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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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.