Despite extensive use, few studies have thoroughly tested competency of the Florist Quality Index (FQI) to assess vegetation quality by comparing it with alternative statistics and with independent measures in large data sets. We compared the efficacy of species richness and floristic quality indices in detecting temporal change and fire effects on quality within and among tallgrass prairie remnants. We calculated species richness at small (1/4-m² plot) and large (total sample) scales, as well from a Species Richness Index (SRI) that integrates these measures. These statistics were compared with FQI, which assesses quality by integrating species richness with estimates of species conservatism (C values) to undisturbed natural vegetation. We made within-site comparisons of temporal change in dry-mesic and mesic prairie vegetation following 22 years of fire exclusion and then after five years of fire management. The among-site comparisons used 33 prairies that were graded as A or B quality and sampled in 1976 by the Illinois Natural Areas Inventory (INAI). We resampled these sites in 2001 and analyzed vegetation change in relation to their fire-management histories.

Within-site comparisons found that significant declines in average plot species richness corresponded to independent measures that documented deterioration of vegetation composition and structure. In contrast, lack of significant change in average C values failed to detect these changes. We found that species richness was greatest in mesic habitats, while C values were biased toward higher values assigned to dry prairie species. Among sites, those ranked as grade A by the INAI had greater species richness than grade B sites. Temporal changes in species richness in these sites were also positively correlated with fire frequency, which in turn was negatively correlated with a shift in vegetation structure toward increasing woody vegetation and loss of grasses. Average C values did not differ significantly between the INAI A and B quality grades, nor were they correlated with fire frequency; however, they were negatively correlated with increasing woody dominance. These results indicate that, for tallgrass prairie, measures of species richness can be very sensitive indicators of vegetation change, and can help gauge differences in vegetation quality within vegetation types. In contrast, indexing floristic quality is inconsistent and problematic due to bias and lack of precision in assigning conservatism scores as well as instability in the FQI formula and potential for circular reasoning in validating its effectiveness.