Stressor Identification and Causal Assessment

Stressor Identification

Relative risk is a common method used in the medical field such as determining your risk factors of cancer if you smoke because it is relatively easy for the public to interpret. Relative risk can be used to determine what instream stressors are associated with poor biological conditions (Van Sickle et al. 2006). Relative risk assesses what proportion of sites with poor biological condition also have poor condition for a given potential stressor. In this example we use fine sediment. 92% of sites with poor sediment conditions also had poor biological conditions. And 60% of sites with poor biological conditions also had poor sediment conditions. If we divide these two proportions, this tells us that a stream is 1.53 times more likely to be in poor biological condition if sediment condition is also poor.

Relative Risk (RR): association between macroinvertebrate condition and the condition of individual stressors:

Relative risk equation of probability poor biological condition given poor sediment conditon divided by probability poor biological condition give good sediment condition

Stream is 1.53 times more likely to be in poor biological condition if sediment condition is also poor.

Potential steps for causal assessment

  • Consider scale and determine what kind of causal analysis is appropriate
    • Control Impact or Before After Control Impact studies
    • Correlative analyses (e.g. Vander Laan et al. 2013)
  • List standards not achieved, reasons for not meeting and indicators used
  • List all possible causes
    • conceptual diagrams
  • Consider natural disturbances
    • covariates and levels of departure from your benchmarks
  • Determine associated land uses
    • Random forest and GIS variables
  • Review ancillary data 
    • Large scale ecological assessment and planning tools
      Local grazing records, project records, local history

Is grazing a causal factor of poor macroinvertebrate condition?

Connected list of grazing factors on Macroinvertebrates. At the top, %Grazing with Riparian vegetation underneath. On the right of riparian vegetation, bank stability, erosivity, slope, and fine sediment which ends at macroinvertebrates. On the left, canopy cover, stream size, elevation, stream temperature, and the connecting to macroinvertebrates.

Literature Cited

  • Van Sickle, J., J. L. Stoddard, S. G. Paulsen, and A. R. Olsen. 2006. Using Relative Risk to Compare the Effects of Aquatic Stressors at a Regional Scale. Environmental Management 38:1020–1030.
  • Vander Laan, J. J., C. P. Hawkins, J. R. Olson, and R. a. Hill. 2013. Linking land use, in-stream stressors, and biological condition to infer causes of regional ecological impairment in streams. Freshwater Science 32:801–820.