Evaluating the efficacy of visual encounter and automated acoustic survey methods in anuran assemblages of the Yungas Andean forests of Argentina

The earliest comprehensive work on Cavite Province’s anurans started in 1998 in Mount Palay-Palay/Mataas na-Gulod Protected Landscape. Annual anuran assessments have been done in this protected area since then, but not for the entire province. The anuran assemblages are also experiencing numerous anthropogenic threats that could affect their diversity. The aim of the study was to determine anuran diversity, richness, and abundance in six secondary lowland forest fragments in Cavite and further contribute to existing knowledge on Philippine anurans. Anuran surveys were conducted from February to September 2010 by employing a combination of strip transect sampling, time-constrained searches, visual encounter survey (VES), and acoustic encounter survey (AES). Utilizing EstimateS v.8.2, species diversity was represented by Shannon’s Diversity Index (H’), species richness by the non-parametric Jackknife1 and sampling efficiency by MaoTau. Twenty-one (21) microhabitats were identified, and species diversity and richness were highest in riparian forests during both dry and wet seasons. A total of 17 anuran taxa, including five representing new records for Cavite, were recorded. Species diversity (H ́) values slightly varied and a comparison of the MaoTau and the Jackknife1 results indicated that ca. 80-100% of the anuran species were detected from the different forest fragments. The anuran assemblage has a high degree of endemism (70.6%) with two yet-to-be described species, Platymantis sp. and Kaloula sp., also known to occur in Cavite.

In recent years, passive acoustic monitoring (PAM) has become increasingly popular. Many acoustic indices (AIs) have been proposed for rapid biodiversity assessment (RBA), however, most acoustic indices have been reported to be susceptible to abiotic sounds such as wind or rain noise when biotic sound is masked, which greatly limits the application of these acoustic indices. In this work, in order to take an insight into the influence mechanism of signal-to-noise ratio (SNR) on acoustic indices, four most commonly used acoustic indices, i.e., the bioacoustic index (BIO), the acoustic diversity index (ADI), the acoustic evenness index (AEI), and the acoustic complexity index (ACI), were investigated using controlled computational experiments with field recordings collected in a suburban park in Xuzhou, China, in which bird vocalizations were employed as typical biotic sounds. In the experiments, different signal-to-noise ratio conditions were obtained by varying biotic sound intensities while keeping the background noise fixed. Experimental results showed that three indices (acoustic diversity index, acoustic complexity index, and bioacoustic index) decreased while the trend of acoustic evenness index was in the opposite direction as signal-to-noise ratio declined, which was owing to several factors summarized as follows. Firstly, as for acoustic diversity index and acoustic evenness index, the peak value in the spectrogram will no longer correspond to the biotic sounds of interest when signal-to-noise ratio decreases to a certain extent, leading to erroneous results of the proportion of sound occurring in each frequency band. Secondly, in bioacoustic index calculation, the accumulation of the difference between the sound level within each frequency band and the minimum sound level will drop dramatically with reduced biotic sound intensities. Finally, the acoustic complexity index calculation result relies on the ratio between total differences among all adjacent frames and the total sum of all frames within each temporal step and frequency bin in the spectrogram. With signal-to-noise ratio decreasing, the biotic components contribution in both the total differences and the total sum presents a complex impact on the final acoustic complexity index value. This work is helpful to more comprehensively interpret the values of the above acoustic indices in a real-world environment and promote the applications of passive acoustic monitoring in rapid biodiversity assessment.

The diversity of frogs and toads in Universitas Gadjah Mada campus area has been studied by Prasintaningrum on the year 2018. The 2018 study found five different species of frogs and toads in the campus area. The diversity and abundance of frogs and toads species in the east side of campus area is monitored in this study. The purpose of this study is to compare the diversity and abundance also the distribution of frogs and toads in the east side of campus area between the year 2018 and 2020. This study conducted using visual encounter survey and time-constrained search as the sampling technique. From the monitoring result we concluded that the diversity and evenness of frogs and toads show reducing value in year 2020 compared to 2018. Duttaphrynus melanostictus species show increase in abundance, instead Kaloula baleata and Polypedates leucomystax show reduce in abundance in year 2020 compared to 2018. The relative distribution is changing in year 2020 compared to 2018.  Key words: Anura; monitoring; diversity; abundance; Universitas Gadjah Mada.

