The management and recovery of threatened and endangered amphibians on Department of Defense (DoD) lands relies on an understanding of their distribution and abundance. Fortunately, most anuran species can be surveyed acoustically using vocalizations during the breeding season. This project demonstrated the use of subsurface passive acoustic monitoring (SPAM) to survey for rare underwater-calling, at-risk anuran species on DoD installations. 

The objective of this project was to demonstrate how SPAM can be used to document and monitor the occurrence and relative abundance of two at-risk, underwater-calling frogs on DoD installations. Specifically, the project aimed to (1) examine the influence of environmental factors (e.g., water depth, temperature, substrate type, vegetation density) on acoustic detectability, (2) demonstrate and validate the ability of SPAM to detect frog calls generated below the water’s surface, (3) demonstrate the effectiveness of SPAM data for estimating relative abundance, and (4) compare the cost-effective-ness of SPAM as a monitoring technology relative to traditional survey methods for amphibians on DoD installations.

This project evaluated the performance of SPAM relative to traditional passive acoustic monitoring (PAM) (microphone) and human manual calling survey (MCS) methods. This project transferred SPAM technology to freshwater systems to survey for two at-risk species: the Chiricahua leopard frog (Lithobates chiricahuensis), a federally threatened species, and the gopher frog (Lithobates capito), under review for federal listing. 

The project team used archival acoustic recording units with two channels of audio in-put, modified from the Swift autonomous recording unit developed by the Center for Conservation Bioacoustics at Cornell University. One channel was connected to a hydro-phone for recording underwater (SPAM component), and the other channel was connected to an airborne microphone for recording sounds in air (PAM component). The recorders stored data on secure digital extended capacity flash cards and were powered by 16 D-cell batteries, enabling continuous recording for up to three months.

Results showed that SPAM outperformed PAM and MCS in validation experiments where calls were generated underwater. SPAM was less successful than PAM and MCS in the field demonstration. Most leopard frog calls were apparently produced in air despite previous reports of extensive underwater-calling behavior. 

This project highlights how acoustic information can help address a data gap in the ecology of at-risk species, which can help refine future survey methodology and management efforts. Ultimately, the utility of SPAM for under-water-calling species will depend on the focal species, the landscape where it occurs, and technological considerations available to the surveyor. SPAM is more expensive than traditional methods but, in some situations, may be the only way to effectively detect species.