Abstract
Biological indicators are useful tools for the assessment of ecosystem condition. Multi-metric and multi-taxa indicators may respond to a broader range of disturbances than simpler indicators, but their complexity can make them difficult to interpret, which is critical to indicator utility for ecosystem management. Floristic Quality Assessment (FQA) is an example of a biological assessment approach that has been widely tested for indicating freshwater wetland condition, but less attention has been given to clarifying the factors controlling its response. FQA quantifies the aggregate of vascular plant species tolerance to habitat degradation (conservatism), and model variants have incorporated species richness, abundance, and indigenity (native or non-native). To assess bias, we tested FQA variants in open-canopy freshwater wetlands against three independent reference measures, using practical vegetation sampling methods. FQA variants incorporating species richness did not correlate with our reference measures and were influenced by wetland size and hydrogeomorphic class. In contrast, FQA variants lacking measures of species richness responded linearly to reference measures quantifying individual and aggregate stresses, suggesting a broad response to cumulative degradation. FQA variants incorporating non-native species, and a variant additionally incorporating relative species abundance, improved performance over using only native species. We relate our empirical findings to ecological theory to clarify the functional properties and implications of the FQA variants. Our analysis indicates that (1) aggregate conservatism reliably declines with increased disturbance; (2) species richness has varying relationships with disturbance and increases with site area, confounding FQA response; and (3) non-native species signal human disturbance. We propose that incorporating species abundance can improve FQA site-level relevance with little extra sampling effort. Using our practical sampling methods, an FQA variant ignoring species richness and incorporating non-native species and relative species abundance can be logistically efficient, easily understood, and effective for wetland assessment.
Highlights
Biological indicators are widely used to indicate environmental condition (U.S EPA, 2006)
Our analysis indicates that (1) aggregate conservatism declines with increased disturbance; (2) species richness has varying relationships with disturbance and increases with site area, confounding Floristic Quality Assessment (FQA) response; (3) non-native species are favored by human disturbance; and (4) proportional abundance of species provides important functional information at the site level
We found that native species richness (N) was not correlated with any measure of wetland condition (OIWI) or stress (RIRAM, Impervious surface area (ISA)), and our work is consistent with other studies that have found that variants excluding species richness more reliably vary with wetland condition (Bried et al, 2013; Cohen et al, 2004; Matthews et al, 2009; Miller and Wardrop, 2006; Vaselka et al, 2010)
Summary
Biological indicators (or bioindicators) are widely used to indicate environmental condition (U.S EPA, 2006). Bioindicators range in complexity from single indicator species to multimetric indices based on multiple attributes of multiple taxa. Multi-metric and multi-taxa indicators are attractive to practitioners interested in assessing ecological integrity because they theoretically integrate a more diverse response to environmental conditions than simpler indicators (Birk et al, 2012; Karr, 1991), but the complexity of these indicators requires additional time and taxonomic expertise over simpler measures, and may be a drawback if the component metrics show interactive or countervailing responses that make the final indicator difficult to interpret (Karr and Chu, 1999). FQA is a relatively simple bioindicator, using one to three attributes of vascular flora viewed as a single taxonomic group, yet it has shown potential to integrate and reflect broad aspects of freshwater wetland condition (DeBerry et al, 2015). Bioindicators in aquatic systems used coefficients to characterize species’
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