Passive plants not so defenseless

Posted: Sunday, March 09, 2008

Much of the world is green-covered with verdant vegetation, so one might think there's plenty of food out there for all the millions of herbivorous animals that eat greenery. All those deer and rabbits and caterpillars, to mention a few, generally do not seem to gobble up the vegetation and eat it down to nubbins. But why not?

There are pretty good reasons why there is so much greenery out there. Virtually all plants have at least one, and often several, means of self-defense against herbivores. Nothing that resembles karate or aikido, no long fangs or slashing claws.

But an impressive array of defensive mechanisms, nevertheless. Here is a sampling:

Any Juneau hiker is well acquainted with our thickets of devil's club, whose stems are loaded with spines that fester in our skin. Don't forget the stinging nettles that commonly guard the start of the Granite Basin trail.

The little hooks on salmonberry stems are minor annoyances to us, but the curved hooks of the closely related blackberry species are far more formidable. Those hooks and stings and spines did not evolve to protect the plants from us, but we get to "enjoy" the consequences as a by-product of the plants' devices to deter herbivores.

Hairs and tiny hooks on leaves or stems are a common defense against insects; the hairs entangle or sometimes puncture the insect herbivore. On some plants, the hairs are sticky, creating an effective trap against many insects. In some cases, a hard outer covering, such as thick bark, is an effective defense against certain herbivores.

Almost all plants have some means of chemical defense. Some chemicals deter the approach of an herbivore, some make the plant poorly digestible, and some are downright toxic.

Certain plant chemicals just smell bad to a potential consumer, perhaps resembling the smell of another plant with which the herbivore had a previous bad experience.

Digestibility may be reduced by packing plant tissues full of cellulose, lignin or silica, which are structural elements that also are hard or impossible for most animals to digest. Digestibility is also reduced by interfering with the activity of digestive enzymes in the consumer. Tannins, which are found in most woody plants and many herbs, have this effect.

There are thousands of plant toxins. They typically work by blocking specific biochemical pathways, such as particular aspects of cellular metabolism. There are alkaloids such as strychnine, caffeine, nicotine and cocaine.

Terpenoids (think "turpentine") include the pyrethrins used in organic insecticides, a monoterpene in red cedar that inhibits insect attack, and compounds that interfere with the molting process in insects. A number of plants generate cyanide when their tissues are damaged. Cyanide is lethal to cellular metabolism.

Curiously, many of these plant toxins have human medicinal uses, usually in small doses.

Some plants "bleed" nasty juices when an herbivore nibbles on them. Milkweeds (which grow in the Lower 48) are well-known examples, oozing white sticky sap when damaged. This gums up the jaws of insects and delivers a good dose of toxins as well.

Resins produced by conifer trees and birches are gummy and presumably also contain terpenoids, and trees respond to damage by producing copious amounts of resin.

On Alaskan birches, there are more resin glands on low branches within reach of snowshoe hares than on higher branches that hares cannot readily reach.

Among the most fascinating means of defense are those that are induced by initial damage by an herbivore. After an herbivore eats some bit of these plants, the rest of the plant generates increased amounts of chemical protection, which helps prevent further damage.

In some cases, there is a chemical message carried within the plant and circulated to other parts of the plant. In other cases, there is no internal circulation of such messages - they are airborne, wafted in the breeze from one branch to another.

Even more fascinating are the plants that "eavesdrop" on their close neighbors.

After an herbivore damages one plant, an airborne chemical response is emitted and picked up by neighboring plants, which react by producing more chemical defense.

Airborne communication is known among birch trees, alders, and in a few other species, such as sagebrush. When one is chomped by an herbivore, its neighbors ramp up their own defenses.

Even though this phenomenon is apparently rare in the plant world, it fascinating in part because we so often think of plants as rather passive organisms.

Plants have this huge arsenal of defenses, but no defense is perfect. Eventually, some consumer evolves a means of breaching each defense.

When humans select strains of soybeans or corn or cotton or any other domesticated crop, they usually produce plants that have fewer defenses than the wild types.

Fewer defenses means it is easier for us to use the plant, but the plants still need protection from their (other) enemies, especially when they are grown in huge monocultures spread over many acres. Hence, the burgeoning of pesticide application, which often ends up poisoning us, in turn. A nice irony!

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.

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