and
emerging seedlings to 50 m 2 or more for large canopy trees.
Plants in terrestrial ecosystems are immobile and compete locally for large-
ly immobile resources such as light and soil nutrients. This leads to the devel-
opment of heterogeneity in local competitive environments, even within sites
heavily invaded by an aggressive exotic species. This interaction heterogene-
ity explains why plant invasions rarely, if ever, directly lead to the loss of a spe-
cies from an entire community. Species may be lost from areas directly
impacted by an invasion, but will persist in spatial refugia not dominated by
the invader (e.g., [20-22]).
While competitive interactions dominate the ecological literature on inva-
sions [23], there are several other direct and indirect mechanisms through
which plant invaders could interact with the resident community. These
include allelopathic interactions [24-26], associational defenses [27], influ-
ences on nutrient dynamics [28, 29] and alterations of soil biotas [30]. Similar