Assessing diversity in plant communities: The importance of within-species variation

While the measurement of plant species diversity continues to play a central role in ecology and conservation, few formal attempts have been made at defining the concept of diversity, and, specifically, at evaluating the utility of taxonomic species as units for diversity indices. Ecological diversity is defined here as a measure of the degree of biotic and/or abiotic dissimilitude within a community that is perceivable and utilizable by organisms. Although indices based merely on 'species' may be useful for cataloging taxonomic diversity in communities, plant taxa are generally unreliable units for quantifying the ecological diversity perceived and utilized by most organisms in communities. Recent research highlights the shortcomings of the unit of species for characterizing ecological diversity, and we advocate the further exploration of alternative diversity classes with which to describe plant communities.