A major challenge when conducting an assessment of natural resources, especially at a continental scale, is determining the appropriate standard (i.e., reference sites) against which to judge ecological condition. This issue occurs because (1) tremendous natural environmental heterogeneity exists within a large landscape and (2) reference sites vary in quality across and within regions. We describe the process to identify reference sites for the US Environmental Protection Agency’s (USEPA) 2011 National Wetland Condition Assessment (NWCA). NWCA defines reference as sites with the best available ecological conditions given the current state of the landscape, i.e., least disturbed sites.
NWCA 2011 employed a probability design to sample 1138 sites across the contiguous US to make an unbiased assessment of wetland condition. Vegetation data were used with ecoregions and wetland type to define ten reporting groups minimizing natural variability associated with continental-wide differences in biogeography. All 1138 sites were then arrayed along a disturbance gradient created for each reporting group. The gradient was defined using ten measures of site disturbance based on land use, hydrologic modifications, soil heavy metals, and alien plants. Disturbance thresholds of least (i.e., reference), intermediate, and most disturbed sites were established for each reporting group, thus customizing the definition of reference by region and wetland type and assigning all sites sampled to a disturbance category.
Ultimately 277 least disturbed reference sites were identified. Of the 277 sites, 170 qualified as minimally disturbed sites (i.e., none of the disturbance measures were present). Amount and type of disturbance found in reference sites varied by region and wetland type. Reference sites in the Coastal Plain and Estuarine reporting groups and the Eastern Mountains and Upper Midwest (EMU) inland woody wetland group were generally minimally disturbed. Those in the West and Interior Plains reporting groups and the EMU inland herbaceous wetland group were largely least disturbed. An effort to handpick reference sites prior to sampling for the NWCA 2011 had a 52% failure rate when evaluated against the definition developed for reference for the appropriate reporting group.
The NWCA provided a unique opportunity to improve our conceptual and technical understanding of how to best apply a reference approach to assess the wetland resource across the US. Moreover, the reference sites identified are the first of a collection of quantitatively defined reference sites that will grow with each subsequent NWCA beginning in 2016. This abstract does not necessarily reflect USEPA policy.