Masthead: Kaweah Range

Sierra Nature Notes, Volume 5, May 2005

Using bat assemblages as a measure of ecosystem health

As predators that rely almost exclusively on insects, bats may provide a reliable measure of ecosystem function.

Left: spotted bat (Euderma maculatum, juvenile shown).
Solar powered Anabat acoustic detection system at right.
Copyright William E. Rainey (2)

Comprising the second largest order of mammals in North America, bats trail only rodents in number of species. The NPSpecies database has records for 77 species and subspecies of bats in 141 units of the National Park System, with an average of 5 species per unit. Seventeen bat species occur in Yosemite National Park. The park is the site of a three-year study of bat activity and overwintering habits. The study tested solar-powered Anabat acoustic detection systems to monitor winter bat activity at 15 sites in Yosemite National Park, including this one along the South Fork of the Merced River.

Photo: California Bat Conservation Fund

IN 2003 THE U.S. GEOLOGICAL SURVEY and University of California–Berkeley began a three-year study to investigate patterns of bat distribution and activity in response to aquatic insect emergence and abundance in Yosemite National Park, California. Called “Project Bats and Rivers,” the study is funded by the Yosemite Fund, a private, nonprofit partner of the park. Using a combination of acoustic sampling and mist (fine mesh) netting, investigators learned that the number of bat species declines as elevation increases; the number of species with reproductive populations also declines with elevation. Preliminary analyses reveal considerable night-to-night variation in activity levels at most sites. Bat activity in the summer and autumn generally correlates with ambient temperature, with the greatest activity occurring on the warmest nights. In 2004 the team made the unexpected discovery that, though activity is low on the coldest nights, insect emergence and foraging continue at temperatures near freezing. Although the Bats and Rivers project started as an investigation of bat distribution and activity, two years of fieldwork suggested that the techniques being used were also applicable to monitoring ecosystem function by detecting changes in bat assemblages (groups of bat species inhabiting specific areas).

Until recently, bats have been hard to study because they are nocturnal and capturing them in mist nets is challenging. Acoustic detection now provides a reliable technique for recording and identifying each species based on its unique echolocation calls. During the 2004 field season the team continued to modify and improve the reliability of the Anabat acoustic detection system so that it can be used for long-term monitoring. Falling prices and lower power requirements for data storage devices have allowed investigators to deploy equipment that can operate without intervention for up to three months. These devices, which turn themselves on at dusk and off at dawn, store bat calls on small memory cards. The solar-powered systems have allowed researchers to sample simultaneously at multiple locations for extended periods with minimal labor. The system automatically identifies the species and records its abundance for a given night.

Cuuuute: Pallid Bat (Antrozous pallidus)
Photo: California Bat Conservation Fund

The team tested the efficacy of using this equipment in July 2003 by deploying 15 detectors simultaneously at selected elevations for six to eight nights. The detector in Yosemite Valley, when downloaded in late January 2004, revealed highly unexpected results: very high levels of bat activity (rivaling levels in summer) over the river, even on nights when the temperature dropped well below freezing. To place this finding in context, there is almost no information regarding where California bats winter. Although very large aggregations hibernate in caves in the eastern and midwestern United States, very little is known about winter behavior or overwintering sites for most western species. The intense bat activity, involving at least three species, suggests that Yosemite Valley, with its combination of granite cliffs and nearby river, may serve as a significant winter refugium for bats.

As predators that rely almost exclusively on insects, bats may provide a reliable measure of ecosystem function. Changes in the composition of bat species in a particular location may reflect ecosystem changes that require more intensive study. Using arrays of 8 to 10 acoustic detectors spread among a variety of habitats enables investigators to “capture” the widest diversity of bat species, increasing the chances of detecting those that are rare or habitat-specific.

The Sierra Monitoring Network is currently inventorying bats in three parks. In Sequoia National Park, two Anabat acoustic detectors are monitoring bat activity over the 2004–2005 winter. A remaining challenge for the Anabat system is developing filters that will fully automate the identification of bats from their calls. Though the Anabat acoustic detection system recognizes the sonogram of the calls of many species, some species still require visual examination of call sequences. The team is also working on designing a statistically valid sampling protocol for detecting real changes in bat assemblages. When the protocol is fully developed, assemblages of bat species could be selected as a “vital sign,” an indicator of ecosystem conditions for monitoring networks where bats abound.
Wildlife Biologist, U.S. Geological Survey Western Ecological Research Center, Yosemite Field Station, California
Consulting Biologist, Berkeley, California
Associate Specialist, Department of Integrative Biology, University of California–Berkeley

Further Reading

California Bat Conservation Fund Photos of bats as well as what to do with a bat in your house or where to take an injured bat. Offers educational outreach.

Watchable Wildlife: Bats  California Department of Fish and Game

California Bats


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Masthead Photo from:
Kaweahs From Trailcrest, Kings Canyon National Park
© 2009, Howard Weamer