Chemical defenses

Chemical defense is the use of chemical compounds by biological organisms to deter herbivory and predation. Chemical defenses can also be used in competitive interactions between species to prevent overgrowth or maintain spatial dominance.

Chemical defenses are one of the most common adaptive strategies employed by plants to lower herbivory. The production of capsaicin in many strains of bell peppers is one such defense familiar to humans. These defenses are common and can be direct or indirect and happen in response to plant damage, genetic modification, and can affect survival, and growth. Chemical defenses can also be in the form of non-volatile compounds that can attract natural predators of the herbivore. These defenses happen by themselves or in conjunction with others. The bark of many species of conifers contain high amounts of phenolics. These chemical defenses act as anti-fungals and inhibit herbivory by producing hydrolytic enzymes that prevent many insects from penetrating the tough exterior found on many conifers.

Chemical defense is most common in insects, but can also occur in mammals. The skunk is a particularly dramatic mammalian example. Other examples include the bombardier beetle which can accurately shoot a predator with a stream of boiling poison, the ornate moth which excretes a frothy alkaloid mixture, and the Pacific beetle cockroach sprays a quinone mixture from modified spiracles.

The chemicals used for defense produced by the plant species depend on the amount of nutrients available in the environment. Fast growing species with lots of nutrients available to use often have a diverse amount of chemicals that have a high turn over rate. Slow growing species with low nutrient levels often have lignins or polyphenolic chemicals that have dosage dependent effects that have slow turn over rates.

Marine invertebrate animals also harbor chemical defenses that protect them from predators, particularly tropical marine sponges,gorgonian corals,nudibranch molluscs, and tunicates.

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