The final step in our water quality program is to determine the presence, if any, and variety of aquatic invertebrates, or creatures without a backbone. Aquatic insects and other invertebrates, because they live in the water, are probably the easiest and cheapest way of getting a rough idea of the water quality. They depend on clean, unpolluted water to live in, and if there is pollution entering the system, insects are usually the first to disappear. Using insects to determine water quality is based on the fact that every species has a certain range of physical and chemical conditions in which it can survive.
Some organisms can survive in a wide range of conditions, and are more tolerant of pollution.
Water quality can be determined by comparing the number of pollution-tolerant organisms a with the number of pollution intolerant species. Remember, though, that pollution tolerant organisms can be found in both polluted and unpolluted waters. These are perhaps the most important component of a healthy aquatic habitat. If there are plants, there will undoubtedly be some forms of insects and other small organisms feeding on them or their remains. Insects, then, are a major link in the food chain.