Niche ecology in Floristic Quality Assessment: Are species with higher conservatism more specialized?

Floristic Quality Assessment (FQA) metrics are used to assess the conservation value of natural areas based on the composition of their plant communities. FQA is based on coefficients of conservatism (C-values). C-values are regional, expert-assigned values, which range from 0 to 10 to reflect a taxon's fidelity to high quality natural areas and its relative tolerance to modern human impacts. Approximately half of FQA publications assert without evidence that plant species with lower C-values are more generalist, whereas species with higher C-values are habitat specialists. The connection between specialization and C-values has never been investigated. Our primary objective was to test the claim that ecological specialization is associated with C-values. We first used the “theta (θ) method,” based on species co-occurrences, to estimate niche width for 274 species in 598 wetlands, grasslands, and forests in Illinois, USA. As a secondary proxy for niche width, we counted the habitat preferences of all Illinois vascular taxa listed from the Illinois Plant Information Network (ILPIN). For θ in grasslands and wetlands, and for ILPIN habitat counts, the lowest and highest C-values were associated with greater ecological specialization, whereas species with low to intermediate C-values were the most generalist. In forests, we found weaker, negatively linear relationships between C-values and θ, indicating that species with a high conservation value (i.e. conservative) were the most specialized. However, when shrub and canopy layers were combined with the herbaceous layer data in forests, we did not find an association between C-values and specialization. We also reanalyzed the data to determine the effect of non-native species on the relationship between niche width and C-values because non-native species, by default, receive C-values of 0. Removing non-native species from the analyses did not influence our conclusions. Both methods to measure specialization suggested there is a weak relationship between C-values and ecological specialization. Our findings support the common claim that some conservative species are more specialized than species with intermediate or low C-values. However, we found the general C-value and specialization relationship is more complex than assumed by many FQA publications because if it exists, it is not necessarily linear, i.e., species with low C-values are often specialists. Specialization and niche width could be latent components of C-values, but FQA users should avoid defining species conservatism by niche and specialization.