WHY ARE STREAM INSECTS SO COMMON?

by Whit Gibbons

October 17, 2004

Species richness is a scientific term used by professional ecologists, as well as being one that is understandable as a measure of biodiversity.

An ecosystem high in native biodiversity, or rich in the number of species, is generally a healthy one, and certain groups of species are good indicators of ecosystem health. Thus, a high diversity of snake species in a habitat in the Southeast is usually an indication that the habitat is functioning in a normal and natural manner. Snakes are at the top of the food chain, and their presence means a diversity of prey animals must be present. If prey animals are present, certain key plants must also be present. Complex ecosystems with many species of one group of organisms are more likely to have many species of another. Species richness, therefore, is a sign of environmental wealth and health.

Species richness of insects is a good measure of the environmental health of a natural area in most instances. Although many people think of mosquitoes or houseflies when they think of insects, most of the six-legged creatures of the world are benign or even useful from a human viewpoint. And a diverse composition of insect species in a region is a sign of a healthy, natural ecosystem. Bringing the concept to a specific situation, the species richness of insects in streams is an indicator of the well-being of the waters and the surrounding watershed.

A study by Neal J. Voelz of St. Cloud State University in Minnesota and J. Vaun McArthur of the University of Georgia's Savannah River Ecology Laboratory explored the factors that influence insect species richness in streams. The researchers examined a variety of stream systems throughout the world to determine what caused the numbers of species present in particular streams. Their findings, which were published in a major ecological journal, indeed gave insight into some of the factors that can lead to high biodiversity. But the most impressive feature of their report to me was a fact they presented about one particular stream, Upper Three Runs Creek (UTRC) near Aiken, S.C. In short, UTRC has the highest species richness of insects of any stream in North America, and possibly the world. In addition, the stream has a variety of other species of aquatic animals, including freshwater shrimp, clams, crayfish, and dozens of species of fishes.

Information that a stream in your county, state, or region might qualify for the Guinness Book of World Records is exciting. The news is even better when the qualifying criterion is that the stream has characteristics that indicate it is a healthy one.

The importance of the high diversity of aquatic animals in UTRC from a conservation standpoint cannot be overstated. High-quality streams and the animals that inhabit them are steadily declining across our country and around the globe. So, having a stream that has all the signs of environmental integrity is a commodity worth keeping. Knowing the factors that make UTRC so special is the first step in maintaining such a system for posterity.

Based on the study conducted, two factors were deemed important in creating high species richness among streams--the stream productivity (such as abundant aquatic vegetation) and a wide diversity of habitats in which animals can live. For example, root masses under banks, sections of shifting sand, and numerous snags create suitable living situations for a variety of aquatic insects and the animals that eat them. Upper Three Runs Creek has a richness of habitats that surpasses most other streams that have been studied.

One explanation for why Upper Three Runs has such a variety of natural habitats to support the animal communities is that parts of the stream flow through more than 20 miles of environmentally protected habitat on a defense facility, the Savannah River Site (SRS). Most of the stream travels through a landscape that suffers minimal impacts from agricultural, urban, or industrial activities. Only the upper reaches of the creek are vulnerable to human influences that could affect its environmental health. If the headwaters, which are outside of the SRS, can be kept intact through responsible development, perhaps Upper Three Runs will one day actually appear in the Guinness Book of World Records.



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