Using citizen science to test for acoustic niche partitioning in frogs

The acoustic niche hypothesis proposes that to avoid interference with breeding signals, vocal species should evolve to partition acoustic space, minimising similarity with co-occurring signals. Tests of the acoustic niche hypothesis are typically conducted using a single assemblage, with mixed outcomes, but if the process is evolutionarily important, a pattern of reduced acoustic competition should emerge, on average, over many communities. Using a continental-scale dataset derived from audio recordings collected by citizen scientists, we show that frogs do partition acoustic space. Differences in calls were predominately caused by differences in spectral, rather than temporal, features. Specifically, the 90% frequency bandwidths of observed frog assemblages overlapped less than expected, and there was greater distance between dominant frequencies than expected. To our knowledge, this study is the first to use null models to test for acoustic niche partitioning over a large geographic scale

Using a novel visualization tool for rapid survey of long-duration acoustic recordings for ecological studies of frog chorusing

Continuous recording of environmental sounds could allow long-term monitoring of vocal wildlife, and scaling of ecological studies to large temporal and spatial scales. However, such opportunities are currently limited by constraints in the analysis of large acoustic data sets. Computational methods and automation of call detection require specialist expertise and are time consuming to develop, therefore most biological researchers continue to use manual listening and inspection of spectrograms to analyze their sound recordings. False-color spectrograms were recently developed as a tool to allow visualization of long-duration sound recordings, intending to aid ecologists in navigating their audio data and detecting species of interest. This paper explores the efficacy of using this visualization method to identify multiple frog species in a large set of continuous sound recordings and gather data on the chorusing activity of the frog community. We found that, after a phase of training of the observer, frog choruses could be visually identified to species with high accuracy. We present a method to analyze such data, including a simple R routine to interactively select short segments on the false-color spectrogram for rapid manual checking of visually identified sounds. We propose these methods could fruitfully be applied to large acoustic data sets to analyze calling patterns in other chorusing species

Passive acoustic monitoring in terrestrial vertebrates: a review

Passive acoustic monitoring (PAM) has become increasingly popular in ecological studies, but its efficacy for assessing overall terrestrial vertebrate biodiversity is unclear. To quantify this, its performance for species detection must be directly compared to that obtained using traditional observer-based monitoring (OBM). Here, we review such comparisons across all major terrestrial vertebrate classes and identify factors impacting PAM performance. From 41 studies, we found that while PAM-OBM comparisons have been made for all major terrestrial vertebrate classes, most comparisons have focused on birds (65%) in North America (52%). PAM performed equally well or better (61%) compared to OBM in general. We found no statistical difference between the methods for total number of species detected across all vertebrate classes (excluding reptiles); however, recording period and region of study influenced the relative performance of PAM, while acoustic analysis method and which method sampled for longer overall showed no impact. Further studies comparing PAM performance in non-avian vertebrates using standardised methods are needed to investigate in more detail the factors that may influence PAM performance. While PAM is a valuable tool for vertebrate surveys, a combined approach with targeted OBM for non-vocal species should achieve the most comprehensive assessment of terrestrial vertebrate communities

Can an acoustic observatory contribute to the conservation of threatened species?

Observatories are designed to collect data for a range of uses. The Australian Acoustic Observatory (A2O) was established to collect environmental sound, including audible species calls, from 344 recorders at 86 sites around Australia. We examine the potential of the A2O to monitor near threatened, threatened, endangered and critically endangered species, based on their vocal behaviour, geographic distributions in relation to the sites of the A2O and on some knowledge of habitat use. Using IUCN and EPBC lists of threatened and endangered species, we extracted species that vocalized in the audible range, and using conservative estimates of their geographic ranges, determined whether there was a possibility of hearing them at these sites. We found that it may be possible to detect up to 171 threatened species at sites established for the A2O, and that individual sites have the potential to detect up to 40 threatened species. All 86 sites occurred in locations where threatened species could possibly be detected, and the list of detectable species included birds, amphibians, and mammals. We have incidentally detected one mammal and four bird species in the data during other work. Threatening processes to which potentially detectable species were exposed included all but two IUCN threat categories. We concluded that with applications of technology to search the audio data from the A2O, it could serve as an important tool for monitoring threatened species

