Bat Use of Giant Sequoia
Prior to this study, little was known about the bat community in Yosemite’s three giant sequoia groves and virtually nothing was known about how bats use the canopy in any of the Parks’ forests. Dr. Elizabeth Pierson, Dr. William Rainey, and Leslie Chow carried out major research to study bat roosting behavior in fire-scarred hollows at the base of sequoia trees, bat feeding behavior in association with a variety of habitats, and bat activity in the giant sequoia canopy. In addition, they combined observations from this study and others to describe the natural history of Yosemite’s 18 bat species.
For two years the researchers studied bats in Mariposa, Merced, and Tuolumne groves, observing them with night vision devices, trapping dropped guano in basal hollows, mist-netting, and interpreting echolocation signals received on radio tracking devices.
At least six species, they found, roost in giant sequoia trees and five of them are known to have reproductive populations in the park. They roost by day in basal hollows created by repeated fires, deep bark furrows, and cavities and crevices of tree crowns. By night, when not hunting insects, they choose basal hollows for shelter.
The finding most significant to the team was that temperatures within the basal hollows remain above freezing throughout the winter and are relatively stable all year. Because bats are small, warm-blooded animals, relatively warm and predictable temperatures are essential for their survival and reproduction. Past studies have shown that bats restrict their winter hibernation to sites with stable temperatures very similar to those found within giant sequoia basal hollows. The researchers are convinced the hollows meet the standards for bat hibernating habitat; because no prior research has described bat hibernation sites in the Sierra Nevada, these results are incentive for deeper study. They indicate that bats may very well make the basal hollows within giant sequoia groves their winter home.
At the end of the study, the researchers made two recommendations for park managers. Based on the observation that many bat species move or abandon roosts frequented by park visitors, they suggest establishing visitor guidelines that discourage entry into the basal hollows. They have also suggested vetting the practice of felling old trees and snags (dead, standing trees) in the park for visitor safety. Mid-to-late stage snags are critical roosting habitat for bats. They recommend evaluating a tree’s potential as a bat roost before removing it, and only doing so when considered absolutely necessary for human safety.