Digital Recorders Increase Detection of Eleutherodactylus Frogs

Improved methods for large-scale and long-term amphibian monitoring projects would aid species status assessments and identification of potentially declining species (e.g., Stuart et al. 2004). Acoustical surveys allow researchers to quickly determine species presence and calling activity level (Lips et al. 2001; Zimmerman 1994). The North American Amphibian Monitoring Program, a large-scale monitoring effort, uses acoustical surveys to monitor species in ponds and wetlands (United States Geological Survey 2004). A challenge of this approach is accurate species identification, which is particularly problematic in a tropical area with high anuran species diversity (e.g., anuran species richness in the Neotropics [2135] was greater than in the Neartic [90], Duellman 1999). Furthermore, abundant or loud species may lower the detection probability of other species. An assumption that needs to be tested in acoustical surveys is that detection is highly correlated with the species presence or activity. Without an appropiate correction for detection probability, data will not accurately reflect the status of the species (MacKenzie et al. 2002). This may result in inappropriate management decisions that could waste time and money. Digital recorders can be used as a standard method for species detection and determination of calling activity level in amphibian choruses in long-term monitoring projects. Digital recordings can be stored and transferred easily and can allow experts to analyze the recordings at their convenience. In addition, loud or abundant species can be filtered using computer software to increase the detection of other species. I tested the hypothesis that detection of species presence and calling activity level in anuran choruses will be higher when recordings are analyzed with the help of computer software compared to only hearing the recordings. The resulting increase in detection should result in improved population status data and reduced false negatives during species inventories (i.e., when a species is not detected due to interference). Materials and Methods. —I recorded amphibian choruses at two sites in the Luquillo Experimental Forest (Tradewinds Trail site, 18.290°N, 65.798°W; Mount Britton Tower site, 18.303°N, 65.795° W), and at one site in the Carite State Forest (18.103° N, 66.035° W), Puerto Rico (Fig. 1). The Tradewinds and Mount Britton sites were located in the Lower Montane Wet Forest lifezone and the Carite State Forest site was located in the Subtropical Wet Forest lifezone (Ewell and Whitmore 1973). Recordings were made using an Automated Digital Recording System (ADRS; Acevedo and Villanueva-Rivera 2006). The recorder was a Nomad Jukebox 3 digital player and recorder (Model DAP-HD0003, Creative Labs, Inc, California), and was set to record in 16-bit wav files with a sampling rate of 48 kHz. Because the recorder had no microphone input, only a line level input, the microphone was connected to a portable preamplifier (Model SP-PREAMP, The Sound Professionals, Inc., New Jersey). To test if digital recordings can be used to increase detection in tropical frog choruses, I compared one minute recordings, made at 2000, 2200, and 0000 h on five consecutive days, for a total of fifteen recordings per site. I analyzed these recordings using two methods: 1) listening to the recordings; and 2) listening and analyzing the recordings using computer software. For the first method I listened to the recordings with headphones, and each species heard was given an Amphibian Calling Index value (ACI; United States Geological Survey 2004). The ACI has four possible values: 0—no animals calling; 1—a few animals calling without overlap; 2—some overlap of animals calling; and 3—a full chorus of the species. For the second method, I listened to the recordings with headphones and analyzed them using computer software to detect the signal of each species’ call by their sound and visual pattern in the auditory spectrum. There is little overlap in the call spectrum of the Eleutherodactylus frogs in Puerto Rico, and thus it was possible to distinguish each species (Drewry and Rand 1983). In addition, recordings of the calls of all the species have been published (Rivero 1998). Each recording was then filtered to remove the range of frequencies of the dominant species, Eleutherodactylus coqui (1–2.4 kHz), and re-analyzed. I gave each species a new ACI value for this second method. All recordings were coded and analyzed in random order to reduce bias. Each pair of results, for each recording, by species, were compared using Wilcoxon signed ranks tests. To evaluate the differences in the level of the recorded sound of the species, which may explain why some species were not heard, I obtained the average signal level by frequency using the values of decibels full scale (dBFS), which corresponds, on a logarithmic scale, to the signal in the digital file. On this scale the maximum is 0 dB and the minimum is -96 dB, which corresponds to the maximum and minimum levels, respectively, of sound that the