Hawaii and the Pacific Islands - U.S. Army Corps of Engineers

ERDC/EL TR-12-5 22Indicator 3: Prevalence indexDescription: The prevalence index is 3.0 or less.User Notes: The prevalence index ranges from one to five. A prevalenceindex of 3.0 or less indicates that hydrophytic vegetation is present. Tocalculate the prevalence index, at least 80 percent of the total vegetationcover on the plot (summed across all strata) must be of species that havebeen correctly identified and have assigned wetland indicator statuses(Reed [1988] or current list) or are upland (UPL) species.Procedure for Calculating a Plot-Based Prevalence Index: Theprevalence index is a weighted-average wetland indicator status of all plantspecies in the sampling plot, where each indicator status category is given anumeric code (OBL = 1, FACW = 2, FAC = 3, FACU = 4, and UPL = 5) andweighting is by abundance (absolute percent cover). It is a morecomprehensive analysis of the hydrophytic status of the community thanone based on just a few dominant species. It is particularly useful in(1) communities with only one or two dominants, (2) highly diverse communitieswhere many species may be present at roughly equal coverage, and(3) cases where strata differ greatly in total plant cover (e.g., total herb coveris 80 percent but sapling/shrub cover is only 10 percent). The prevalenceindex is used in this supplement to determine whether hydrophyticvegetation is present on sites where indicators of hydric soil and wetlandhydrology are present but the vegetation initially fails the dominance test.The following procedure is used to calculate a plot-based prevalence index.The method was described by Wentworth et al. (1988) and modified byWakeley and Lichvar (1997). It uses the same field data (i.e., percent coverestimates for each plant species) that were used to select dominant speciesby the 50/20 rule, with the added constraint that at least 80 percent of thetotal vegetation cover on the plot must be of species that have been correctlyidentified and have an assigned indicator status (including UPL).For any species that occurs in more than one stratum, cover estimates aresummed across strata. Steps for determining the prevalence index are asfollows:1. Identify and estimate the absolute percent cover of each species in eachstratum of the community. Sum the cover estimates for any species that ispresent in more than one stratum.