The reference-degraded continuum - assessing biological condition relative to anthropogenic disturbance

Assessment of biota typical entails binary classification of a test site relative the reference condition (RC), whose limits are determined empirically by sampling many reference sites. The RC is defined by physicochemical characteristics of 'best available' sites and associated biota. We operationally defined a complementary “degraded condition”, wherein physicochemical characteristics (“disturbances” - D ) are deemed unacceptable by consensus. A test site's relative quality (Q) can thus be estimated from its position along a reference-degraded continuum (RDC) (D). Two alternative patterns may be postulated when Q is plotted vs. an RDC scale. The value of D may determine the value of Q  (i.e., D is a dose-like factor, and Q vs. D is best analysed by linear or logistic regression), or D may merely limit Q (D is one of many bounding factors; Q vs. D is best analyzed by methods that identify upper and lower bounds of Q). Studies of Great Lakes fish assemblages, Detroit R. zoobenthos, and wetland bird, herptile, and plant species richness suggest that threshold responses are common manifestations of environmental variation (D). Piecewise quantile regression can quantitatively identify such thresholds.