What Bugs Tell Us

What Bugs Tell Us

This spring, find your way to a nearby steam and turn over a large rock from a riffle. The scurrying or clinging little life forms you uncover are called benthic macroinvertebrates by scientists who are trying to tell us that these are the spineless animals that we can see living at the bottom of the creek. Invertebrates, such as mayflies, are critical to the aquatic ecosystem; without them there would be no fish, no heron, and no fisherman. Like earthworms in the soil, they eat dead organic matter like leaves, and themselves become food for larger predators. By bridging death and life, they form an essential link in the nutrient cycle.

Playing in the creek as a boy, I was amazed by the confusing array of different critters I could find. This probably meant I was playing in a healthy stream. As we learn to identify these creatures, we find that cleaner waters support more biodiversity. This general observation has been distilled by science:

"Freshwater invertebrates are used more often than any other group of organisms to assess the health of freshwater environments. Some kinds are very sensitive to stress produced by pollution, habitat modification, or severe natural events, while others are tolerant of some types of stress. Taking samples of freshwater invertebrates and identifying the organisms present can reveal whether a body of water is healthy or ill, and the likely cause of the problem if one exists, much like an examination by a physician." (Voshell, A Guide to Common Freshwater Invertebrates of North America, p. 11)

When we find a diverse array of invertebrates that cannot survive in a polluted stream, we know we have an intact ecosystem. On the other hand, upon pulling a seine net from a stream to find only a homogenous sample of a certain fly larva or snail, we may conclude that this body of water is in distress.

Assessing ecosystem health by sampling the diversity of life it supports, calledbiomonitoring, shows scientists the big picture. While a chemical test of the water may yield precise numbers, it offers only a limited snapshot; recent weather conditions and seasonal variation can easily distort the picture. Biomonitoring, on the other hand, tells a more complete and subtle story of the life and times of the particular stream of interest.

In spring many freshwater invertebrates are getting ready to swim to the surface and undergo metamorphosis, much like a caterpillar becoming a butterfly. Emerging from their old skin, they forego the swimming, clinging existence of a nymph for the life of a flying adult. These are the dragonflies, damselflies, mayflies, stoneflies, caddis flies, crane flies, and even the gnats and mosquitoes. All these creatures are as aquatic as they are airborne. It’s a dramatic season of change, and the perfect time to head for the creek to look under rocks and explore the world below. By learning to identify these fascinating animals, you’ll soon be predicting the health of your local creek.

Learn more:
Online key for freshwater invertebrates. >>The best book to learn to identify and understand freshwater invertebrates. >>