The “full species list” fallacy in Floristic Quality Assessment

Biological assessments typically involve field surveys that are time-consuming and require taxonomic expertise. Floristic Quality Assessment, a popular bioassessment method for wetlands and other ecosystems, generally assumes a comprehensive or representative species list for accurate implementation. We explored this long-held belief by analyzing an essential floristic quality metric (mean conservatism) across real and simulated gradients of species representation in two disparate case studies. In one study, we incrementally removed species at random from an exhaustive floristic survey of a suburban wetland complex in northeast Ohio. Bootstrapping mean conservatism at each removal step, precision scarcely decreased with 10%–30% species loss, becoming noticeable only when about 50% or fewer species remained. For the other study, we exploited varying percentages of dominant species available from hundreds of single-visit wetland determination surveys throughout Illinois. Comparing dominants-only mean conservatism with total species mean conservatism, the relationship steadily improved as dominants covered progressively larger fractions of native richness, ranging from r² = 0.12 at ≤10% dominants to 0.74 at >40% dominants. Both exercises suggest that community size is more important than taxonomic representation or inventory completeness per se in determining accuracy. Our results indicate that full or representative checklists are not a prerequisite for reliable Floristic Quality Assessment, supporting the investigation and potential use of taxonomic shortcuts and empowering a wide range of users beyond expert field botanists.