Evaluating non-native species and wetland indicator status as components of wetlands floristic assessment

Abstract

We evaluated a potential index for quantifying wetland floristic quality, based on the Floristic Quality Assessment Index (FQAI) developed and tested in other regions of the United States. Principal reasons for this study were 1) FQAI is based on plant species’ coefficients of conservatism, which are unavailable for most of the world and 2) FQAI value calculation mathematically neglects exotic (non-native) species. The index that we evaluated, termed the Floristic Assessment Quotient for Wetlands (FAQWet), incorporated components of total species richness, wetland affinity of species present, and the contribution of native versus exotic species to wetland vegetation quality. Thus, this index incorporated factors demonstrated to be influenced by the degree of human activity on the landscape while including both native and non-native plant species, and it used information that is readily available for most plant species encountered in wetlands of the United States. Adequacy of this index at representing perceived human influence on wetlands was evaluated by sampling vascular plant assemblages in 53 wetlands across northern Mississippi, USA. Correlation of FAQWet scores with semi-quantitative disturbance indicators suggested that FAQWet Index calculations weighted by proportion and frequency of native species performed best overall and were comparable with the currently used FQAI, although all indices displayed low correlation with indices of human activity. Analyses of individual components of the disturbance index, along with information on hydrogeomorphology and hydrologic alteration, helped account for some unexplained variation in the relationship between floristic quality and disturbance. For example, there were more exotic species and a lower overall degree of wetland affinity in vegetation of depressional wetlands than in lake fringe and riverine wetlands. The most important benefits of the FAQWet index in these studies were a lack of correlation with total plant species richness (unlike the FQAI), a stronger correlation with non-native species richness than was the case with the FQAI, and the ease of obtaining wetland indicator status and nativity status for FAQWet calculations. Results also highlighted the potential danger of ignoring exotic species in floristic assessments because of the relatively strong correlations of non-native species richness with human activity, hydrologic impairment, and floristic index scores.