Spectral overlap and temporal avoidance in a tropical savannah frog community

Vocalizing animals often occur in complex acoustic environments where there is a high chance of acoustic interference from co-occurring species with spectrally overlapping vocalizations. Acoustic interference impedes detection and discrimination of signals, and may ultimately reduce fitness. The aim of this study was to determine the relationship between spectral overlap and temporal overlap in a tropical savannah frog community. We predicted that species pairs that overlap spectrally would behaviourally respond to one another by avoiding calling at the same time, to minimize acoustic interference. We determined the calling activity of 14 frog species at four locations across 4 months during peak breeding season. We then determined the difference between observed and expected temporal overlap for all species pairs as a function of spectral overlap at three temporal scales (night-minute-call). There was potential for acoustic interference between many of the species studied with the calls of up to half of the species pairs at each site overlapping spectrally. There was no relationship between spectral overlap and observed–expected temporal overlap at either the night or minute level. Instead, calling activity was aggregated at both the night and minute level. Evidence of a negative relationship between spectral overlap and temporal overlap was found at the call level, with species pairs with greater mean spectral overlap calling together less frequently than expected. The observed aggregated calling at the night and minute level suggest that at gross timescales, factors other than avoiding spectral overlap are driving calling activity in tropical savannah frog communities. The call level avoidance found in this study may be sufficient for spectrally overlapping species pairs to avoid the negative effects of acoustic interference, allowing them to call during the same nights and at similar times to take advantage of favourable environmental conditions

Contrasting fundamental and realized niches: two fishes with similar thermal performance curves occupy different thermal habitats

Human alteration of thermal regimes of freshwater ecosystems is creating an urgent need to understand how freshwater ectotherms will fare under different thermal futures. Two key questions are: 1) how well do the fundamental thermal niches of ectotherms map to their realized thermal niches, and 2) which axes of the fundamental thermal niche must be modeled to predict temperature-dependent fitness in real ecosystems? The first question is particularly challenging in riverine systems, where gradients in temperature are strongly confounded by gradients in other biotic and abiotic drivers. To address these questions, we compared the realized and fundamental thermal niches of 2 congeneric riverine fish: Gadopsis marmoratus and Gadopsis bispinosus. We characterized their realized thermal niches by examining their distributions in relation to environmental temperature at multiple scales. We characterized their fundamental thermal niches by doing laboratory experiments on the thermal sensitivity of swimming performance and metabolic rates, particularly aerobic scope. The distributions of the 2 species supported the idea that they have different realized thermal niches, with G. bispinosus occupying cooler habitats than G. marmoratus. Despite this, we detected no significant differences in the shapes of thermal performance curves defining 2 axes of their fundamental niches: swimming performance and aerobic scope. Our results suggest that either the distributions of these 2 species are driven by factors other than temperature or that swimming performance and aerobic scope were not suitable proxies of their fundamental thermal niches. Our study shows that modeling the thermal niches of ectotherms along the river continuum is not straightforward. If we are to forecast effects of thermal futures effectively and efficiently, we must do more to decipher the relative influence of temperature and other abiotic drivers on the fitness of riverine ectotherms

Effects of current and future climates on the growth dynamics and distributions of two riverine fishes

1. To facilitate conservation planning, there is a need for improved confidence in forecasts of climate change impacts on species distributions. Towards that end, there have been calls for the development of process‐based models to test hypotheses concerning the mechanisms by which temperature shapes distribution and to corroborate forecasts of correlative models. 2. Models of temperature‐dependent growth (TDG) were developed for two Australian riverine blackfishes with disjunct longitudinal distributions: Gadopsis marmoratus (occupies lower, warmer elevations) and Gadopsis bispinosus (occupies higher, cooler elevations). The models were used to (a) predict blackfish monthly and annual growth dynamics under current and future climate scenarios within different elevation bands of their current distribution, and (b) test the hypothesis that, under the current climate, the distributions of each species would be positively correlated with predicted TDG. 3. Increases in mean annual growth were forecast for both species under all warming scenarios, across all elevation bands. Both species currently occupy annual habitat temperatures below those optimal for growth. Under certain warming scenarios, the predicted increases in annual growth belie forecasts of within‐year dynamics that may interact with the phenology of blackfish to impair recruitment. 4. There was not a significant positive linear relationship between predicted TDG and observed abundance among river segments for either species. Both species were strongly under‐represented where annual growth rates were forecast to be optimal and over‐represented where growth rates were forecast to be intermediate. 5. Confidence in forecasts of climate change impacts based on correlative models will increase when those forecasts are consistent with a mechanistic understanding of how specific drivers (e.g. water temperature) affect processes (e.g. growth). This process‐based study revealed surprises concerning how future climates may affect fish growth dynamics, showing that although the blackfish distributions are correlated with temperature the temperature‐dependent mechanisms underpinning that correlation require further investigation

