A national-scale vegetation multimetric index (VMMI) as an indicator of wetland condition across the conterminous United States

Teresa K Magee,M Siobhan Fennessy,Karen A Blocksom

doi:10.1007/s10661-019-7324-4

Abstract

In 2011, the US Environmental Protection Agency and its partners conducted the first National Wetland Condition Assessment at the continental-scale of the conterminous United States. A probability design for site selection was used to allow an unbiased assessment of wetland condition. We developed a vegetation multimetric index (VMMI) as a parsimonious biological indicator of ecological condition applicable to diverse wetland types at national and regional scales. Vegetation data (species presence and cover) were collected from 1138 sites that represented seven broad estuarine intertidal and inland wetland types. Using field collected data and plant species trait information, we developed 405 candidate metrics with potential for distinguishing least disturbed (reference) from most disturbed sites. Thirty-five of the metrics passed range, repeatability, and responsiveness screens and were considered as potential component metrics for the VMMI. A permutation approach was used to calculate thousands of randomly constructed potential national-scale VMMIs with 4, 6, 8, or 10 metrics. The best performing VMMI was identified based on limited redundancy among constituent metrics, sensitivity, repeatability, and precision. This final VMMI had four broadly applicable metrics (floristic quality index, relative importance of native species, richness of disturbance-tolerant species, and relative cover of native monocots). VMMI values and weights from the survey design for probability sites (n = 967) were used to estimate wetland area in good, fair, and poor condition, nationally and for each of 10 ecoregion by wetland type reporting groups. Strengths and limitations of the national VMMI for describing ecological condition are highlighted.

Highlights

The National Wetland Condition Assessment (NWCA) (USEPA 2016m) provided an unprecedented opportunity to characterize the quality of wetlands at a continental-scale
We found in preliminary analyses of NWCA data that candidate vegetation multimetric index (VMMI) based on various sets of the highest performing metrics, did not perform as well as the best VMMIs built using Van Sickle’s approach (USEPA 2016n)
We developed a VMMI applicable to diverse wetland types across the conterminous United States (US) (Table 1), and which provides a standardized approach to assessing wetland biological condition based on vegetation

Summary

Introduction

The National Wetland Condition Assessment (NWCA) (USEPA 2016m) provided an unprecedented opportunity to characterize the quality of wetlands at a continental-scale. Ecological condition is estimated using biological indicators, typically multimetric indices (MMI) based on the combination of a few measured and interpreted metrics that describe different attributes of a particular biotic assemblage (e.g., USEPA 2006; Stoddard et al 2008; USEPA 2009). These indices vary with human-mediated disturbance (hereafter, disturbance), reflecting departure from reference expectations that are typically defined based on the least disturbed sites within a study area (Stoddard et al 2006; Herlihy et al 2008). For wetlands, the term IBI has often been applied to both the traditional IBI and the more objective MMI approaches

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