Photo: USFWS/Ann Froschauer A Mexican free-tailed bat in Texas's Bracken Cave

Caves can be pretty bleak places. Things live in them, sure, but nutrients are so scarce that most cave-dwellers top out at less than a half inch long. The life of a troglobite is often one of constant famine, its existence a daily toil on the precipice of death.

But all of that changes with the addition of one magical ingredient: bat poop.

As the days get shorter and colder, bats across the country are seeking out caves in which to spend the winter. And where they go, life follows.

Bats are like tiny, hairy harvest nymphs for cave ecosystems. Each night, these mammals fly out into the darkness and hunt down energy in the form of insects, fruit, or blood. Each morning, they return to the cave, digest their bounty, and then let life-giving poo rain down on the inhabitants below. We’re only starting to learn just how many species benefit from this nutrient bonanza, but recent studies show bats nourish everything from frogs to ants.

Turd by turd, bat guano can slowly stack up to gargantuan heights. In Texas’s Bracken Cave, summer home to the world’s largest bat colony, experts estimate the guano piles to be at least 59 feet deep. That’s what 20 million Mexican free-tailed bats can do over the course of several thousand years.

Entire ecosystems spring up around this fecal fecundity. (People benefit from guano, too. Historically, we’ve used it as fertilizer to grow stuff, and as gunpowder to kill stuff.)

“Guano is a food source for a lot of cave critters,” including crickets, fleas, mites, and many other invertebrates, says Fran Hutchins, cave manager for Bat Conservation International. “In Bracken Cave, we actually have six different species of dermestids,” he adds. “Those are your flesh-eating beetles.”

And, of course, where there are insects, there are insect-eaters.

Photo: Vladimir Dinets A Caucasian parsley frog

Across the world in Turkey, Georgia, and Russia lives a little green amphibian known as the Caucasian parsley frog. Until recently, scientists thought these hoppers spent their winters roughing it in leaf litter. But a recent report in the Herpetological Bulletin revealed that some of the frogs wait out winter within caves, where the species had never before been found. But they won’t hole up in just any caves—Caucasian parsley frogs seem to prefer caves with bats.

“There are places in the Caucasus where life exists in caves at the depth of half a mile, thanks to guano nutrients percolating down from bat colonies near cave entrances,” says Vladimir Dinets, a research assistant professor at the University of Tennessee and author of the report.

The International Union for the Conservation of Nature lists Caucasian parsley frogs as near threatened. Knowing that the animals spend time in bat caves is great for scientists who want to learn more about their life cycle in order to breed them and try to boost the amphibian’s numbers. (Dinets says previous efforts to breed the frogs in captivity never made it past the tadpole stage.)

But news of Dinets’s discovery is also a little worrying since it might mean the amphibian’s survival relies on bat survival, and those mammals have historically gotten the guano kicked out of them by humans. Dinets says bat colonies have disappeared all across the Caucasus thanks to development, tourism, unregulated use of pesticides, and deforestation.

“The Sochi Olympics, in particular, were a huge ecological disaster,” he says. “Pristine valleys clear-cut, roads blasted through precious canyons, tree pathogens introduced to local forests with exotic trees imported for landscaping, Europe's tallest trees cut down to build ski routes and lifts.”

On this side of the pond, U.S. bat populations have their own version of Sochi—namely, white nose syndrome. The fungal disease continues to spread through North America with mortality rates reaching 100 percent in some populations. And as bats go, so do the ecosystems that rely on their guano. “There are hundreds of cave-adapted species in the U.S.,” says Dinets. “We have the richest cave fauna in the world. And now we are at risk of losing most of it.”

Good news for bats, however, is starting to seep through the bad, albeit slowly. Researchers recently detected the disease in China, where it does not seem to cause population collapses. Studying those infected bats might help pathologists understand what makes WNS tick and perhaps learn how to fight it. Studies have also shown that some species may be capable of building up resistance to the disease. In addition, biologists are looking into evidence that a type of fungus-fighting bacteria could help win the war against WNS.

In the fight to save bats, these are just small glimmers of hope—much like what a glistening drop of guano is to a troglobite in a dark, stark cave.

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