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 lichens mycobiont via hydrocarbons produced by the lichens photobiont. However, a single defensive chemical may serve multiple purposes. Usnic acid, for example, is implicated across anti-bacterial, -viral, and -fungal actions. 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.
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.
In 2004 the death of hundreds of elk near Rawlins, Wyoming was linked to consumption of tumbleweed shield lichen (Xanthoparmelia chlorochroa). This strangely powerful chemical defense is irregular given that such poisoning is very rare while the consumption of this lichen is fairly regular.