Chemical defense is an anti-predator defense strategy employed by many organisms to avoid consumption by other organisms, by producing toxins or irritant metabolites or chemical deterrents that incite avoidance behaviors in the latter. The production of defensive chemicals occurs naturally in plants, fungi and both invertebrate and vertebrate animals, as well as in microorganisms such as bacteria and protists. The class of chemicals produced by organisms that are considered defensive may be considered in a strict sense to only apply to those aiding an organism in escaping herbivory or predation. However, the distinction between types of chemical interaction is subjective and defensive chemicals may also be considered to protect against reduced fitness by pests, parasites and other competitors.
Repellent rather than toxic metabolites are allomones, a sub-category signaling metabolites known as semiochemicals. Many chemicals used for defensive purposes are secondary metabolites derived from primary metabolites which serve a physiological purpose in the organism. Secondary metabolites produced by plants are consumed and sequestered by a variety of arthropods and, in turn, toxins found in some amphibians, reptiles and even pitohui birds can be traced back to arthropod prey. There are a variety of special cases for considering mammalian anti-predator adaptations as chemical defenses as well.
Prokaryotes and fungi
Bacteria of the genera Chromobacterium, Janthinobacterium, and Pseudoalteromonas produce a toxic secondary metabolite, violacein, to deter protozoan predation. Violacein is released when bacteria are consumed, killing the protozoan. Another bacteria, Pseudomonas aeruginosa, aggregates into quorum sensing biofilms which may aid the coordinated release of toxins to protect against predation by protozoans. Flagellates were allowed to grow and were present in a biofilm of P. aeruginosa grown for three days, but no flagellates were detected after seven days. This suggests that concentrated and coordinated release of extracellular toxins by biofilms has a greater effect than unicellular excretions. Bacterial growth is inhibited not only by bacterial toxins, but also by secondary metabolites produced by fungi as well. The most well-known of these, first discovered and published by Alexander Fleming in 1929, described the antibacterial properties of a "mould juice" isolated from Penicillium notatum. He named the substance penicillin, and it became the world's first broad-spectrum antibiotic. Many fungi are either pathogenic saprophytic, or live within plants without harming them as endophytes, and many of these have been documented to produce chemicals with antagonistic effects against a variety of organisms, including fungi, bacteria, and protozoa. Studies of coprophilous fungi have found antifungal agents which reduce the fitness of competing fungi. In addition, sclerotia of Aspergillus flavus contained a number of previously unknown aflavinines which were much more effective at reducing predation by the fungivorous beetle, Carpophilus hemipterus, than aflatoxins which A. flavus also produced and it has been hypothesized that ergot alkaloids, mycotoxins produced by Claviceps purpurea, may have evolved to discourage herbivory of the host plant.
Lichen Lichens demonstrate chemical defenses similar to those mentioned above. Their defenses act against herbivores and pathogens including bacterial, viral, and fungal varieties. To that end, a variety of chemicals are produced by the lichen's mycobiont via hydrocarbons produced by the lichen's photobiont.Bhattacharyya, S., Deep, P. R., Singh, S., & Nayak, B. (2016). Lichen secondary metabolites and its biological activity. Am. J. PharmTech Res, 6(6), 1-7. However, a single defensive chemical may serve multiple purposes. Usnic acid, for example, is implicated across anti-bacterial, -viral, and -fungal actions.Emmerich, R., Giez, I., Lange, O. L., & Proksch, P. (1993). Toxicity and antifeedant activity of lichen compounds against the polyphagous herbivorous insect Spodoptera littoralis. Phytochemistry, 33(6), 1389-1394. Such defensive chemicals may be stored in various tissue types of the lichen thallus, or they may accumulate on the mycobiont hyphae as extracellular crystals.Molnár, K., & Farkas, E. (2010). Current results on biological activities of lichen secondary metabolites: a review. Zeitschrift für Naturforschung C, 65(3-4), 157-173.
Mycobiont-produced acids, including but not limited to, evernic, stictic, and squamatic acids exhibit allelopathy, more specifically, lichen defensive chemicals may inhibit a primary metabolic pathway within competing lichens, mosses, microorganisms, and vascular plants. Documented allelopathic targets include jack pine, white spruce, and garden variety tomato, cabbage, lettuce, and pepper plants. Antimicrobial efforts of lichen are also mediated by various mycobiont-produced acids such as lecanoric and gyrophoric. Similar defensive chemicals were found to inhibit herbivores and insects. Some of these lichen defensive compounds show pharmaceutical potential, too.Furmanek, Ł., Czarnota, P., & Seaward, M. R. (2022). A review of the potential of lichen substances as antifungal agents: the effects of extracts and lichen secondary metabolites on Fusarium fungi. Archives of Microbiology, 204(8), 1-31.
