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Nature's Feisty Partnerships
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13326 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
10 / 1987 |
850 Words |
| Author
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Allen M. Young
Allen M. Young is curator and head of the Invertebrate Zoology
Section of the Milwaukee Public Museum. |
The world's fields, forests, and gardens are battlegrounds for nature's intriguing games of survival between insects and the plants they eat. For millions of years, the ever-changing environment has designed and shaped combative associations between plant-feeding insects, called herbivores, and green plants. When walking through the field or forest, we stand in this living theater of struggle, where plants deploy tactics to resist insect attack, and insects counter by cleverly penetrating the green world's lines of defense. The crux of these struggles is the basic axiom of life that all creatures need energy. Plants make energy and insects use it as food.
A major discovery of biological research in recent years is that biologically active molecules mediate insect-plant interactions, antagonistic partnerships in which each participant struggles to outwit the other. Because insect grazing damages leaves and other living tissues, plants evolve mechanisms to free themselves from herbivores, while insects evolve the means to overcome these plant defenses. Plant defense against herbivores has been a prime factor in shaping the tremendous diversity of insect life on Earth.
Because plants are immobile and highly nutritious, they are attractive, accessible food sources for insects. When insects strip a plant of its leaves, flowers, and fruits, the plant dies. Even with moderate grazing, insects mobilize valuable nutrients, energy that would otherwise be used by the plant. Through photosynthesis, plants combine carbon dioxide and water, in the presence of chlorophyll in the leaves and stems, to make sugars, the prime source of energy and the building blocks for growth and reproduction.
Herbivorous insects actively destroy plant tissues where nutrients are synthesized and stored. To counter these attacks, plants have evolved elaborate structural and chemical defenses. Structural defenses such as thorns, resin ducts, and sticky hairs form mechanical barriers to many herbivores.
Far more effective and widespread in nature are insect-repelling chemicals or "plant poisons." These natural insecticides repel, retard growth, or kill many insects. Whereas all plants are nutritious, they differ greatly in the kinds and amounts of chemical defenses. Some of these substances are widely distributed among many groups of plants, while others are very restricted. The most common natural insecticides in plants are the alkaloids, including nicotine, caffeine, cocaine, and many others. Alkaloid-rich plants include tomato, tobacco, and hundreds of
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