Amphibians are among the most endangered groups of vertebrates worldwide; therefore, effective population monitoring is crucial for understanding the factors driving these unfavourable trends. In this study, we evaluated the effectiveness of fully automated passive acoustic monitoring in estimating the distribution and population size of the European green toad (Bufotes viridis)-an anuran species in which males vocalize intensively during the breeding season. We conducted visual counts with human observers, recorded the soundscape using autonomous sound recorders, applied automatic call detection, and calculated soundscape complexity using acoustic indices adjusted to the frequency range of European green toad vocalizations. Our algorithm generated a calling intensity index, which detected 36% of calls with 96% precision. The calling intensity index showed a significant correlation with the number of calls detected manually (r = 0.727). Additionally, we found moderate but significant correlations between acoustic indices (Acoustic Complexity Index and Bioacoustic Index) and both the manually and automatically detected call counts. The calling intensity index and acoustic indices did not significantly correlate with the number of males counted by human observer, but they reflected the phenological changes in species' reproductive activity. Our study demonstrates that passive acoustic monitoring enables the assessment of presence and distribution of the European green toad, as well as daily and seasonal patterns in calling activity. However, due to the specific biology and ecology of the species, calling intensity alone is insufficient for accurately estimating male abundance.

Mixed-species forest stands may provide suitable habitats for more bird species than tree monocultures by increasing the number of niches and resources available. However, assessing the effects of forest diversity on birds over time and across multiple sites using traditional bird survey methods can be expensive, time-consuming and impractical. Alternatively, we used passive acoustic monitoring (PAM) and acoustic indices to investigate how tree species diversity, relative density, and other plot characteristics affect the species richness of avian communities in temperate forests. Acoustic monitoring was conducted in Nationalpark Hainich, Germany, using plots which differed in tree species diversity and relative species density. Using a subset of our data, five acoustic indices, the normalised difference soundscape index (NDSI), acoustic complexity index (ACI), bioacoustic index (BI), the acoustic evenness index (AEI) and temporal entropy (Ht), were initially assessed using audio recordings of different soundscape elements to determine the degree to which they were influenced by non-biophony soundscape elements. Of these, NDSI, ACI and BI were considered least biased and displayed significant positive relationships with bird species richness assessed using audio recordings. However, NDSI values plateaued at around 5 bird species when measuring bird species richness and ACI values for dawn choruses were lower than expected, potentially due to the high density of birdsong saturating the soundscape. Plot tree species diversity had significant positive effects on ACI and BI during dawn, dusk and daytime periods. Increased basal area of sycamore (Acer pseudoplatanus) and, to a lesser degree, beech (Fagus sylvatica), had a negative effect on acoustic indices. These results could potentially be due to the earlier leaf out timings of these tree species in relation to the dates when surveys were performed. While the use of PAM and acoustic indices enabled surveys across multiple sites over longer time periods than would previously have been achievable, our results show that care should be taken when selecting indices and interpreting acoustic index results.

Passive acoustic monitoring (PAM) is a sampling technique that has gained increasing popularity in the field of wildlife monitoring and research since it allows for non-invasive and cost-effective collection of acoustic information. Retrieving biological information from PAM recordings can often involve time-consuming sound annotation methodologies, but the advent of acoustic indices can help expedite this process. While correlations between acoustic indices and species richness have been observed in a variety of ecological contexts, these relationships faulter in environments with increased vehicular noise. Here, we assessed the direct impact of vehicular noise on nine acoustic indices through controlled manipulation of vehicular noise within computer-generated bird assemblage soundscapes. Our results demonstrate that recording distance from roadsides and number of passing cars per minute have notable and persistent impacts on acoustic index values, but the magnitude of the effect varies across indices. Four acoustic indices demonstrated greater resilience to vehicular noise interference and may therefore be better suited for developed areas: Bioacoustic Index, Acoustic Complexity Index, Acoustic Diversity Index, and Acoustic Evenness Index. By contributing to the collective understanding of acoustic index behaviours under anthropogenic noise pollution, we hope to better inform their ecological application in human-developed contexts.

