2.4 Target and reference condition selection

Comparing indicator condition against a fixed/reference value is a critical requirement for using indicators to inform condition assessments. This fixed value could be an historical condition, a desired future condition, a legal threshold, or some other reference value. It provides the context for interpreting indicator results — a number against which current status and trends can be compared. For instance, a high water temperature or an increasing trend in water temperature only tells us something meaningful about the risk of this condition to fish if we know at what temperature fish will be adversely affected, and whether the current trend is moving closer to or further away from that temperature threshold. A reference value is a quantity/value of an indicator that reflects some threshold, desired goal or target, or historic and/or pristine condition, according to what is most meaningful for the assessment and reporting purpose, and supported by science. The selection of reference values is as important as the selection of the indicator itself because, without this baseline, it is difficult to assess the magnitude of change objectively, whether the magnitude of change is important, or if any efforts at improving conditions are succeeding (National Research Council, 2000).

The term reference condition also may have multiple meanings. Stoddard et al. (2006) suggest that the term ‘‘reference condition’’ is reserved for referring to the ‘‘naturalness’’ of the biota (structure and function) and that naturalness implies the absence of significant human disturbance or alteration. They further propose specific terms to characterize the expected condition to which current conditions are compared: ‘‘minimally disturbed condition,’’ ‘‘historical condition,’’ ‘‘least disturbed condition,’’ and ‘‘best attainable condition.’’ A similar concept of reference conditions is considered in the USEPA-SAB reporting framework (Young and Sanzone, 2002): “Reference conditions that attempt to define a ‘healthy’ ecological system are often derived from either the conditions that existed prior to anthropogenic disturbance or conditions in a relatively undisturbed but comparable system in the ecoregion. Alternatively, reference conditions can be inferred from a combination of historical data, a composite of best remaining regional conditions, and professional judgment.”

Ideally, reference conditions will include sites with little or no indication of stressors associated with human disturbance. However, this is not always the case and most landscapes have already been altered. Where undisturbed sites are absent, Stoddard et al. (2006) propose a combination of methods to determine reference conditions: (1) sampling biota from least disturbed sites (reference sites), (2) interpreting historical records to deduce which characteristics occurred at times with substantially less human disturbance, (3) developing models that incorporate the best ecological knowledge, and (4) using best professional judgment.

For each indicator in the Feather River Watershed Report Card, targets are described (Section 3). These range from an estimate of historical condition, to legal guidance for endangered species, to comparisons against the best or worst condition in the watershed. Targets were set for both the good reference condition (score = 100) and for the poor reference condition (score = 0). Indicators were evaluated on this range from poor to good reference conditions.