Biological Role of Lichen Substances

The bizarre phenolic substances produced by many lichen-formingfungi are assumed to have important biological roles, an idea that has led various investigators to speculate on topics ranging from light-screening to rock mineralization. At the present time, three proposed roles- antimicrobial, allelopathic and antiherbivore-are best supported by the available evidence. Anti- microbial: The fact that lichens grow slowly and yet are resistant to decay microorganisms suggests that lichen substances, known to be clinically effective antibiotics, also have a protective role in nature. Indeed, this may be their principal role. However, there is little direct evidence from field studies. Allelopathic: Recent studies show that lichen substances can inhibit seed germination of vascular plants and spore germination of mosses and lichens, demonstrating a possible allelopathic role for these substances. The ecological significance of such inhibitory effects in natural species assemblages that include lichens, however, requires more research. Antiherbivore: Despite their availabilty as food to generalist herbivores, lichens are seldom eaten, suggesting that lichen sub- stances deter feeding. Recent research, done primarily with invertebrate consumers, indicates that some lichen compounds are capable of defending lichen thalli from predation. The secondary chemicals produced by lichens have attracted the attention of investigators for over 100 years, and information about their structure, bio- genic origin and phylogenetic significance has ac- cumulated steadily over this period. Indeed, the sec- ondary chemistry of few other groups of organisms is better known. Yet our understanding of their pos- sible biological roles is meager. This is unfortunate because the extent to which lichen secondary sub- stances can be expected to contribute to the solution of evolutionary problems depends on a better un- derstanding of their adaptive value. What do these compounds do? Most lichenolo- gists assume that they do something, since it is dif- ficult to imagine how mechanisms resulting in the production of such a wide array of energetically ex- pensive compounds could have evolved in so many groups of energetically-depauperate lichen-forming fungi. If these substances have no adaptive value at all, their relatively high energetic cost would ulti- mately lead to their disappearance over evolution- ary time. Just the opposite appears to be happening, however. In the most rapidly speciating lichen