Cavers meet annually to scoop the poop.
By Katie Armstrong
As the sun sinks below the hills on summer evenings, a natural wonder occurs at Bracken Cave. Mexican free-tailed bats — 20 to 40 million of them — whirl out from the mouth of the cave for their nightly insect feast. The bats, mostly females who have migrated from Mexico to give birth, eat more than 200 tons of insects every night during their summer stay at Bracken.
The end result of so much eating? Guano. Lots of it.
“We have anecdotal measurements that the guano is 70 feet deep in the back of the cave,” says Jim Kennedy, a biologist and cave resources specialist with Bat Conservation International (BCI), an Austin-based group that protects bats and their habitats.
Guano, the dried excrement of bats and birds, is a superb natural fertilizer thanks to its high concentrations of nitrogen and phosphorus. It has been so highly prized that countries have gone to war over it.
In the 1800s, chemists discovered that the same concentrated nitrates in guano that make it a gardener’s dream could also be used to manufacture gunpowder. This doubled its value, and bat caves were raided for their guano stores during the Civil War.
“Guano was the biggest mineral export in Texas before oil was discovered,” says Kennedy.
These days, explosives are made with artificial nitrates and the guano market has shifted to organic gardening. Texas still boasts a huge store of it, though — Bracken Cave, near San Antonio, is the largest habitat for Mexican free-tailed bats in the world. Each year the bats deposit between 85 and 100 tons of guano on the cave floor. That makes for a lot of fertilizer.
As BCI’s designated “cave guy,” caver friends turned to Kennedy when they wanted some guano for their gardens and to take a look inside Bracken Cave. Thus, the Guano Gathering was born.
Every year Kennedy and a group of 40 to 70 friends and family descend on Bracken Cave during late winter, before the bats have come to roost. Armed with shovels, bandanas and five-gallon buckets, they form a line called “The Bucket Brigade” that stretches from the mouth of the cave to whatever they have brought to fill, from trash cans to truck beds.
Two brave souls are actually inside the cave, knee-deep in the brown, powdery guano, scooping and passing as fast as they can. After a hot and stinky two hours, the Brigade will have removed close to two tons of guano from the first 40 to 50 feet of the cave.
Afterwards, the group gets the rare opportunity to explore the inside of the cave. When bats are present, the cave floor teems with dermestid beetles, which feed on the guano and unfortunate bats that fall off the wall. The beetles emit ammonia gas as a waste product, and levels can get so high inside the cave that they would kill a human. Luckily for the cavers, the beetles die off when the bats leave, allowing them to venture into the habitat in hot and smelly safety.
“It was quite the experience,” says Mike Quinn, a TPWD invertebrate biologist who participated in 2006. Despite the overwhelming smell of ammonia, he says, “I had a lot of fun and I’d like to explore the cave some more.”
While no more guano gatherers are needed, BCI does operate Bracken Cave as a preserve. BCI members can observe the bats during limited scheduled visits in the summer. For information on other programs and becoming a member, visit the BCI Web site at <www.bat con.org> or call (512) 327-9721.