BCI continues to fund research into the bats' eating habits. They also eat things like cucumber beetles. Those are two of the most costly pests in America. All that dining translates into a whole lot of bat waste, called guano. The Bracken population produces enough of the stuff that you can't go into the cave without boots and a respirator, although the real danger is ammonia produced by dermestid beetles, flesh-eating bugs that feast on those bats that don't fly through the air with the greatest of ease.
Young bats can become skeletons within minutes of hitting the floor.
The unpleasant by-product of this diet, when combined with water vapor, becomes ammonium hydroxide. In some caves the concentration is so high that the bats' fur is bleached. The guano that drops into the Colorado River from the bridge doesn't significantly affect the water, largely because the droppings, made mostly of digested moth parts, dilute and break down pretty quickly. The bats under the bridge were the prime suspects. They sampled the water upstream.