Herbivory and Plant Defenses

Herbivory is the consumption of plant tissues by animals. This usually has a negative impact on plant growth and reproduction, and so imposes nat­ural selection on plants, thereby favoring the evolution of traits that reduce losses (defenses).

Terrestrial plants generally lose less than 10 percent of annual produc­tion to herbivores. Fish can consume 100 percent in some shallow-water marine systems in days or weeks, and single-celled algae are often eaten as fast as they reproduce. A few terrestrial herbivores (gypsy moths or locusts, for example) occasionally experience population explosions during which they may totally defoliate their preferred food plants over wide areas A loss of even 15 percent of annual production can reduce plant growth and fit­ness.

The fact that some herbivores occasionally can consume most or all veg­etation suggests that something prevents this most of the time. Two dom­inant opposing hypotheses are that “top-down” forces (predators, parasites, disease) limit herbivore populations so all plants are not consumed, or that “bottom-up” forces (plant quality) prevent herbivores from eating some plant tissues. In reality both forces combine to limit herbivores and herbivory. Herbivore damage is greater in simplified managed systems, re­flecting the loss of these interactions.

Plants have developed numerous physical (spines, thorns, tough tissues, sticky resins, and hairs) and chemical defenses that may reduce herbivory. Some of these are fixed (constitutive), whereas others are only produced (in­duced) when the plant is attacked. A tremendous diversity of plant biochemical are toxic, repellent, or antinutritive for herbivores of all types. Many of these chemicals have been called “secondary metabolites” because roles have not been found for them in primary plant functions like growth and reproduction. Examples include alkaloids (such as caffeine and nico­tine), terpenoids (terpene and pinene), glucosinolates (sinigrin), and phenolics (tannin and chlorogenic acid).

Although many of these compounds appear to have specific activity in animal systems, such as interfering with neurotransmission, recent research suggests that they may have other functions in plants. Nonetheless, it is clear that plant chemistry is a major barrier to herbivory. It is also ex­ploited by humans in medicinal plant use and pharmaceutical develop­ment.

While all plants produce chemical defenses continuously, all plants so far studied also change or increase production of both physical and chem­ical defenses when attacked by herbivores. Defenses can be induced throughout a plant, even in unattacked tissues or tissues produced after the attack, producing systemic resistance. New physical and chemical de­fenses may be synthesized, enzymes may activate preexisting defenses, or tissues may be dropped (abscised) to remove pests. This requires detec­tion, coordination, and response. Plants can discriminate among and re­spond differentially to wounding (such as wind damage) and herbivores, or even among herbivores.

Chemicals found in insect regurgitant (mouth juices) trigger plant re­sponses; these cues may be produced by the insect or by bacteria living in their guts. Fatty acid signals made by the plant (especially jasmonic acid) in response to attack circulate widely, stimulating defense gene expres­sion and providing systemic resistance. Methyl jasmonate is volatile and escapes wounded plants, triggering defense responses in nearby unwounded plants.

Because producing defenses requires materials (such as carbon and ni­trogen) and energy that presumably could be used for growth or repro­duction, many believe that defense may be costly for plants. Clear evidence of this is difficult to obtain, but some plants grow or reproduce less when producing maximum defenses, providing indirect support. The types of de­fenses employed by fast-growing versus slow-growing, or early- versus late- successional plant species differ in consistent ways, but why this is so is not clear. One benefit of induced defenses could be cost-saving, since they are only produced when needed. Induced or constitutive, plant defense chem­istry influences litter quality, decomposition and nutrient cycling in ecosys­tems.

Herbivores, especially insects, have developed behavioral and biochem­ical mechanisms that reduce the effects of plant defenses. Plant evolution includes ongoing development of new defenses, which is thought to favor the evolution of new herbivore adaptations and promote speciation in both plants and herbivores. This kind of reciprocal evolutionary impact is called coevolution. While all plants gain some protection from defenses, no plant escapes herbivory by at least some adapted herbivores. Plant defenses and consumer adaptation can limit consumption, and thus can determine the length and shape of food webs.

References

Agosta, William C. Thieves, Deceivers and Killers: Tales of Chemistry in Nature. Prince­ton, NJ: Princeton University Press, 2000.

Karban, Rick, and Ian T. Baldwin. Induced Responses to Herbivory. Chicago: Univer­sity of Chicago Press, 1997.

Rosenthal, Gerald, and May R. Berenbaum. Herbivores: Their Interaction with Secondary Plant Metabolites. San Diego, CA: Academic Press, 1991.