Who eats ferns?

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 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.

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Some odd vegetarians

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.

July observations

–Out of the corner of my eye, I noticed a cottonwood branch twitching strangely. I looked up, expecting to see a bird. Instead, there was a red squirrel, bouncing out along the branch, stopping every so often. When it stopped, pieces of leaf fluttered down to the ground. A squirrel eating cottonwood leaves?? But why by-pass most of them, then? I reached for my binoculars and zoomed in.

I could see that some of the leaf pieces were yellow, not green. Then I could also see that the squirrel was not nibbling on leaf stems or leaf blades but rather it seem to be briefly manipulating each chosen leaf. Aha! Yellow leaf bits falling, squirrel picks only certain leaves…That squirrel was foraging for leaf rollers! This seems to be a good year for leaf roller moths, whose caterpillars use silk to bend leaf blades into protective tubes in which they live and feed. But there was little protection from this hungry squirrel, which cruised branch after branch, foraging all the way on juicy morsels of fat and protein.

Mine was not the only such observation: A naturalist friend observed another enterprising squirrel selecting rolled-up alder leaves. The squirrel noisily chewed open the leaf roll and ate the delicacy within, then moved on to more branches and more leaf rolls.

–It’s bear-watching season on Steep Creek near the visitor center, and one day I saw a yearling about twenty feet up in a cottonwood, in an odd pose with its rear end up and head down. Its hind feet were on one branch and its fore feet were on another, lower branch. Those front feet were deftly manipulating a salmon carcass, adeptly turning it first one way and then they other, occasionally flipping it over. The little bear eventually stripped that carcass down to spine and tail and let these remnants drop. Then it spent several minutes cleaning up its front paws and scampered up another fifteen feet to have a nap.

Young black bears usually separate from their mothers in their second summer. By then, they have learned a good deal about suitable foods and foraging, but they sometimes have a little trouble getting enough to eat. This little guy seemed to be doing just fine. However, it looks to be a rather poor year for berry crops, so it will be interesting to see how yearlings do this fall.

–While I was at Steep Creek one day, I watched the sockeye as the females were tail-flapping to disperse the sediments so they could lay their eggs in clean gravels, and the males were jockeying for position near nest-building females. Breeding males are deeper-backed than females, because they develop a slight hump on the back. The hump is probably a visual signal to other males, making its owner look big and hefty. Male pink salmon commonly develop such large humps that one of their other names is ‘humpy.’ But both sockeye and pinks can use the hump in the same way: when two males are side by side, contending for access to a female, the male with the taller hump leans over the smaller male in a literal put-down.

The first time I saw this behavior was while I was watching pink salmon coming into Sawmill Creek in Berners Bay. The male pinks in that creek seemed to have unusually tall humps, perhaps in part because the accessible part of the stream is quite short and flat, so a streamlined body is not so important. But it could also be partly because competition among males in that stream is, for whatever reason, particularly intense, making a big hump especially advantageous.

It was in Sawmill Creek that I watched a male pink that had such a huge hump that its body was shaped more like a dinner plate than a fish. This male would come closely alongside another male and lean that tall body over the less well-endowed male, forcing the smaller male to lie on its side until it could flap away. Since that time, I’ve seen this behavior several times, in sockeye as well as pinks. It seems to me that this is a form of physical domination, perhaps just short of a direct attack with toothy jaws.

–A friend and I are learning how to identify the local ferns. On a recent walk with that goal, my friend noticed a sizable brown caterpillar on a northern wood fern. The caterpillar was gnawing away at the fern frond, and nearby we saw several other chewed wood ferns. No other ferns on our walk showed signs of insect damage, but a botanist friend recalled seeing severe damage on lady fern on Admiralty Island a few years ago.

Most ecologists seem to agree that, in general, relatively few plant-eating insects specialize on ferns, and ferns get less damage from insects than flowering plants, even though there have been many millions of years for insects to evolve toward eating ferns. So how do ferns avoid heavy damage by insects? One suggestion is that ferns have general chemical defenses that reduce their value as food (just as tannins, for example, make many tree tissues hard to digest) that could be more difficult for insects to overcome than specific toxic defenses such as alkaloids; insects have evolved many specific detoxification mechanisms that allow them to utilize flowering plants that contain toxins.