Striving for consistency in the National Wetland Condition Assessment: developing a reference condition approach for assessing wetlands at a continental scale

Abstract

One of the biggest challenges when conducting a continental-scale assessment of wetlands is setting appropriate expectations for the assessed sites. The challenge occurs for two reasons: (1) tremendous natural environmental heterogeneity exists within a continental landscape and (2) reference sites vary in quality both across and within major regions of the continent. We describe the process used to set reference expectations and define a disturbance gradient for the United States (US) Environmental Protection Agency’s National Wetland Condition Assessment (NWCA). The NWCA employed a probability design and sampled 1138 wetland sites across the conterminous US to make an unbiased assessment of wetland condition. NWCA vegetation data were used to define 10 reporting groups based on ecoregion and wetland type that reduced the naturally occurring variation in wetland vegetation associated with continent-wide differences in biogeography. These reporting groups were used as a basis for defining quantitative criteria for least disturbed and most disturbed conditions and developing indices and thresholds for categories of ecological condition and disturbance. The NWCA vegetation assessment was based on a reference site approach, in which the least disturbed reference sites were used to establish benchmarks for assessing the condition of vegetation at other sites. Reference sites for each reporting group were identified by filtering NWCA sample data for disturbance using a series of abiotic variables. Ultimately, 277 least disturbed sites were used to set reference expectations for the NWCA. The NWCA provided a unique opportunity to improve our conceptual and technical understanding of how to best apply a reference condition approach to assessing wetlands across the US. These results will enhance the technical quality of future national assessments.

Using a vegetation index to assess wetland condition in the Prairie Pothole Region of North America

“…This process may include the sampling of wetlands across a broad degradation gradient to validate existing metrics, or to identify new metrics, including abundance metrics, that display a predictable response to disturbance. Reference wetlands (i.e., least disturbed, best available; Herlihy et al, 2019 ) also may have to be sampled to facilitate development of index scoring ranges for the various condition categories (e.g., poor, fair, good).…”

Enhanced indicator species performance with increasing contextualization

“…Our results imply that spatial‐environmental contextualization at least be carefully considered when extracting indicator species for conservation purposes, complementing other types of context study of indicator species (e.g., Tulloch et al, ). Sample classification schemes involving regionalization and typology (e.g., Herlihy, et al, ; Stoddard, ) could enhance species potential for indicating biodiversity loss/status, ecological disturbance, environmental change, and management progress (Stewart et al, ; Wiens et al, ; Zettler et al, ).…”

“…Ecoregional and hydrogeomorphic stratification is commonly used in wetland assessment research to limit interference from natural heterogeneity (e.g., Herlihy et al, ; Jog et al, ). We divided the sample between prairie ecoregions (85 sites) and forest ecoregions (32 sites) (Figure ).…”

Considering the Adaptive Cycle and Resilience of the Ecosystem to Define Reference Conditions for Wetland Restoration

“…The Ramsar Convention has provided general principles and guidelines for wetland restoration based on many different projects worldwide (Ramsar Convention, 2002). “Historical documents” (Hughes et al., 1998; Kress et al., 2019), “reference sites” (Cui et al., 2009; Herlihy et al., 2019), and “paleoecology” (Finlayson et al., 2016; Muxika et al., 2007) are standard methods for establishing reference conditions for wetland ecological restoration. These methods usually set the pre‐disturbed natural dynamic ecosystem states and interactive factors as reference conditions for ecosystem restoration.…”