Who eats ferns?

there aren’t many who do!

Ferns are not a very popular food item for the animal kingdom. Compared to the herbivorous insects on flowering plants and conifers, relatively few insects eat ferns. One estimate is there is about one insect species for every twenty species of fern, compared to one insect per one species of flowering plant. The disparity varies regionally, however; Hawaii, for example, has more ferns and more fern-eating insects than some other places.

The insect community on ferns is different from that on other plants. Although many beetles and moths are herbivorous, these taxa are under represented among the fern-eaters. Instead, sawflies and two taxa of true bugs (such as aphids) that typically suck plant juices (rather than chewing the tissues) are more common.

The reasons for the relative paucity of insects that eat ferns are not fully understood. One factor is surely the lack of flowers and seeds, which many kinds of insects use. Another factor probably is the defensive chemistry of ferns. Although they lack many of the defensive compounds found in flowering plants, they have considerable chemical resistance to attack by herbivores.

Bracken fern is notorious for its toxins, although toxin levels vary among bracken populations. This species has been studied intensively, because domestic livestock sometimes eat bracken. If cows and horses eat a lot of bracken, over a period of time the cumulative effects of the toxins can be lethal. Bracken turns out to be loaded with compounds that cause various blood disorders, depress levels of vitamin B1 (potentially leading to blindness), and cause cancer. The most toxic parts of the plant are the rhizomes (underground stems), followed by the fiddleheads and young leaves. A survey of toxins in other ferns would help our understanding of who eats ferns (lady fern, a common local species, is known to be toxic, to dogs, humans, and presumably others, at least if large amounts are eaten; in small quantities, the filicic acid in it help control tapeworms).

hoary-marmot-eating-ferns
Hoary marmot taking a risk on bracken fern

Vertebrates seem to avoid eating ferns, in general. Among the mammals, white-tailed deer sometimes eat them, and feral pigs in Hawaii eat the starchy tree-fern trunks. Beavers dig up and eat the very toxic rhizomes (how do they deal with the toxins?).

The champion fern-eater is the so-called mountain beaver, a burrowing rodent living in the Pacific Northwest. It is not a true beaver; probably related to squirrels, it is the last survivor of a group that once contained many species, now extinct. More than seventy-five percent of its diet consists of ferns, mostly bracken and sword fern. Female mountain beavers shift away from ferns to a higher protein diet of grasses and forbs when they are lactating, however. Mountain beavers must have a very special way of dealing with all the toxins!

A few vertebrates nibble the spores from the spore-containing packets (called sori) commonly produced on the underside of fern fronds. The European wood mouse does this in winter. The endemic short-tailed bat of New Zealand often forages close to the ground and collects spores. There’s a little parrot in Indonesia that eats fern spores. And the Azores bullfinch eats both spores and leaves in winter and spring. Interestingly, perhaps, I have found no indication that the closely-related Eurasian bullfinch does this.

Humans eat ferns too, sometimes as a springtime change of diet, sometimes more regularly. But there are potential risks to eating very much fern tissue. Clearly, learning more about toxins in a variety of ferns would be useful. And we might learn something from how mountain beavers deal with the toxins. But in the meantime, even though careful preparation might diminish toxicity, it is best to be very cautious about eating ferns.

Some odd vegetarians

subsisting on the tough stuff

Ferns have a reputation of being difficult to digest, but that does not mean that nobody eats them. Certain insects have adaptations to deal with fern tissue and can apparently handle them just fine. And a local bird expert has seen young sooty grouse eating ferns on Mt. Roberts. Humans even eat the young fronds before they unfurl (the fiddleheads).

Euell Gibbons said we could eat anything if it were prepared with enough butter and garlic. But not all humans can handle fiddleheads, however well prepared.

Down in the Pacific Northwest, there lives a creature for which ferns are a mainstay of the diet, along with other hard-to-digest things such as fir needles. It is called the mountain beaver, although it is not a beaver, is not closely related to true beavers, and does not even look like a beaver; it is also called a sewellel. Mountain beavers live in dense vegetation, usually near a stream, and make extensive systems of tunnels. Some of the vegetation they harvest is stacked up in ‘hay piles’ in front of a burrow entrance. When the hay is partially dry and wilted, it is stored inside the burrow. The hay stays greener and decays more slowly than fresh vegetation.

Some seasonal shifts in diet reflect a need for or availability of foods with high protein. Adult males eat red alder leaves in early fall, at a time when protein content is high. Lactating females need protein to make milk for the young, and they shift from fern to new conifer needles in spring, and to herbaceous vegetation (such as valerian, fireweed, twisted stalk) in summer, to get the needed protein.

Mountain beavers share with many other rodents and hares the habit of re-ingesting their feces, which allows them to extract additional nutrients from their food. They produce two kinds of pellets—soft ones that are eaten immediately and hard ones that are discarded.

Thinking about ferns and the mountain beaver got me thinking about other plant material that is difficult for many animals to eat. The first one the popped into my mind was conifer needles, which are loaded with resins and other difficult stuff. As always, some animals have adapted to using these difficult foods. Spruce grouse and sooty grouse both eat lots of conifer needles, and we’ve all seen Sitka spruce trees around here whose tops have been raddled by hungry porcupines.

Arguably the champion needle-eater is a vole down in the Pacific Northwest. Called the red tree vole, it is found only in coastal Oregon, but a rather similar species lives in northern coastal California. The red tree vole is almost entirely arboreal, living its entire life in the canopy of Douglas firs (or occasionally Sitka spruce). However, to escape a pursuing predator, a tree vole may deliberately launch itself into the air, sometimes falling over fifty feet to the ground, where it picks itself up and scampers up a tree again.

The red tree vole specializes on Douglas fir needles, handling them in a very specific way. The vole cuts twigs and brings them back to its nest. There it carefully chews the resin ducts out of each needle before eating the rest.

The tree vole lives in its own midden. Food twigs accumulate in a little platform, packed down by trampling and by waste products of digestion. As additional twigs pile up, the vole crawls under them to feed. The twigs are shoved around to make a small chamber enclosed by twigs. As the vole eats, discarded resin ducts pile up. The discarded resin ducts are rearranged so that eventually they line the chambers and tunnels that the vole carves out of the ever-growing heap of twigs. Each nest has two kinds of chambers, one for living, and one for latrine; old nest chambers end up as latrines.

In large Douglas firs, the vole’s nest may be far out on a limb, but in smaller trees, the nest is close to the trunk. Trees younger than twenty-five or thirty years old are too small to support the vole’s nest. Some nests grow to enormous size, wrapping all the way around the trunk, if the tree is fairly small, and expanding upward.

The red tree vole is probably endangered. Its preferred habitat contains numerous large Douglas firs, a habitat now much reduced in western Oregon.

These creatures captured my fancy, even though they don’t live in Juneau. But perhaps it is worth thinking about why they are so restricted in distribution, and why the Pacific Northwest is home to such specialized species, but Juneau is not.