Bioacoustic monitoring of frogs
Creators: name: CASEY, Brendan
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 410299
subject: Ecological applications not elsewhere classified

description: Frog species within Australia are declining in abundance and range. The primary threats to frogs are disease and loss of habitat.  Determining the presence of frogs is difficult as they can be cryptic within their habitat or have long periods of dormancy.  Information about the status and ecology of frogs is urgently required to support actions directed towards their survival. The aim of this thesis was to use bioacoustic technology to determine the call activity of frog species at the monitoring sites and advance knowledge of their ecology.

This project developed a novel, species-specific acoustic monitoring program that addresses knowledge gaps about two frog species threatened with extinction. An important discovery was that the daily call activity of the Growling Grass Frog Litoria raniformis varied among adjacent habitats within the seasonal period of calling. Water temperature was confirmed as a significant variable associated with L. raniformis call activity. The effect of the discharge of treated effluent on L. raniformis calling was understood for the first time when stream level and flow rate were found to significantly correlate with periods of continuous calling. Acoustic recording using a hydrophone was a novel monitoring method that led to the first documented occurrence of an endemic Australian frog species calling underwater. Statistical analyses confirmed that the annual program of flooding an artificial habitat had a significant and positive correlation on call activity for five frog species. An important contribution to frog survey methods was made when the calls of the Giant Burrowing Frog Heleioporus australiacus were heard using passive, automated technology. Calls emitted by H. australiacus from a pool within a cleared landscape was new information about the species' habitat. Quantitative and qualitative descriptions of frog call characteristics improve the phonetic method commonly used to describe frog calls.

Monitoring the calls of frogs at carefully selected sites can support answering important research questions including whether the target species is present at the monitoring location; under what conditions are calls emitted; do human activities affect calling; and can frogs and their habitats be protected by listening to their calls?  

A Song Meter (SM4) was used at three modified habitats along Merri Creek to record the call activity of L. raniformis (Chapter 2). Calling was heard between mid-October to early February from a stormwater pond; mid-October to late January from a pool in a creek with modified hydrology; and late September to late January from a modified pond that was deliberately flooded during the program. Calling was mostly nocturnal at the lentic sites and spread throughout the diel period at the lotic site. Calling simultaneously ceased at the three sites during three synoptic scale weather events.

To investigate factors that influence L. raniformis call activity, an SM4 was positioned beside a pool located downstream from an effluent discharge point (Chapter 3). Linear discriminant analysis correctly predicted 82 % of call days, and quadratic discriminant analysis predicted 95 % of call days. Multilinear regression identified water temperature, evaporation and ultraviolet radiation as significant variables for predicting call days. Spearman correlation coefficients identified that sustained calling occurred when the water level and flow rate were steady or reducing. The effect of the discharge from the upstream sewage plant on L. raniformis call behaviour requires further investigation.
A discovery was made when Litoria raniformis was identified emitting calls underwater (Chapter 4). Underwater calling occurred at the lentic habitat six times and was within call groups that included above water calling. Underwater calling was heard on two occasions at the lotic habitat, firstly in the absence of above water calling and later when above water calls were made. The lentic habitat underwater calls had a lower peak frequency (av. 476.6 Hz) compared to above water calls (av. 1066.6 Hz), while they were similar at the lotic habitat (904.6 Hz compared to 953.4 Hz). Territorial type calls were heard underwater in all call groups at the lentic habitat and only heard above water at the lotic site. Underwater calling was hypothesised to be a response to territorial incursions by conspecifics at the lentic site, however visual observations are required to confirm this. 

To determine the benefit of the annual program of deliberately flooding an artificial pond to provide frog habitat, frog call activity was monitored continuously for a 20-month period (Chapter 5). Multilinear regression analyses using standardised variables found that the flooding program had a significant and positive effect on the daily call activity of C. signifera (model coefficient = 0.3784), L. tasmaniensis (0.8423), L. raniformis (0.3784), L. dumerilii (0.8653) and L. peronii (0.1311). Daylight hours, ultraviolet radiation and air temperature were variables having a significant association with the daily call activity of all frogs at the pond habitat. Linear discriminant analyses correctly predicted between 81.6 and 98.0 % of call days, and quadratic discriminant analyses predicted between 87.1 and 98.0 %. Knowledge of the environmental conditions when frogs call is crucial to support the design of monitoring surveys.

A stream transect of four acoustic recorders was positioned along pools within creek gullies to determine if calls of the Giant Burrowing Frog Heleioporus australiacus could be recorded (Chapter 6). The status of H. australiacus within Victoria was uncertain, as recent surveying has failed to detect the species, possibly due to cryptic behaviour or absence from the survey area. Heleioporus australiacus calls were heard on three occasions following 11700 acoustic sampling events. A total of sixty calls were heard from one site during early May 2020. Calls were comprised of one to seven notes, with 74 % of call notes having a peak frequency of 281.25 Hz, and 24 % being 234.38 Hz. Heleioporus australiacus calls came from a degraded pond within a cleared landscape, providing new information about the habitat of the species.
Examples of frog species calls heard during this program are presented as a catalogue of calls (Appendix 2). Calls are described visually (wave form plot of relative amplitude and spectrogram of call harmonics), quantitatively (peak frequency, call bandwidth, call or note length and inter-note interval) and qualitatively (using acoustic ecology terminology). The method used to describe the calls expands on the phonetic descriptions commonly used to describe frog calls.