Lichen is a unique, composite organism resulting from a symbiotic partnership between a fungus and a photosynthetic partner, typically an alga or cyanobacterium. This association allows lichens to colonize and thrive in challenging environments, from arctic tundra to scorching deserts. Lichens form an important base of the food web in many extreme ecosystems. Consumers relying on this resource have developed specific behavioral and biological traits to handle its challenging nutritional profile.
Large Mammals as Primary Lichen Consumers
Caribou and reindeer (Rangifer tarandus) are the most recognized large herbivores that depend heavily on lichens. These animals inhabit the Arctic and subarctic regions, relying on lichens as a primary food source during the winter months when vascular plants are dormant or covered by snow. The ground lichen species they consume, often called “reindeer moss” (Cladina species), is a highly digestible carbohydrate source.
During winter, a reindeer may consume between three and five kilograms of lichen daily, often using their hooves to dig through snow to reach the forage. Musk oxen (Ovibos moschatus) also graze on lichens, scraping away snow to access terrestrial species. Though lichens provide necessary energy, they are low in protein and certain minerals, meaning these large mammals often lose body condition over the winter.
Smaller Animals and Invertebrate Lichen-Eaters
Lichen consumption is not limited to large mammals; a diverse array of smaller animals and invertebrates also feed on this organism. Snails and slugs are common consumers, using their rasping mouthparts, called a radula, to graze the surface layers of foliose and crustose lichens. This scraping action often leaves distinct, visible tracks on the lichen thallus.
Mites and springtails are microscopic arthropods that feed directly on the lichen structure, often sheltering within colonies growing on rocks or bark. The larvae of specialized insects, such as the lichen moth (Clemensia albata), rely on lichens as their sole food source during their caterpillar stage. These invertebrates play a significant role in the decomposition and nutrient cycling of lichen communities. Arboreal lichens, like Bryoria (horsehair lichen), form up to ninety percent of the diet for flying squirrels in certain areas.
Specialized Adaptations for Lichen Digestion
Lichen presents challenges for digestion because it is poor in protein and contains complex, often indigestible, carbohydrates, along with unique chemical defenses. Most lichens contain only about two percent protein, requiring animals to seek out high-protein forage when available. The bulk of the thallus is composed of complex polysaccharides, such as lichenin and isolichenin, which are difficult for most digestive systems to break down.
The most significant hurdle is the presence of secondary metabolites, commonly known as lichen acids, such as usnic acid. These compounds act as chemical defenses and are toxic to many unadapted herbivores, including domestic sheep and elk. Reindeer and caribou overcome this by harboring a highly specialized microbial community in their four-chambered stomach, or rumen.
This adapted gut flora, which includes bacteria like Eubacterium rangiferina, detoxifies and metabolizes the harmful usnic acid. These microbes also produce enzymes to efficiently break down complex lichen carbohydrates into simple sugars, which the ruminant absorbs for energy. This allows reindeer to achieve a high lichen digestibility, often around 74%, a rate superior to that of unadapted animals. The efficiency of this microbial digestion fluctuates seasonally, peaking in winter when the diet is almost exclusively lichen.