Guideline 14: Assessment Thresholds 



To facilitate interpretation of indicator results by the user community, threshold values or ranges of 

 values should be proposed that delineate acceptable from unacceptable ecological condition. Justification 

 can be based on documented thresholds, regulatory criteria, historical records, experimental studies, or 

 observed responses at reference sites along a condition gradient. Thresholds may also include safety 

 margins or risk considerations. Regardless, the basis for threshold selection must be documented. 



Performance Objectives 



1 . Present and justify approach used to describe expected conditions under a regime of minimal 

 human disturbance. 



2. Present and justify proposed threshold values for the indicator to distinguish among classes 

 of ecological condition. 



The approach to scoring individual metrics is based on comparison of an observed metric response at a 

 sampling site to the response expected under conditions of minimal human disturbance (see Table 4-6). 

 Expectations for individual metrics (Table 4-1 9) that are based on measures of species richness are derived 

 from a large number of sample sites from the MAHA study, as opposed to using a set of representative 

 "reference" sites believed to be minimally impacted by human activities. For metrics based on the percentage 

 of individuals, expectations are based primarily on values developed for similar indicators in other areas 

 (e.g., Karr 1986, Yoderand Rankin 1995). 



Initial threshold values of the final indicator score have been proposed to classify different states of ecological 

 condition (Table 4-20). Four classes of condition are proposed, based in part on the examination of the 

 distribution of values within resource populations of the 1993-1994 MAHA study. Impaired condition was 

 operationally defined as any score less than 50, which represents a level of biotic integrity less than one-half 

 of that score expected under minimal human disturbance. This number of classes is consistent with the 

 potential power of the indicator to distinguish differences in condition (Table 4-18). These thresholds are 

 also somewhat consistent with those proposed by other groups using similar multimetric indicators (e.g., 

 Fore etal. 1996). 



These threshold values have not been quantitatively examined, a process that requires a better understanding 

 of indicator responsiveness (Guideline 12). Independent confirmation of appropriate threshold values is 

 also necessary to achieve the performance objectives established for this guideline and implement this 

 indicator in the proposed monitoring framework. Confirmation can be achieved by applying the indicator to 

 an independent set of sites of known levels of impairment. Peer review of the proposed thresholds by 

 professional ecologists and resource managers familiar with the development and interpretation of multimetric 

 indicators is also required to complete the evaluation of the indicator with respect to this guideline. 



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