What moves the score
Context Pollution-sensitive animals drive the swings: they add 2 to 18 points per survey.
The SOS score counts three points for each pollution-sensitive group found, two for each less-sensitive group and one for each tolerant group.
Finding or missing one or two sensitive groups such as mayflies, stoneflies or riffle beetles moves the total by several points. Tolerant animals such as midges and worms add little in any year, which is a good sign.