Plants show numerous responses to being grazed. Two are the most commonly studied. Of these, morphological changes are usually obvious. The plants produce thickened bark or tough leaves, making them difficult to chew and thus preventing some of the damage done by grazers. Plants can also produce thorns or spikes, preventing some grazers from being able to climb on in order to eat their soft tissues, or dissuading animals to bite them because of the damage inflicted by the plant's defenses.
The second major form of defense is chemical. The plants respond to grazing by producing bad-tasting or toxic chemicals. These either dissuade an actively foraging herbivore from continuing to feed or cause it to learn to avoid that type of plant (when the herbivore gets other, correlated cues from the plant so that it can recognize it without having to bite it).
The chemicals used in defense seem to be of two general types. First, there are compounds that are produced in the plant's normal metabolism that have noxious or toxic properties when contacted or ingested by animals. The plants respond to herbivory by accumulating these otherwise waste products, so that they are present in their tissues in sufficient quantities to deter grazers.
Second, the chemicals may be produced specifically in response to attacks by grazers, but would otherwise not be part of normal metabolism. Most of such chemicals would be produced at an energetic cost to the plants, in contrast to the former types, which are produced whether or not grazers attack them.
Both groups of compounds are called secondary substances or secondary metabolites. They are found in some species or populations of plants, but not others and are not an absolute necessity for metabolism of all plants. The diversity of types of compounds is enormous and the ecology of chemical defense by plants is a vast topic in its own right, so will only briefly be illustrated here.
A well-known example of mechanical defense is that of oak trees in Europe. This species is attacked by larvae of more than 200 species of butterflies and moths, but most attack is in spring when the leaves of the trees are young. The leaves change color and toughness during spring. Experimental rearing of larvae showed greater growth on a diet of young leaves (those first produced in the northern spring) than of those fed on older leaves (those collected in the third and subsequent week of the appearance of leaves). The older leaves suffered from reduced grazing, primarily because they are tougher, with a thicker cuticle.
If this were all that happened, larvae should be able to grow well when fed older leaves that have been broken up, so that the outer layers are no longer a barrier. Experimental provision of groundup older leaves demonstrated that, indeed, once the physical defenses of the leaves were removed, larvae grew well on a diet of older leaves.
A particularly well-studied example of chemical defense is the production of indole glucosinolate compounds by cabbages when they are attacked by herbivores. The chemicals are a deterrent to attack by some herbivores, such as slugs and birds. When grazed, the plants start to produce more of the defensive chemicals and grazing is subsequently reduced or prevented.
Induction of defensive chemicals rather than the plants always containing them is believed to be response by the plants to the behavior of specialist species of grazers. Thus, experimental evidence demonstrates that production of glucosinolates causes increased attack by larvae of cabbage-white butterflies and some other species. Permanent production of the defensive chemicals would lead to increased capacity of the specialist grazers to find and attack the plants. Producing the chemicals only in response to grazing would lessen this risk.
There are other influences on production of chemical defenses. For example, insects are quite often found destructively grazing trees which are normally attacked only in minor ways. One explanation for this is that the secondary metabolites can only be produced at some costs in terms of energy. When plants are stressed by environmental factors such as drought or reduced amounts of nutrients (e.g., nitrogen), the production of the energetically expensive chemical compounds is reduced. Attacks by insects are then much more severe. The interaction of excessive grazing on top ofother environmental stresses is then a serious problem for the survival of the plants.
Another aspect of chemical defenses that is currently receiving ecological attention is that of tri- or multi-trophic defense. This occurs when plants subjected to grazing release chemicals to which other organisms respond. Some of the responding animals are predators, which arrive and start to eat the grazers, thereby reducing the intensity of grazing. These mechanisms of defense are considered to be indirect or extrinsic, in contrast to the more straightforward direct or intrinsic defenses occurring when the plant itself reduces the intensity of grazing.
The principle of indirect defense is best illustrated by an example involving plants that are attacked by spider mites (Tetranychus urticae). When the mites begin to eat a plant, it releases volatile compounds from the damaged tissues. These are attractive to predatory mites (Phytoseiulus persimilis) which arrive at the damaged plant and proceed to eat the plant-eating mites. On the plant that is being attacked, undamaged leaves can also begin to produce chemicals that signal attractiveness to the predatory mites. Adjacent, unattacked plants can also start to emit the signaling chemicals. The chemicals used for such tritrophic (plant-herbivore-carnivore) interactions are of two different types. Some are specific - they are only produced and released from the plant when it is attacked. Others are the same chemicals normally found in undamaged or mechanically damaged plant tissue. These compounds are, however, released in much greater quantities when the plants are attacked by herbivores. Experimental removal of grazers from damaged plants has demonstrated that the chemicals which attract predators are produced by the plants and not by the grazers themselves.
A final aspect of defense against grazers concerns the biology of plants that cause predators of their grazers to live permanently with them, thus deterring or reducing the activities of grazers.
Some features of plants that encourage predators to live with them are the construction of sheltering galls or special sized 'domatia' which are inhabited by predators or fungal-feeding arthropods. These structures are modified veins of leaves or specialized pits or crevices in the cuticle. They provide shelter from wind and weather and can also prevent attacks by enemies of the predators (hyperpredators: consumers of other predatory species) of the animals that live in them. Grazing species that might otherwise use these refuges are generally consumed by the predatory species that live there. The presence of permanent populations of their predators clearly reduces the number and effectiveness of grazers. Grazers are rarely found in the domatia on the plants.
Other plants maintain populations of defensive predators by providing food for them. A good example are the acacia ants (Pseudomyrmex spp.) in tropical regions of America. Swollen-thorn acacias have hollow thorns in which the ants live permanently, feeding on sugars, proteins, and oils produced by the plants. When ants are experimentally removed from plants, the acacias grow much more slowly and more of them die as a result of grazing. The ants also forage on the plants, but attack any grazing species that they encounter. They also remove seeds and any other new vegetation that appears on the ground around the base of their host tree. This potentially reduces competition from surrounding plants, resulting in yet more protection for the host acacias.
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