CADDIS Volume 1: Stressor Identification
Step 2: List Candidate Causes
After the case has been defined in Step 1, you begin to consider the candidate causes, or stressors, which may be responsible for the observed biological effects. Listing these candidate causes further refines the scope of the causal analysis, and provides a framework for assembling available data and determining what data are lacking for the causal analysis.
The initial list of candidate causes should include all stressors that could be causing the biological impairment. These stressors may be chemical (e.g., elevated concentrations of metals or ammonia), physical (e.g., increased sediment or water temperature), and/or biological (e.g., increased abundance of an invasive species) in nature. A candidate cause simply can be a proximate stressor, or the agent that organisms contact or with which they co-occur (e.g., low dissolved oxygen concentrations). Alternatively, a candidate cause may include more detailed information about how that proximate stressor produces a response (e.g., low levels of dissolved oxygen asphyxiating fish), or more details of the precursors of the proximate stressor (e.g., increased nutrients leading to increased algal biomass, resulting in low dissolved oxygen once the algae die). More detail is better if it helps identify ways of distinguishing among various candidate causes.
The candidate cause list can be based on many things, including existing data from the site (including information on possible sources), existing knowledge of biological processes or mechanisms, or stakeholder input. Because biological effects usually involve processes at multiple spatial and temporal scales, listing of candidate causes also should involve explicit consideration of the scales at which each candidate cause operates. The initial map delineating the geographic scope of the case may be refined here, by adding pollution sources or environmental conditions that affect which candidate causes are listed.
After the initial list of candidate causes is completed, conceptual model diagrams can be developed for each candidate cause. These models are simple graphics showing the linkages between potential sources, stressors or candidate causes, and biological effects in the case. These models can help you think about potential mechanisms causing the observed effects, which can help refine the list of candidate causes. Because these diagrams can show where different candidate causes may interact, they may be especially useful when multiple stressors contribute to the impairment. The models also provide a framework for keeping track of what data are available and relevant to each candidate cause, as well as an effective way to communicate to stakeholders the working hypotheses and assumptions about how and why effects are occurring.
Once the list of candidate causes has been completed and conceptual models have been drawn, you can begin analysis of the candidate causes. To do this, proceed to Step 3: Evaluate Data from the Case.