Judy Sielkie, age 17, of St. Vital, Manitoba, Canada, for her question:

Is symbiosis harmful to plants or animals?

Symbiosis means sharing life on a give and take basis. In all the realm of nature, surely there can be nothing harmful in such a sensible theme. On an individual scale, a lichen is a perfect example of a symbiotic relationship. It is a partnership of two small plants, an alga and a fungus. The fibrous fungus traps enough plant food for both. Other symbiotic partnerships may be between a plant and an animal or between two animals. In your own intestines, certain bacteria thrive on your food and in the process help you to digest it. This too is symbiosis.

This sort of give and take works for all living things. In a way, our global ecology is a network of exchanges between countless systems of symbiosis. The green plant worls needs the waste carbon dioxide given away by the animal kingdom. It uses this gas to manufacture its food and in the process returns fresh supplies of breathable oxygen to the air. This is symbiosis on a planetary scale    and no plant or animal could survive without it.