Heat and hypoxia give a global invader, Gambusia holbrooki, the edge over a threatened endemic fish on Australian floodplains

Deciphering the mechanisms by which climate change interacts with invasive species to affect biodiversity is a major challenge of global change biology. We conducted experiments to determine whether the global invader, Gambusia holbrooki, was more resistant to high water temperature (heat) and low dissolved oxygen (hypoxia) than a threatened native fish, Nannoperca australis. Metabolic experiments conducted at 25 and 29 °C showed that G. holbrooki had at least four times the capacity for metabolic depression during hypoxia than N. australis. An increase in environmental temperature from 25 to 29 °C had no significant impact on the critical oxygen tension, Pcrit, of G. holbrooki, but significantly and strongly increased Pcrit of N. australis. Gambusia holbrooki also had a lower Q10 of standard metabolic rate than N. australis. Our results indicate that G. holbrooki have physiological traits conferring greater resistance to hypoxia than N. australis, and as temperature increases, the resistance of N. australis to hypoxia was more eroded than that of G. holbrooki. Intensive monitoring of the temperature and dissolved oxygen dynamics of wetlands showed that contemporary heat waves are already causing conditions that might give G. holbrooki the edge over N. australis on Australian floodplains. Our study adds weight to recent anecdotal reports of drought and heat waves causing localised extinction of N. australis, but the proliferation of G. holbrooki

Automated species identification of frog choruses in environmental recordings using acoustic indices

Acoustic monitoring provides opportunities for scaling up bioacoustic study of vocal animals to greater temporal and spatial scales. However, the large amounts of audio that can be easily and efficiently collected necessitates automated methods of analysis to extract useful ecological data. Acoustic indices have been used in spectrographic visualisation of long environmental recordings to successfully identify many biological sounds from their acoustic patterns and features. In particular, the choruses of several frog species are conspicuous in these spectrogram images which suggests that acoustic indices may be useful for detecting species in automated sound classification algorithms. The aim of this study was to investigate the use of acoustic indices as predictors in classification models for automated identification of frog species in environmental sound recordings from breeding habitats in north Queensland, Australia. Three types of classification models (random forests, support vector machines and gradient boosting) were trained and validated on a data set of 3274 1-minute audio segments labelled for the presence or absence of calling of 12 target frog species, and a feature set of 11 acoustic indices calculated on frequency bins of bandwidth 43.1 Hz. Classification performance was high for all 12 target species on the validation data set held out from the labelled training data (precision range 0.90-1.00 and recall range 0.83-0.99). However, performance declined for most target species when predicting frog calling on a further test data set taken from unseen recordings from the same sites. Best prediction results on the test data were achieved for species with the most training data, indicating accuracy may be improved by increasing training data, and this method is best suited to predicting chorusing of common species

Passive acoustic monitoring in terrestrial vertebrates: a review

Passive acoustic monitoring (PAM) has become increasingly popular in ecological studies, but its efficacy for assessing overall terrestrial vertebrate biodiversity is unclear. To quantify this, its performance for species detection must be directly compared to that obtained using traditional observer-based monitoring (OBM). Here, we review such comparisons across all major terrestrial vertebrate classes and identify factors impacting PAM performance. From 41 studies, we found that while PAM-OBM comparisons have been made for all major terrestrial vertebrate classes, most comparisons have focused on birds (65%) in North America (52%). PAM performed equally well or better (61%) compared to OBM in general. We found no statistical difference between the methods for total number of species detected across all vertebrate classes (excluding reptiles); however, recording period and region of study influenced the relative performance of PAM, while acoustic analysis method and which method sampled for longer overall showed no impact. Further studies comparing PAM performance in non-avian vertebrates using standardised methods are needed to investigate in more detail the factors that may influence PAM performance. While PAM is a valuable tool for vertebrate surveys, a combined approach with targeted OBM for non-vocal species should achieve the most comprehensive assessment of terrestrial vertebrate communities