9.16B Evaluate the use of indicator species as evidence to assess the…

The idea

Indicator species are organisms whose presence or absence reveals pollution levels, because each tolerates a particular range of conditions. In water, bloodworms and sludgeworms tolerate low oxygen and organic pollution, so their abundance suggests polluted water. Freshwater shrimps and stonefly larvae need high oxygen, so they indicate clean, unpolluted water. For air, lichen species differ in sensitivity to sulfur dioxide: some lichens survive only in clean air, while others tolerate pollution. Blackspot fungus on roses is damaged by sulfur dioxide, so its presence suggests cleaner air. Evaluating this evidence means judging reliability: indicator species give a biological signal of overall conditions but can be affected by other factors, so results should be interpreted alongside direct measurements.

What earns the marks

• Bloodworms and sludgeworms tolerate polluted, low-oxygen water, so their presence indicates organic pollution.
• Freshwater shrimps and stonefly larvae require high oxygen levels, so they indicate clean water.
• Different lichen species vary in tolerance to air pollutants such as sulfur dioxide, so lichen communities indicate air quality.
• Blackspot fungus on roses is sensitive to sulfur dioxide, so its presence suggests relatively clean air.
• Evaluation should recognise that indicator species give indirect evidence affected by other factors, so conclusions are strengthened by combining them with direct physical or chemical measurements.

Common mistakes and how to put them right

• Assuming any living organism indicates clean water.

  Put it right: Some species such as bloodworms and sludgeworms indicate polluted water.

• Treating a single indicator species as definitive proof.

  Put it right: Evidence is stronger when several indicator species and direct measurements agree.

• Reversing the lichen and blackspot fungus signals.

  Put it right: Pollution-sensitive species indicate cleaner air, while tolerant species indicate poorer air quality.

Exam tips

• Match each named species to the condition it indicates rather than listing species generically.
• Explain the reasoning (oxygen tolerance or pollutant sensitivity) behind each indicator.
• When evaluating, state a limitation of indicator evidence and how direct measurements could confirm it.