NATURAL AND UNNATURAL TOXINS

Righ-proof alcohol is only one of the many novel hazards to which we are exposed. Agricultural pests are con- trolled mainly by insecticides that did not exist before 1940. Silos are perfused with poisonous vapors to protect grain from insects and rodents. Demonstrably toxic chemicals such as nitrates are used to extend the shelf life of our foods. Many workers inhale toxic dust or fumes, and suburbanites spray insecti- cides such as lindane into their trees, often with little regard to the pos- sible effects on themselves or their neighbors. There are heavy metals in our water, pollutants in our air, and radon gas rising from our base- ments. Obviously our modern age is especially hazardous, with respect to poisons in the food we eat and the air we breathe. Right?

Wrong. While we are now exposed to many toxins that did not exist in even the recent past, our exposure to many natural toxins has greatly decreased since the Stone Age and early agricultural times. Recall from the chapters on infectious disease that the contest between consumer and consumed can generate an evolutionary arms race. Plants can’t protect themselves by running away, so they use chemical warfare instead. People have always known that some plants are toxic. Gardening books routinely list plants known to have caused illness or death from being eaten. These lists merely deal with the worst offend- ers. Most plants contain toxins that would be harmful if eaten in more than a minimal amount. Scientists have only recently realized that the toxic substances are not by-products that just happen to be toxic to certain potential consumers; they are the plants’ essential defenses against animals that want to eat them (herbivores), and they play a key role in the ecology of natural communities. People who live in the east- ern United States needn’t look far for an example. Most lawns there are of tall fescue, a grass species popular because it grows fast and resists pests. The fantasy of getting rid of our lawn mowers and letting horses graze our lawns once a week is appealing, but the horses would soon get sick. Most tall fescue is infected at its base with a fungus that makes potent toxins. The grass protects itself by transporting these toxins to the tips of the blades of grass, the perfect location for dis- couraging herbivores. Tall fescue and its fungus help each other.