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The study of ecological interactions mediated by the chemicals that organisms produce. These substances, known as allelochemicals, serve a variety of functions. They influence or regulate interspecific and intraspecific interactions of microorganisms, plants, and animals, and operate within and between all trophic levels—producers, consumers, and decomposers—and in terrestrial, fresh-water, and marine ecosystems.
Function is an important criterion for the classification of allelochemicals. Allelochemicals beneficial to the emitter are called allomones; those beneficial to the recipient are called kairomones. An allomone to one organism can be a kairomone to another. For example, floral scents benefit the plant (allomones) by encouraging pollinators, but also benefit the insect (kairomones) by providing a cue for the location of nectar.
The chemicals involved are diverse in structure and are often of low molecular weight (<10,000). They may be volatile or nonvolatile; water-soluble or fat-soluble. Proteins, polypeptides, and amino acids are also found to play an important role.
Plant allelochemicals are often called secondary compounds or metabolites to distinguish them from those chemicals involved in primary metabolism, although this distinction is not always clear.
Chemical defense in plants
Perhaps to compensate for their immobility, plants have made wide use of chemicals for protection against competitors, pathogens, herbivores, and abiotic stresses. A chemically mediated competitive interaction between higher plants is referred to as allelopathy. Allelopathy appears to occur in many plants, may involve phenolics or terpenoids that are modified in the soil by microorganisms, and is at least partly responsible for the organization of some plant communities. See Allelopathy
Chemicals that are mobilized in response to stress or attack are referred to as active or inducible chemicals, while those that are always present in the plant are referred to as passive or constitutive. In many plants, fungus attack induces the production of defensive compounds called phytoalexins, a diverse chemical group that includes isoflavonoids, terpenoids, polyacetylenes, and furanocoumarins. See Phytoalexins
Defensive chemicals can be induced by herbivore attack. There has been increasing evidence that inducible defenses, such as phenolics, are important in plant-insect interactions.
Constitutive defenses include the chemical hydrogen cyanide. Trefoil, clover, and ferns have been found to exist in two genetically different forms, one containing cyanide (cyanogenic) and one lacking it (acyanogenic); acyanogenic forms are often preferred by several herbivores. See Alkaloid, Flavonoid
Chemical defenses frequently occur together with certain structures which act as physical defenses, such as spines and hairs. While many chemicals protect plants by deterring herbivore feeding or by direct toxic effects, other defenses may act more indirectly. Chemicals that mimic juvenile hormones, the antijuvenile hormone substances found in some plants, either arrest development or cause premature development in certain susceptible insect species.
Plant chemicals potentially affect not only the herbivores that feed directly on the plant, but also the microorganisms, predators, or parasites of the herbivore. For example, the tomato plant contains an alkaloid, tomatine, that is effective against certain insect herbivores. The tomato hornworm, however, is capable of detoxifying this alkaloid and can thus use the plant successfully—but a wasp parasite of the hornworm cannot detoxify tomatine, and its effectiveness in parasitizing the hornworm is reduced. Therefore, one indirect effect of the chemical in the plant may be to reduce the effectiveness of natural enemies of the plant pest, thereby actually working to the disadvantage of the plant.
Most plant chemicals can affect a wide variety of herbivores and microorganisms, because the modes of action of the chemicals they manufacture are based on a similarity of biochemical reaction in most target organisms (for example, cyanide is toxic to most organisms). In addition, many plant chemicals may serve multiple roles: resins in the creosote bush serve to defend against herbivores and pathogens, conserve water, and protect against ultraviolet radiation.
It is argued that there are two different types of defensive chemicals in plants. The first type occurs in relatively small amounts, is often toxic in small doses, and poisons the herbivore. These compounds may also change in concentration in response to plant damage; that is, they are inducible. These kinds of qualitative defensive compounds are the most common in short-lived or weedy species that are often referred to as unapparent. They are also characteristic of fast-growing species with short-lived leaves. In contrast, the second type of defensive chemicals often occurs in high concentrations, is not very toxic, but may inhibit digestion by herbivores and is not very inducible. These quantitative defenses are most common in long-lived, so-called apparent plants such as trees that have slow growth rates and long-lived leaves. Some plants may use both types of defenses.
There is accumulating evidence that marine plants may be protected against grazing by similar classes of chemicals to those found in terrestrial plants. One interesting difference in the marine environment is the large number of halogenated organic compounds that are rare in terrestrial and fresh-water systems.
Through evolution, as plants accumulate defenses, herbivores that are able to bypass the defense in some way are selected for and leave more offspring than others. This in turn selects for new defenses on the part of the plant in a continuing process called coevolution.
Animals that can exploit many plant taxa are called generalists, while those that are restricted to one or a few taxa are called specialists. Specialists often have particular detoxification mechanisms to deal with specific defenses. Some generalists possess powerful, inducible detoxification enzymes, while others exhibit morphological adaptations of the gut which prevent absorption of compounds such as tannins, or provide reservoirs for microorganisms that accomplish the detoxification. Animals may avoid eating plants, or parts of plants, with toxins.
Some herbivores that have completely surmounted the plant toxin barrier use the toxin itself as a cue to aid in locating plants. The common white butterfly, Pieris rapae, for example, uses mustard oil glycosides, which are a deterrent and toxic to many organisms, to find its mustard family hosts.
Chemical defense in animals
Many animals make their own defensive chemicals—such as all of the venoms produced by social insects (bees, wasps, ants), as well as snakes and mites. These venoms are usually proteins, acids or bases, alkaloids, or combinations of chemicals. They are generally injected by biting or stinging, while other defenses are produced as sprays, froths, or droplets from glands.
Animals frequently make the same types of toxins as plants, presumably because their function as protective agents is similar. Other organisms, particularly insects, use plant chemicals to defend themselves. Sequestration may be a low-cost defense mechanism and probably arises when insects specialize on particular plants.
Competitive microbial interactions are regulated by many chemical exchanges involving toxins. They include compounds such as aflatoxin, botulinus toxin, odors of rotting food, hallucinogens, and a variety of antibiotics. See Antibiotic, Toxin
Microorganisms also play a role in chemical interaction with plants and animals that range from the production of toxins that kill insects, such as those produced by the common biological pest control agent Bacillus thuringiensis, to cooperative biochemical detoxification of plant toxins by animal symbionts.
A large area of chemical ecology concerns the isolation and identification of chemicals used for communication. Pheromones, substances produced by an organism that induce a behavioral or physiological response in an individual of the same species, have been studied particularly well in insects. These signals are compounds that are mutually beneficial to the emitter and sender, such as sex attractants, trail markers, and alarm and aggregation signals. Sex pheromones are volatile substances, usually produced by the female to attract males. Each species has a characteristic compound that may differ from that of other species by as little as a few atoms.
Pheromones are typically synthesized directly by the animal and are usually derived from fatty acids. In a few cases the pheromone or its immediate precursors may be derived from plants, as in danaid butterflies.
Very little work has been done in identifying specific pheromones in vertebrates, particularly mammals. It is known, however, that they are important in marking territory, in individual recognition, and in mating and warning signals. Chemical communication may also occur among plants and microorganisms, although it is rarer and less obvious than in animals. See Reproductive behavior, Territoriality