Al Peters, age 13, of Houston, Texas, for his question:

What do they mean by the Balance of Nature?

Ecologists mention the balance of nature, especially when warning us not to upset it. Naturally, we wish to strive for a better world by preserving our environment. But this balance of nature theme may seem somewhat mysterious. Actually, the expression is much older than the word ecology. People used it ages ago, when most of them were not too sure how it really worked.

Let's take a science fiction journey into the far future. A couple of billion years from now, earthlings will be seeking a new home planet because their glorious sun will be aging. Space travelers will scout the Galaxy to find earth type planets near younger suns. Now and then, one of these space explorers will be lucky. Suppose he locates an uninhabited planet with land and oceans, earth type gravity and an earth type atmosphere. It is about 93 million miles from a medium star which is several billion years younger than the sun.

This would be just the beginning. Our lucky space scout would need to make preparations for human habitation. Let's say he leaves a million or so plant and animal species to make their way in the new world. His work is done and the folks back home do not revisit the place for five million years. My, how things had changed! Several species failed to survive and almost all the others moved themselves to new locations.

The original grasses struggled along until the winds blew their seed to more suitable soil. Forests established themselves in deep, moist soil. Naturally the animals west to live with their favorite plants. Rabbits thrived on the grassy prairies and provided meaty food for the coyotes. Birds and squirrels found food and shelter in the woods.

Groups of plants and animals found suitable homes    homes they had to share with friends and foes. They settled various ecology systems in valleys and mountains, meadows and marshes. In each ecosystem, every species depended on the local geography. But to keep an ecosystem going, each inter dependent species must give and take only a fair share.

For example, the right number of coyotes consume just enough rabbits to keep their numbers from stripping away all the grasses. If this happened, both rabbits and coyotes would starve. This is where the balance of nature takes control. Coyotes multiply when rabbit food is plentiful and decline when their meat supplies run low. The hunters and the hunted balance each other and all local species are limited.

All goes well until this balance of nature is upset, then everything goes wrong. For example, when a marsh is drained many birds lose their natural nesting grounds. When they perish, their diets of insects and weed seeds are left to multiply unchecked. On our imaginary planet, many such upsets occurred while the ecosystems were being established. On the planet Earth, such tragedies happen every time we upset the balance of nature.