Self-Defense for Plants
Pity the poor plant! While the plant-eating animals (herbivores) can move at will from plant to plant, taking bites where they choose, the plants must stand and endure the indignities. Recent research, however, shows the plants are not quite as defenseless against herbivores as their immobility might suggest.
Biologists at the University of Alaska's Institute of Arctic Biology are studying the interactions between snowshoe hares and the plants upon which they feed. The abundance of snowshoe hares changes in a cyclic pattern, with peak densities reached every ten years or so. Populations in various areas of interior Alaska are now nearing their ten-year peak. At such times, the effect of hares on their food plants is near catastrophic. There is strong selection in favor of any plant that has developed a means of escaping the herbivores. Thorns are one obvious means of discouraging herbivores. Many other plants have evolved various forms of chemical defense. Some simply taste awful to discourage the herbivores. Others inhibit digestion by the herbivores, making it unprofitable for the herbivore to eat them. Yet others are outright toxic, and cause illness or death to the herbivore that over-indulges. Many of the plant-derived chemicals that we use, for example quinine and aspirin (which was originally derived from willows), apparently originate as chemical defenses in plants.
All plants are not equally defended. Those that grow, use and drop their leaves in a single season (deciduous plants) seem to invest less in defense than those that retain their leaves over a number of growing seasons (evergreen plants). There are even differences within a single plant: young shoots and branches growing close to the ground, within easy access of the hares, tend to be heavily defended. Older shoots and branches growing out of reach of hares are protected by position and are much less protected chemically. When a birch tree is felled, making the upper branches available to hares, they are quickly attacked with gusto. Penned hares fed branches cut from the tops of trees thrive; the same hares fed only root suckers growing near the ground die of starvation.
The deterrent chemical in birch is a resin that is contained in resin glands along the stem. Stems from the top of a tree have few resin glands, but the well-defined lower branches appear grainy because of them. The resin may make up 44% of the total weight of root suckers, but only 3% of the weight of top branches. This large difference points out the lengths to which plants will go to avoid being eaten.