In 2004, the death of hundreds of elk near Rawlins, Wyoming was linked to consumption of tumbleweed shield lichen (Xanthoparmelia chlorochroa). Given that such poisoning is very rare though consumption of this lichen is common, this case of poisoning was unusual.Cook, W. E., Raisbeck, M. F., Cornish, T. E., Williams, E. S., Brown, B., Hiatt, G., & Kreeger, T. J. (2007). Paresis and death in elk (Cervus elaphus) due to lichen intoxication in Wyoming. Journal of wildlife diseases, 43(3), 498-503
Plants
A wealth of literature exists on the defensive chemistry of secondary metabolites produced by terrestrial plants and their antagonistic effects on pests and pathogens, likely because human society depends upon large-scale agricultural production to sustain global commerce. Since the 1950s, over 200,000 secondary metabolites have been documented in plants. These compounds serve a variety of physiological and allelochemical purposes, and provide a sufficient stock for the evolution of defensive chemicals. Examples of common secondary metabolites used as chemical defenses by plants include alkaloids, phenols, and terpenes. Defensive chemicals used to avoid consumption may be broadly characterized as either toxins or substances reducing the digestive capacity of herbivores. Although toxins are defined in a broad sense as any substance produced by an organism that reduces the fitness of another, in a more specific sense toxins are substances which directly affect and diminish the functioning of certain metabolic pathways. Toxins are minor constituents (<2% dry weight), active in small concentrations, and more present in flowers and young leaves. On the other hand, indigestible compounds make up to 60% dry weight of tissue and are predominately found in mature, woody species. Many alkaloids, pyrethrins, and phenols are toxins. Tannins are major inhibitors of digestion and are polyphenolic compounds with large molecular weights. Lignin and cellulose are important structural elements in plants and are also usually highly indigestible. Tannins are also toxic against pathogenic fungi at natural concentrations in a variety of woody tissues. Not only useful as deterrents to pathogens or consumers, some of the chemicals produced by plants are effective in inhibiting competitors as well. Two separate shrub communities in the California chaparral were found to produce phenolic compounds and volatile terpenes which accumulated in soil and prevented various herbs from growing near the shrubs. Other plants were only observed to grow when fire removed shrubs, but herbs subsequently died off after shrubs returned. Although the focus has been on broad-scale patterns in terrestrial plants, Paul and Fenical in 1986 demonstrated a variety of secondary metabolites in marine algae which prevented feeding or induced mortality in bacteria, fungi, echinoderms, fishes, and gastropods. In nature, pests are a severe problem to plant communities as well, leading to the co-evolution of plant chemical defenses and herbivore metabolic strategies to detoxify their plant food. A variety of invertebrates consume plants, but insects have received a majority of the attention. Insects are pervasive agricultural pests and sometimes occur in such high densities that they can strip fields of crops.
Animals
Terrestrial arthropods
There are many strategies terrestrial arthropods employ in terms of chemical defense. The first of these strategies include the direct use of secondary metabolites. Many insects are distasteful to predators and excrete irritants or secrete poisonous compounds that cause illness or death when ingested. Secondary metabolites obtained from plant food may also be sequestered by insects and used in the production of their own toxins. One of the more well-known examples of this is the monarch butterfly, which sequesters poison obtained from the milkweed plant. Among the most successful insect orders employing this strategy are beetles (Coleoptera), grasshoppers (Orthoptera), and moths and butterflies (Lepidoptera). Insects also biosynthesize unique toxins, and while sequestration of toxins from food sources is claimed to be the energetically favorable strategy, this has been contested. Passion-vine associated butterflies in the tribe Heliconiini (sub-family Heliconiinae) either sequester or synthesize de novo defensive chemicals, but moths in the genus Zygaena (family Zygaenidae) have evolved the ability to either synthesize or sequester their defensive chemicals through convergence. Some coleopterans sequester secondary metabolites to be used as defensive chemicals but most biosynthesize their own de novo. Anatomical structures have developed to store these substances, and some are circulated in the hemolymph and released associated with a behavior called reflex bleeding.
The use of chemical alarms and detection is another strategy of chemical defense. Identifying predators and responding swiftly and appropriately is advantageous and leads to higher fitness. These defensive responses can include (but are not limited to) avoidance and escape responses, safeguarding offspring, aggressive behaviors, and applying "direct defenses" (i.e. toxins or defensive chemicals similar to the strategy of the monarch butterfly discussed above). For example, the fruit fly (Rhagoletis basiola) can chemically detect a nearby parasitoid (an organism that acts as both a parasite and a predator) and halt its egg-laying. Delaying oviposition can reduce the risk of predation and falls under the category of protecting offspring. The spider mite (Tetranychus urticae) can respond to predator volatiles in the environment and will choose to feed in areas without predator cues. Similarly, spider mites are also able to sense damaged body parts of individuals of the same species, or conspecifics, and present the same avoidance behavior as with predator cues. Furthermore, spider mites exhibit a similar behavior with egg-laying as the fruit fly and will elect to move to areas absent of predator cues before oviposition. Spider mites will not avoid areas with other, non-predator volatiles meaning these organisms are able to chemically distinguish threats from non-threats. Parasitic wasps (Aphidius uzbekistanicus) also sense volatiles of their predator, a hyperparasitoid (a parasite whose host is another parasite), and fly to new areas devoid of the chemical cues, displaying similar avoidance behaviors as the spider mite.