Most of a lichen’s structure is the fungus. The alga lives with it, and in return for shelter it provides photosynthesis, producing sugars that sustain the fungus. But the two are not alone.
“In many ways, lichen are miniature universes,” Dr. Allen and Dr. Lendemer write, as a diverse community of bacteria, non-lichen fungi, nematodes and tardigrades (also known as water bears) live in and on a lichen.
Many other creatures also rely on lichens, from moths whose larvae use them as food to hungry caribou, deer and moose. Hummingbirds and flying squirrels incorporate bits of lichen into their nests.
And take note, gardeners: Lichens help with soil formation by accelerating the breakdown of rocks. They perform nutrient-cycling, too, as the cyanobacteria in them fix nitrogen from the atmosphere, converting it into a more usable form.
And when lichen falls on the soil and breaks down, Dr. Allen said, “it’s a little packet of fertilizer — a fertilizer application.” (It’s also a lichen-hunter’s prize: “Fallen branches often harbor a wealth of lichen,” she said, so keep an eye out during garden cleanup for any to set aside and examine with a magnifying lens.)
Many plants, including epiphytic ones, depend on lichens for humidity and moisture.
“They’re like sponges in the environment, soaking up moisture quickly and releasing it slowly into the area,” Dr. Lendemer said. “Without lichens, the forest is drier and sadder.”
Lichens are highly sensitive to pollution, making them excellent indicators of air quality, and of habitat quality in general. The industrial revolution’s impact reduced lichen diversity, especially in cities, until the clean-air legislation of the 1960s and 1970s gradually made even urban areas hospitable again.