The relationship between community diversity and exotic plants: cause or consequence of invasion? - Invasive Plants: Ecological and Agricultural Aspects - page 91

The value of long-term monitoring of natural plant communities is clearly
evident in the above case studies. The non-experimental nature of this and sim-
ilar studies actually increases the range and breadth of questions that can be
addressed with the resulting data, although direct experimental control has
been sacrificed. In unmanipulated systems, the types of relationships and inter-
actions that can be addressed are not limited by experimental protocols
designed to separate out a small suite of effects. For this simple reason, a study
began to document successional dynamics has been very useful in addressing
plant invasions and their impacts.
Conclusions
Invasion ecology has suffered from the artificial separation of invasibility and
impact processes in understanding the relationship between diversity and plant
invasion. By studying these independently functioning stages of invasion in
concert, we can gain great insight into the biological causes and consequences
of invasions, and develop crucial information for the generation of adequate
management strategies. Our conceptual framework provides a structure to syn-
thesize the current body of research, suggests research needed to fill the gaps
in understanding and to organize results from future research. The framework
is a powerful tool to guide ecological understanding of the relationship
between invasion and diversity across systems, species, and scales.
The case studies discussed here clearly show how both the cause and con-
sequence of diversity may operate simultaneously within an invasion to gener-
ate the community associations often noted in static studies. Currently, it is not
possible to make generalizations about which mechanism is the most impor-
tant because of the extreme lack of information for most plant invasions. To
understand the nature of the relationship between diversity and invasion, both
of these processes must be assessed to determine their relative contribution.
Acknowledgments
Previous versions of this manuscript benefited from comments by SN Handel, JS Johns, STA Pickett,
S Ruhren, BE Wachholder and KA Yurkonis. This material is based upon work supported by the fol-
lowing organizations: Council on Faculty Research, Eastern Illinois University; CSREES, US
Department of Agriculture, under Agreement No. 99-35315-7695; and the National Science
Foundation under award DEB-0424605.
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