Functional diversity has been seen as the key to predicting the stability, invasibility, resource capture, nutrient cycling and productivity of communities. However, it has been unclear how to estimate it. Ten criteria for an index of functional diversity are developed. These include that it should reflect the range of characters present and the abundance of the species with those characters in the community, and be unaffected by the measurement units used or by the number of species.
An index that meets all ten criteria, FDvar, is investigated. It is based on the variance in characters, weighted by the abundance of the species with those characters. Tested with artificial and randomly generated data, it showed reasonable use of the 0 - 1 range (mean 0.60, range 0.0009 - 0.975) and intuitive behaviour. Tested with field data from eight sites in New Zealand, it gave a good spread of values (mean 0.65, range across sites 0.34 - 0.84), showed good ability to distinguish between the communities and its performance was ecologically intuitive. Illustrative correlations are made with mean annual temperature and soil fertility, determined by a bio-assay. FDvar is recommended for general use.
Abbreviations: GFDI = Group Functional Diversity Index; PSU = Photosynthetic unit.