In freshwater health assessments, which organism group is commonly used as indicator species due to their sensitivity to pollution?

• A. Fish
• B. Macroinvertebrates
• C. Mammals
• D. Reptiles

Answer: (B)

Macroinvertebrates are ideal indicators because their presence and absence directly reflect the water quality of a stream over time. They include species with a wide range of tolerance to pollutants, so pollution tends to reduce sensitive groups (like mayflies and stoneflies) while tolerant ones persist, creating a clear signal of ecological condition. They are abundant and relatively easy to sample from the stream bottom, and their life cycles are long enough to integrate water quality over weeks to months, not just a single moment. Because many stay close to their habitat and don’t move far, their communities accurately reflect local conditions. While fish can be informative, they’re more mobile and influenced by multiple factors, and mammals or reptiles are not practical indicators for standard freshwater assessments; macroinvertebrates provide a practical, reliable measure of health.