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 posts 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 offenders. 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 eastern 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 discouraging herbivores. Tall fescue and its fungus help each other.
Only very recently have a few pioneers, such as Timothy Johns and Bruce Ames and his collaborators, made us aware of the enormous medical importance of the plant-herbivore arms race. We can heartily recommend Johns’s book With Bitter Herbs Thou Shalt Eat It for an introduction to the role of plant toxins in human history.
Here we are again dealing with an arms race, this time between animals such as ourselves, who eat plants, and the plants, which need to protect themselves from being eaten. When Stone Age inhabitants of central Europe died of starvation late one winter instead of happily filling up on oak buds and acorns, they were losers in the contest with oak trees. Oak buds and acorns are loaded with nutrients, but, unfortunately for potential consumers, they are also loaded with tannins, alkaloids, and other defensive toxins. Early Europeans who filled up on unprocessed oak tissues died even sooner than their starving companions did.
Animals that eat other animals may have to deal with venoms or other harmful materials manufactured by their prey, and they will certainly have to deal with at least traces of the plant toxins eaten by the prey. The monarch butterfly caterpillar, mentioned earlier, feeds on milkweed not only because it has machinery that makes it invulnerable to the milkweed’s deadly cardiac glycosides but also because it becomes poisonous itself by consuming the plant and is therefore avoided by potential predators. Many insects and arthropods protect themselves with venoms and poisons. Many amphibians are poisonous, especially the bright-colored frogs that Amazonian peoples use to poison their arrowheads. The vivid colors and patterns of such poisonous animals protect them from predators, who have learned from bitter experience that such prey are not pleasant food items. If you are starving in a rain forest, eat the camouflaged frog that is hiding in the vegetation, not the bright one sitting resplendent on a nearby branch.
How do plant toxins work? They do whatever will keep herbivores from eating the plants. Why are there so many different toxins? Herbivores would quickly find a way around any one defense, so the arms race creates many different ones. The list of different toxins and their diverse actions is impressive. Some plants make precursors of cyanide, which is released either by enzymes in the plant or by the intestinal bacteria of the consumer. The bitter almond is noteworthy in this regard, but apple and apricot seeds use the same strategy, as do cassava roots, which are used for food in many cultures.
All adaptations, however, have costs, and plants’ defensive chemicals have theirs. Toxin manufacture requires materials and energy, and the toxins may be dangerous to the plant that produces them. In general, a plant can have high toxin levels or rapid growth, but not both. To put it from the herbivore’s point of view, rapidly growing plant tissues are usually better food than stable or slowly growing structures. This is why leaves are more vulnerable than bark and why the first leaves of spring are especially vulnerable to caterpillars and other pests.
Seeds are often especially poisonous, because their destruction would thwart the plant’s reproductive strategy. Fruits, however, are bright, aromatic packets of sugars and other nutrients specifically designed to be attractive food for animals that can disperse the seeds contained in them. The seeds within the fruit are designed either to be discarded intact (like peach pits) or to pass safely through an intestinal tract (like raspberry seeds) to be deposited at some distant place surrounded by natural fertilizer. If the fruit is eaten before the seeds are ready, the whole investment is wasted, so many plants make potent poisons to discourage consumption of immature fruits, thus the proverbial stomachache caused by green apples. Nectar is likewise designed to be eaten, but only by whatever pollinators are best for the plant that makes it. Nectar is an elaborate cocktail of sugar and dilute poisons. The recipe has evolved as an optimal trade-off between the need to repel the wrong visitors and not discourage the right ones.
Nuts represent a still different strategy. Their hard shells protect them from many animals, and some, like acorns, are also protected by high levels of tannin and other toxins. Though many acorns are eaten, some are trampled into the ground, while others are buried by squirrels and thus have a chance to sprout into new trees. It takes such elaborate processing to turn acorns into human food that we wonder if the tannin may be too much even for squirrels. Perhaps it leaches out when acorns are buried in moist soil. If so, the squirrels are processing as well as hiding their food, a neat ploy in their arms race with the oak. If you find yourself starving in an unknown wilderness, seek your nourishment in soft sweet fruits, the nuts with the hardest shells, and perhaps some inaccessible tubers. Avoid seemingly unprotected fleshy plant materials like leaves; they are much more likely to be poisonous, as they must be to protect them from your own or any other hungry mouth.
Plants’ escalations of the arms race are numerous and varied. Some plants make little defensive toxin until they are mechanically damaged, after which toxin rapidly accumulates in or near the injured part. Damage to a tomato or potato leaf induces production of toxins (proteinase inhibitors) not only at the site of the wound but throughout the plant. A plant has no nervous system, but it does have electrical signaling and a hormone system that can keep all its parts informed about what takes place in a small region. Some aspen trees have even more impressive communication. When a leaf is damaged, a volatile compound (methyl jasmonate) evaporating from the wound can turn on the proteinase response in nearby leaves, even those on other trees. The usual result of such defenses is that insects are discouraged after feeding even briefly. Some particularly adept insects, however, begin their meal by cutting the main supply vein to a leaf so the plant cannot deliver more toxins. And so the arms race goes on.
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