Fungi and Wildlife

animal harvesters of fungal delights

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Fungi are made up of vegetative parts, which are filamentous structures that typically lie underground, and reproductive parts, which take various forms, sometimes finger-like or shelves, or commonly as mushrooms with a stalk and a cap. But some fungi never produce above-ground parts; the small, round or lumpy reproductive structures are underground—these are known as truffles.

Fungi reproduce by means of spores—each tiny spore containing the makings of a new individual. Most fungi, such as ordinary mushrooms, disperse their spores aerially—releasing the mature spores from the mushroom cap to vagrant breezes in the forest understory. Truffles do it differently: they rely on small mammals to dig them up and eat them, passing the spores through the digestive tract and depositing them in feces.

Here in Southeast, there are two major harvesters of truffles: the red squirrel and the flying squirrel. The red-backed vole also does this and other small mammals may do so occasionally. These rodents also harvest typical mushrooms, sometimes caching them, to be eaten later, after they dry. Red squirrels do this very regularly; flying squirrels in other regions cache many kinds of food but apparently they have not been recorded to do so in Alaska.

squirrel-with-mushroom-by-bob-armstrong
Photo by Bob Armstrong

The caloric content of fresh mushrooms is low, far lower than that of nuts and seeds. However, dried mushrooms compare more favorably, although they still average only about two-thirds of the caloric content of conifer seeds. Mushrooms are very low in fat, compared to spruce and hemlock seeds, but they can be a pretty good source of carbohydrates and protein, especially when dried. They may also provide an assortment of micronutrients such as vitamins and minerals. Fresh mushrooms can be a source of water during season dry periods.

However, the actual food value of fungi to rodent consumers depends in part on the intake rate. Taking a bite of mushroom or truffle is quick and easy. But when eating conifer seeds, squirrels have to peel back the cone scale and trim the membranous ‘wing’ from each seed before eating it. Although squirrels are remarkably fast at extracting conifer seeds from a cone, it still seems that the food intake rate would be slower than when eating fungi. The actual food value also depends on how efficiently a squirrel’s digestive process extracts energy and nutrients from the material that is ingested. Some studies of squirrel diets suggest that the digestibility of fungal tissue is considerably less than that of conifer seeds. Nevertheless, squirrels regularly eat fungi, so there must be sufficient reward to make it worthwhile.

The relationship between truffles and squirrels (and voles) is mutualistic—the mammals get dinner and the truffles disperse their spores. For truffles, the relationship is obligatory; they are dependent on small mammals for spore dispersal. For the rodents, the relationship is more variable, depending in part on the availability other food sources (for example, squirrels might eat more truffles in years when the cone crop is poor). Mushroom-producing fungi sometimes get spores dispersed by rodents that harvest mushrooms; the spores ingested are viable after passing through the rodent’s guts, so in addition to the normal, aerial means, the fungi benefit from rodent assistance. This amounts to a casual sort of mutualism in which both parties benefit but the relationship is not obligatory for either one (in contrast to that for truffles).

Now the plot thickens! Many fungi, both truffles and mushrooms, are mycorrhizal—forming mutualistic relationships with the roots of various plants. The plants provide carbohydrates to the growing fungi and the fungi supply various nutrients to the plants. In most cases, both participants in the relationship grow better with the partner than they do alone; in some cases the relationship is obligatory to at least one of the partners.

Thus, in this network of interactions, one well-developed mutualism (the fungus-plant relationship) intersects with another one (the rodent-fungus relationship). Something for everybody! A nicely tangled web!

Winter explorations

little snow stories, a fungophile squirrel, and long-leg aggregations

Wonderful snow! Brightening our short December days; under that full moon, it was spectular.

All that glorious snow drew me out, day after day, looking for little stories writ there. There were all the usual perpetrators: deep, winding furrows made by wide-bodied, peripatetic porcupines, lots of snowshoe hares (especially near the visitor center), a mink bounding over a frozen rivulet, red squirrels tramping back and forth between trees, a deer or two—or lots in some places, an ermine, some good otter slides, a few voles and shrews (and a couple of mysteries that I shall gloss over).

There were also a few real highlights. Out near Peterson Creek, after watching a hungry dipper searching for some open water, my friend glanced up and spotted something odd-looking about ten feet up a spruce, right next to the trunk. This looked like a mushroom, stuck in a tuft of twigs. Really? So my friend climbed up, to look more closely. Indeed, not just one but five or six mushrooms were wedged in a tight stack, in among the twigs. Ha!—a squirrel cache. Red squirrels are known to store mushrooms in dry places, such as the tops of well-drained stumps or logs (but cones are usually stored in damp places, so they don’t open and drop the seeds). This squirrel clearly thought that the twiggy tuft was good enough to warrant stashing several valuable food items there.

