Bryophytes are small, non-vascular plants that form dense mats across nearly every terrestrial ecosystem on Earth. With approximately 12,000 species, they thrive from tropical forests to exposed Arctic rocks. Despite their global abundance, mosses are consumed by animals far less frequently than vascular plants. This widespread avoidance, known as the paradox of bryophyte herbivory, stems from nutritional deficiencies combined with physical and chemical defenses. The few animal species that rely on moss have developed specialized methods to overcome these biological barriers.
The Nutritional and Structural Barriers of Moss
Moss offers poor caloric return due to extremely low concentrations of protein and fat, the main energy sources for most herbivores. Moss can be composed of up to 90% water by weight. This means an animal must consume a massive volume of plant material just to ingest a small amount of dry matter.
The structural composition of moss also creates a significant digestive challenge. Up to 80% of the dry mass of moss is composed of fiber, which is highly indigestible for most animals. This toughness results from cell walls containing complex polymers, including specialized forms of cellulose and lignin-like compounds. These materials make it difficult for standard digestive enzymes to access and break down the limited nutrients present.
Confirmed Bryophages: Animals Specialized in Moss Consumption
A small number of specialized animals, known as bryophages, have evolved unique adaptations to make moss a viable part of their diet. The American Pika, a small mountain-dwelling mammal, is one example, with some populations relying heavily on moss for survival. The Pika employs coprophagia, re-ingesting its own feces, which allows the food to pass through the digestive tract a second time for maximum nutrient extraction.
Microtine rodents, including the Canadian and Wood Lemmings, incorporate moss into their winter diets. Moss makes up nearly 40% of the Canadian Lemming’s food intake during the coldest months. Among birds, certain populations of the Barnacle Goose in the Arctic, such as those on the Svalbard archipelago, consume moss as a significant portion of their winter forage when other grasses are unavailable.
Invertebrates also include specialized moss-eaters, such as the larval stages of some beetles in the family Byrrhidae, mites, and slugs. For Arctic herbivores like the Muskoxen, moss consumption may be driven by the high concentration of specific fatty acids, such as arachidonic acid, which is thought to aid in thermal regulation in extremely cold environments. These animals have often developed unique gut microbiomes that assist in breaking down the tough cell walls.
Chemical Defense Strategies in Bryophytes
Beyond the physical and nutritional limitations, mosses employ chemical defenses to deter herbivores. Bryophytes, particularly liverworts, synthesize a wide variety of specialized secondary metabolites distinct from those found in vascular plants. These compounds function as anti-feedants, making the plant unpalatable or toxic to non-adapted consumers.
One major group of these defensive chemicals includes terpenoids and lipophilic aromatic compounds, which are often concentrated in specialized oil bodies within the plant cells. Specific examples include terpenes, phenolics, and flavonoids, which have been shown to deter insects. Furthermore, many mosses produce complex polyphenols and condensed tannins, which are known to bind to proteins in a herbivore’s digestive tract. This chemical action interferes with the absorption of nutrients and digestive enzymes, effectively reducing the already limited nutritional value of the moss for the consumer.