Walking along the East Glacier trail one day, I noticed a scattering of gray lichen scraps on the snow next to a small tree trunk whose lichens were the source of the scraps. Some small creature had ripped off much of the lichen, leaving the bits on the snow. A narrow groove led away from the lichens to a small burrow that led down under the snow blanket. That made me think a shrew had been rooting around in the folds of the lichen for insects or spiders, but it was also possible that a small vole had been selectively feeding on the lichen itself.
A few days later, on the Spaulding Meadow trail, I was following some deer tracks, to see where they were going, when my more observant companion called my attention to where the tracks had come from.
There, not far from the trail, there was a noticeable browse line in the arboreal lichens (witches’ hair or Alectoria) hanging from the tree branches, and underneath there was a multitude of deer tracks. All the Alectoria lichens below a certain height, within reach of a foraging deer, were missing from that patch, although they could be found not far away where the deer had not walked.
These observations made me think about lichens as a food source for animals, and I have a very helpful local lichenologist to thank for some great references (as well as instruction on lichen identification). Some lichens are not very tasty, some are actually very poisonous, many have defensive chemicals to deter consumers, and yet a great many critters eat them, often quite selectively.
Somewhere in the world, lichens are eaten by certain snails and slugs, as well as certain kinds of mites and moth larvae; some of these consumers actually specialize on particular kinds of lichen. Although lichen-eating invertebrates have not been well studied in Alaska, we do have a local snail or slug that grazes on the outer layers of leafy lichens. (Recall that a lichen is a composite of a fungus and an alga or bacterium—and some lichens have all three components). Occasionally, if you look closely at the leafy lichens, you might see tiny white trails on the greenish or brownish lichen surface. These meandering trails reveal the white fungal layer below the outer layer, showing where a snail or slug has scraped off the nutritious outer layer that includes the algae or bacteria that also make food for the lichen.
We know a bit more about vertebrates that probably or actually do eat lichens in Alaska.
Caribou are surely the most well-known consumers of lichen all across the North. They use their hooves to dig craters in the snow so they can reach terrestrial lichens, particularly the shrubby, branching ones such as Cladina (reindeer lichens), Cetraria (Iceland lichens), and Stereocaulon (foam lichens), or leafy ones such as Peltigera (pelt lichens). Where there are trees and arboreal lichens, Alectoria (mentioned above) and Bryoria (horsehair lichens) are favorites. Caribou eat lichens all year long, but especially in fall and winter; in spring and summer they consume more willows and herbaceous plants. Apparently, their digestive enzymes change with the seasons, becoming better able to deal with lichens in winter. But lichens are mostly carbohydrate, with low values of protein, so caribou have to find some other source of protein or a means of coping with low levels of nitrogen.
Some reports indicate that each caribou has to consume ten or twelve pounds of lichen per day, so a whole herd can make big inroads into lichen populations. Intensive caribou grazing can wipe out whole communities of such lichens, leaving only the low-growing or crust-like ones, and it can take decades for the lichen community to be restored. The most famous example concerns the introductions of reindeer (Eurasian caribou) to St Paul Island in the early 1900s. From just a few animals, the population grew by leaps and bounds, and they ate up virtually all the edible lichens, so their population crashed dramatically. They truly ate themselves out of house and home.
In Southeast, Sitka black-tailed deer eat arboreal lichens, as we observed, and so do moose. When mountain goats descend from the peaks into the forest in the winter, they eat arboreal and leafy lichens; in the alpine zone in summer, they may eat branching, terrestrial lichens.
There are several species of red-backed voles in western North America and two of them are known to eat both arboreal lichens and branching, terrestrial ones; I found no reports specifically for the species that occurs in Southeast, but it is likely that this species does also. Flying squirrels are important consumers of arboreal lichens (as well as using them to build nests).
All these lichen-eating animals are important parts of the ecosystems of Southeast, so it is useful it is useful to consider a few of the ecological interactions that ramify from this lichen base. Some lichens accumulate certain organic pollutants, which are then passed on up the food chain to caribou and deer, and thence to predators such as wolves and humans, where the pollutants become concentrated. Industrial pollution and extensive logging can devastate lichen communities, drastically decreasing the food supply for all of the many consumers. Loss of this important food supply has repercussions in many directions: for example, flying squirrels and voles also eat truffles and disperse the spores when they defecate; truffles are mycorrhizal, making nutrient exchanges with the roots of many trees and supporting healthy tree growth. Caribou, moose, and deer help support predator populations (predators that also prey on other species such as hares and salmon); the browsing of these herbivores can alter plant communities and affect the reproduction of shrubs such as willows and blueberries, which in turn affects bumblebees (that feed on willow and blueberry flowers and go on to pollinate many other kinds of flowers) and birds and bears (that eat blueberries and disperse the seeds).
Humans, too, have made extensive use of lichens as food. Indigenous people of the North ate partially digested lichen from caribou stomachs; this material was mixed with fish eggs to make what was considered to be a delicacy (presumably an acquired taste?). In some parts of the world, lichens have been fed to domestic animals, including pigs, dogs, sheep, and cattle, and humans themselves have used lichens as flavoring, thickener for soups, and emergency food.
Historically, the biblical manna from heaven, which is said to have sustained the migrating Hebrews in the desert, could have been one of the so-called vagrant lichens, which grow loosely on the ground and can be blown hither and yon by a wind. Many things have been called ‘manna’, including plant resins, honeydew from scale insects and aphids, and certain mushrooms, but vagrant lichens seem especially plausible as the biblical manna, because they would take up water (and expand) after a rain or morning dew, and they can accumulate in some density after a wind (and dew, rain, and wind are all mentioned in one biblical account or another, in association with the appearance of manna). Lichen-manna is quite edible (to humans, camels, and sheep in the Old World, as well as pronghorns in the Idaho high desert) and can be made into bread, but some reports say that it should be eaten in small quantities.
Although lichens seldom claim much of our attention, it should be clear from these considerations that they should not be neglected!