A biotic index is a tolerance value of macro invertebrates that measures how much pollution the animal can withstand. Scientists use biotic indices to measure water quality. They do this by counting the number of macro invertebrates with a low biotic index and then count the number of macro invertebrates with a high biotic index and compare the two numbers. This comparison indicates the pollution level of a body of water.
| Aquatic Insect | Biotic Index |
|---|---|
| Amphipods: scuds | 5 |
| Chironomidae: midge larva | 8 |
| Coleoptera: beetles | 6 |
| Diptera: true flies | 6 |
| Ephemeroptera: mayfly | 3 |
| Gastropoda: snails | 8 |
| Hemiptera: true bugs | 8 |
| Hirudinea: leeches | 9 |
| Isopods: sowbugs | 8 |
| Megaloptera: dobsonfly larva | 4 |
| Odonata: damselfly nymph | 6 |
| Odonata: dragonfly nymph | 5 |
| Oligochaetes: segmented worms | 9 |
| Plecoptera: stonefly nymph | 1 |
| Trichoptera: caddisfly larva | 3 |
To get the biotic index score, you take 100 macro invertebrates from a water sample and multiply the number of each different macro invertebrates that you find by its biotic value, add all these numbers together, and divide this number by 10. A biotic index score above 70 is excellent, between 60 and 79 is good, between 40 and 59 is fair, and below 40 is poor.
