Life Under the Roost
1Every spring and summer, about 20,000 gray bats live in Hollow Rock Cave in the Ozark hills of Missouri. Most are females that raise one pup each in a large roost on the ceiling of a dark chamber. Each evening the bats fly out to hunt mayflies, moths, beetles and other night-flying insects over the river and the forest, and they return before dawn. In the fall, the colony leaves to hibernate in a colder cave many kilometers away, and the chamber stays empty until spring. The chamber stays near 13 °C all year. Seventh grader Theo Lindahl joined state cave biologist Dr. Marguerite Esposito on two survey trips, one in May, before the pups were born, and one in January. The team stayed only a short time, kept quiet and used dim red lights so they would not disturb the bats.
2No green plants grow in the dark chamber, so there are no producers there. Dr. Esposito's model starts outside the cave: plants use energy from sunlight to make sugars from carbon dioxide and water, insects eat the plants, and bats eat the insects. Under the roost, the bats' droppings, called guano, pile up on the floor. Fungi and bacteria break down the guano, and tiny mites, springtails and guano beetles eat the guano and the fungi growing on it. Cave spiders and centipedes hunt these small animals. Every living thing in the model uses some of the energy in its food for its life processes, and that energy ends up as heat.
3On each trip, the team took measurements under the roost and at a spot on the same chamber floor 50 m away, where no guano falls. They measured the dry mass of guano added each week, counted the small animals in floor samples and measured the carbon dioxide in the air just above the floor (Table 1). Air moves slowly through the cave and out of its entrance.
4Dr. Esposito also used measurements from other studies of gray bats to build an energy model for one May night (Table 2).