Causal Analysis/Diagnosis Decision Information System (CADDIS)

Step 3: Evaluate Data from the Case

• 3.1. Overview
• 3.2. In-Depth Look
• 3.3. Results and Next Steps

3.2.2. Evidence of Exposure or Biological Mechanism

Concept
Measurements of the biota show that relevant exposure to the cause has occurred, or that other biological mechanisms linking the cause to the effect have occurred.

Figure 3-4. Evidence of Exposure or Biological Mechanism. The candidate cause (low dissolved oxygen) of the impairment (dead fish) causes surviving fish to swim at the surface and gulp. Observations of that behavior strengthen the candidate cause and observations that surviving fish are swimming below the surface weakens the candidate cause.
(General explanation of symbols)

Examples
Consider increases in an invasive predator as a candidate cause of decreased native fish abundance. What findings support or weaken the case for increased invasive predators as the cause, based on evidence of exposure or mechanism?

• Supporting evidence - Examination of the invader's gut contents shows that many of the invasive individuals have native fishes in their stomachs.
• Weakening evidence - Examination of the invader's gut contents shows that no native fishes are found in the invader's stomachs.

Other measurements which may provide evidence of exposure or mechanism include:

• Body burden measurements of toxic substances or parasites
• Biomarkers of exposure, such as cytochrome P450 levels
• Behavioral observations, such as avoidance or behaviors such as convulsive swimming), and
• Comparison of responses by organisms with different feeding or life history strategies, which may provide useful mechanistic evidence for causes that rarely leave internal evidence.

How do I analyze the data?
Data relevant to evaluating exposure or a particular mechanism are analyzed by comparing measurements from impaired versus unimpaired sites. Whereas spatial/temporal co-occurrence deals only with measures of the candidate causal agent, or proximate stressor, evidence of exposure or mechanism explicitly considers surrogate measures or measures of other steps in the causal pathway. The analytical challenges inherent in evaluating spatial/temporal co-occurrence also apply here.

• Example analysis worksheet for evidence of exposure or mechanism:
  • Little Scioto River, OH, USA

What evidence would support or weaken the case for a candidate cause?
Supports

• Data showing that organisms at impaired sites have accumulated or had contact with the candidate cause, while organisms at unimpaired sites have not
• Data showing that a specific causal mechanism is acting on organisms at impaired sites, but not at unimpaired sites

Weakens

• Data showing that exposure or a specific causal mechanism has not occurred at impaired sites

Refutes

• Data showing, with a high degree of certainty, that exposure or a specific causal mechanism has not occurred at impaired sites

How do I Score the Evidence?

Helpful tips

• Evidence of exposure or biological mechanism considers intermediate steps between the proximate stressor and observed biological effect, whereas evidence of a causal pathway considers intermediate steps between sources and the proximate stressor.
• Mere spatial/temporal co-occurrence does not establish the occurrence of exposure.