The Stream Doctor

January 27th, 2003

Your questions and answers about everything stream related.


Q. From Bruce in Wisconsin...I saw a pyramid, once, showing that there were more pounds of ants per square mile, than grizzly bears, in a given area where grizzlies lived. Do stream habitats follow that kind of pyramid, with more pounds of mayflies per mile, than trout, for example?

A. The answer to your question is "yes," with qualifications. Let me explain. The general pyramid scheme you refer to holds true as long as you are making broad classifications of the organisms in the pyramid. For instance, if you use the classifications of plants, herbivores, carnivores, the pyramid exists whether you base it on numbers, weights, or even calories of energy. Where the pyramid generalization starts to break down is when you start breaking up these classifications into their constituents, e.g., the different kinds of plants (algae, vascular plants), or even further into the different species. Or breaking down the herbivores into all the different insects and other macroinvertebrates that feed on plants in different ways. Then, when you start listing these, and drawing arrows connecting who feeds on what, you come up with a food-web, rather than a simple pyramid. Both can be accurate; one just tells you more about how the individual parts of the ecosystem fit together.

In my book, I use a quote from a colleague who states that the general rule of thumb says that it takes 10 times more energy to support each succeeding food level in a stream, so this pyramid would decrease by 10 times for each level. There are some interesting anomalies found. In a study of brown trout in a New Zealand stream many years ago, K.R. Allen found that there was not enough food in the stream to support the trout population, yet there it was! The answer was that he was measuring what was present at a given time of sampling; he should have considered the fact that the insect food population reproduced itself many times during the year. Thus, the food available to the fish was more than he measured at a given time. This became known as Allen's Paradox, and still receives attention from researchers.

This is probably more than you wanted to read about this, but you asked a good question.

~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584
Email: streamdoctor@aol.com

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at streamdoctor@aol.com.


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