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DETERMINING LEAST- AND MOST-IMPACTED STREAM CONDITIONS USING SURVEY DATA ACROSS THE RANGE OF NATURAL GRADIENTS
Citation:
Whittier, T R. DETERMINING LEAST- AND MOST-IMPACTED STREAM CONDITIONS USING SURVEY DATA ACROSS THE RANGE OF NATURAL GRADIENTS. Presented at Northwest Bioassessment Workgroup meeting, Long Beach, WA, November 6-8, 2002.
Description:
One of the challenges to developing indicators of ecological condition is to determine what reference conditions are for a given ecosystem. Because human activity often co-varies with natural gradients (e.g., elevation) one cannot simply extrapolate from, for example, small streams in high elevation wilderness areas to large lowlands streams. For many geographic areas and ecosystems, where unimpacted conditions do not exist, one needs to use least-impacted sites to help define reference condition. It is also useful to determine what the likely worst conditions for a given system are. However, there are currently no composite measures of the overall anthropogenic stress on ecosystems. Here, I propose a method that uses survey data to select a set of sites in the survey that represents the least impacted conditions and a set that represents the worst conditions across the range of the natural gradients. As a test of the concept, for 108 stream and river sites from the EMAP-Western Pilot survey, in the Forested Mountains aggregate ecoregion, I plotted values for each of four water chemistry, three watershed-scale, and three site-scale indicators of anthropogenic stress against one of three measures of natural gradients (elevation, slope and stream size). For each plot I drew two lines along the length of the natural gradient below and above which fell about 10% to 20% of the "best" and "worst" sites respectively (for that indicator), such that these points were spread fairly evenly along the gradient. The number of "bests" and "worsts" were tallied for each site. These tallies were then plotted against all three natural gradients to evaluate their distributions. Gaps in the distributions of least- and most-impacted sites along the gradients were alleviated by making minor adjustments to the lines. The final lists also achieved reasonably good geographic distributions of least- and most-impacted sites. Finally, vertebrate assemblage data from these sites were compared and contrasted to assist in the development of biotic indicators of ecological condition.