Choosing Plant Diversity Metrics: A Tallgrass Prairie Case Study

Diversity metrics are used to compare sites and to track trends within sites. Many metrics have been proposed, from simple species counts to complex indices like the Floristic Quality Index (FQI). Using a large dataset (n = 452 quadrats) from high-quality remnant prairie, degraded remnant prairie, and restoration sites, we examined correlations among ten diversity metrics and explored which metrics provide useful information about trends and differences among site types. We also created a conservatism list for 492 plant species of the Texas Blackland Prairie. Shannon diversity, Simpson diversity, and FQI were strongly correlated with species richness (r2 > 0.53). Average conservatism varied greatly at low levels of richness and less at high levels of richness. This metric, which is usually considered to be independent of sample area, should be used with caution in sites with low richness. When assessing change in high-quality remnant prairie plots, native species richness increased the most. Average conservatism changed little, because an increase in specialist species (coefficient of conservatism ≥ 7) was matched by an increase in generalist forbs (coefficients 4–6). FQI increased, driven by changes in species richness rather than conservatism. When comparing site types, only FQI differed among all three site types; average conservatism and the number of specialist species also differed between remnant and restoration sites. We recommend using the number of specialist species and native species richness to examine trends within a site. For differences among sites, we recommend the number of specialist species, average conservatism, or FQI.