The sloping base of a huge spruce in Amalga Meadows had been an active site. Small prints of a critter that could bound six or eight inches decorated the trunk up about three feet, as well as all the surrounding snowy ground. Hmm, not a vole, which usually scuttles along, nor a jumping mouse, which hibernates, but probably a deer mouse.

We looked out over the wind-swept open area of the meadow, where the snow had been draped over tussocks and small conifers. We saw immediately that these looked like sea lions lunging up from the water. There was a big, thick-necked bull, and a whole squadron of juveniles, not far behind. No great imagination needed to provide some sound effects as well!

Some friends like to explore old mines in the Juneau area and recently explored one near the base of Thunder Mountain. The adit was barricaded by stout icicles, but a few were persuaded to break, allowing human entrance. Not too far back in the tunnel, observers spotted a number of harvestmen (a.k.a. daddy longlegs) on the wall. Near the end of the eighty-foot adit, many of these were clustered into a close group, with their long legs sticking out in all directions.

Harvestmen are related to spiders, but not very closely. In contrast to spiders, they are not predatory; they feed chiefly on bits of plant and animal debris. They are also not venomous and cannot bite you with their weak jaws. They are often gregarious, gathering in bunches, but why? One suggestion is that they are behaving like what is known as a ‘selfish herd’. Each animal tries to put as many others as possible between itself and the edge of the group, where the risk of predation is highest. For these harvestmen, the most likely predators are spiders, or possibly mice.

Sheep Creek Valley

trailside discoveries and memories of field work

I go up into Sheep Creek Valley several times a year; it’s one of my favorite places in Juneau. I was there in mid November with Parks and Rec hikers, and we spotted several things of interest. It had snowed recently, so tracking was good. We found tracks of squirrel, deer, mountain goat, a possible weasel, a large canid that could have been a wolf or just a big dog, and lots of porcupine tracks. Two porcupines scuttled off into the brush as we walked by.

A big conifer tree had a large squirrel midden around its base; discarded cone scales and cores covered many square yards. The main cache of full cones was underground, but this red squirrel was not content with that—it had also wedged cones into every available space between the roots and in grooves of the trunk.

We found a beautiful orange and yellow fungus growing on a dead branch. It is a type of jelly fungus, possibly the one called “witches’ butter”.

Some of the more enterprising hikers went up the slope at the back of the valley, far enough that they were wading in thigh-deep snow. Other, less energetic perhaps, were content to perch at streamside for a relaxed lunch break. The creek was running crystal clear and wide open, so we had hopes that a dipper might show up. Indeed, one did, prospecting for aquatic insects along the edge of the water and moving quickly upstream.

Sheep Creek Valley is among the first places I worked when I came to Juneau over twenty years ago. My first big project was to census nesting birds in various habitats; there seemed to be no previous studies of breeding bird communities that would provide an estimate of avian diversity and abundance in different habitats around here—very basic information for future ecological studies.

So for several years, in spring and summer, my field techs and I studied bird communities in Sheep Creek Valley and elsewhere in Juneau. We counted birds, using a standard protocol, by sight and by songs and calls. We found that this valley has a very rich community of nesting birds, arguably the richest one in our area. For example, we counted several kinds of warblers, sparrows, and thrushes—more kinds than in the spruce-hemlock forest.

Along with the standard censuses by sight and sound, we regularly mist-netted birds in the understory. Our black nylon nests were twelve meters long, and we would set up an array of about ten nets in various places. Then we’d walk the array of nets every hour or so, extract and weigh the birds, and release them. Among other things, the net captures helped us detect birds that were quiet and secretive.

There were a few bears in the valley. Occasionally, we would glance up as we extracted a bird from a net and see a calm bear sitting near the end of the net and observing all of our actions! They didn’t seem to have designs on us or on the birds; apparently they were simply curious.

Our other main activity was nest-searching. This is hard work and lots of fun, rather like a continual treasure hunt. By following a bird for a while, often on several occasions, eventually one deduces the approximate nest location, and then careful searching reveals a nest. It often takes several hours of detective work, perhaps over several days, to locate a nest this way. Once a nest was found, we monitored its progress, from incubation of eggs to care of nestlings to fledging—or until the nest failed. Then, for each species, we could calculate the percentage of nests that successfully produced young. For example, about sixty-five percent of yellow warbler nests were successful but only about thirty percent of robin nests and roughly twenty-five percent of fox sparrow nests were successful.

A principal cause of nest failure was predation on eggs or chicks. By installing small cameras that were triggered by removal of an egg, for instance, we learned that predators include Steller’s Jays, red squirrels, mice, and even shrews (see accompanying photo). But Sheep Creek had fewer egg and chick predators than conifer forest.

All of that work required us to begin at dawn, because bird activity is generally greatest early in the morning. The days are wonderfully long in spring and summer, so that meant we started work by three-thirty or so (and had to get up around two a.m., to get to the study sites; this was not so wonderful!). Nevertheless, I look back on those days with much pleasure (perhaps especially because I no longer have to crawl out of bed at crazy hours).