High-pass filters do not consistently strengthen associations between acoustic indices and avian species richness

Evaluating acoustic indices in the Valdivian rainforest, a biodiversity hotspot in South America

Mt. Palay-Palay is Cavite’s only protected landscape and yet very few herpetological studies have been done in this area. Thus, the study aims to provide robust ecological data on different anuran species so that an effective conservation and management plan could be formulated. Five habitats were sampled using a combination of cruising transect, stratified random strip transect sampling, time-constrained searches, visual encounter survey (VES) and acoustic encounter survey (AES). A total of 1528 individuals belonging to 12 species was recorded from the study area. In addition to previous works, 2 Platymantis spp. were new records bringing the total species richness to 16. Of the 16 species, 10 (62.5%) are endemic to the Philippines. Among the species, Platymantis mimulus was the most abundant and also had the highest density of 174 frogs ha-1. Among the habitats riparian forests had highest species diversity, Mau Tao and Jackknife1 values which showed dependence of anurans on water. Of the 17 microhabitats, forest floor litter was the most occupied by anurans.

In recent decades, climate change has significantly influenced the frequency and intensity of wildfires across Mediterranean pine forests. The loss of forest cover can bring long-term ecological changes that impact the overall biodiversity and alter species composition. Understanding the long-term impact of wildfires requires effective and cost-efficient methods for monitoring the postfire ecosystem dynamics. Passive acoustic monitoring (PAM) has been increasingly used to monitor the biodiversity of vocal species at large spatial and temporal scales. Using acoustic indices, where the biodiversity of an area is inferred from the overall structure of the soundscape, rather than the more labor-intensive identification of individual species, has yielded mixed results, emphasizing the importance of testing their efficacy at the regional level. In this study, we examined whether widely used acoustic indicators were effective at capturing changes in the avifauna diversity in Pinus halepensis forest stands with different fire burning histories (burnt in 2001, 2009, and 2018 and unburnt for >20 years) on the Sithonia Peninsula, Greece. We recorded the soundscape of each stand using two–three sensors across 11 days of each season from March 2022 to January 2023. We calculated for each site and season the following five acoustic indices: the Acoustic Complexity Index (ACI), Acoustic Diversity Index (ADI), Acoustic Evenness Index (AEI), Normalized Difference Soundscape Index (NDSI), and Bioacoustic Index (BI). Each acoustic index was then assessed in terms of its efficacy at predicting the local avifauna diversity, as estimated via two proxies—the species richness (SR) and the Shannon Diversity Index (SDI) of vocal bird calls. Both the SR and SDI were calculated by having an expert review the species identification of calls detected within the same acoustic dataset by the BirdNET convolutional neural network algorithm. A total of 53 bird species were identified. Our analysis shows that the BI and NDSI have the highest potential for monitoring the postfire biodiversity dynamics in Mediterranean pine forests. We propose the development of regional-scale acoustic observatories at pine and other fire-prone Mediterranean habitats, which will further improve our understanding of how to make the best use of acoustic indices as a tool for rapid biodiversity assessments.

In recent years, passive acoustic monitoring (PAM) has been widely applied in the field of soundscape ecology. Meanwhile, acoustic indices, as an important tool for rapid biodiversity assessment (RBA), have also attracted increasing attention. However, considerable evidence demonstrates that existing acoustic indices are susceptible to noise, which poses a challenge for wider applications of acoustic indices. In this work, we firstly provide insights into how noise affects the widely used acoustic diversity index (ADI), revealing that the global threshold scheme (i.e., −50 dBFS for all time–frequency bins) to generate the binary spectrogram (spectrogram containing time–frequency bins denoted as ‘1′ with power above the threshold and ‘0′ below) is the key factor that makes ADI sensitive to noise. To address this problem, we propose a revised ADI version named frequency-dependent acoustic diversity index (FADI), which employs a new threshold scheme to reduce noise impacts on its values. Specifically, considering that the noise floor along frequency axis usually varies in practice, i.e., a colored noise rather than a white noise exists in field recordings, floating thresholds adapted to the noise level at each frequency bin were employed and further incorporated with the global threshold of ADI. Controlled computational experiments using field recordings were conducted where manipulated noise levels were introduced as different signal-to-noise ratios (SNR). It was shown that the proposed FADI was significantly robust to noise within a much wider SNR range as compared with ADI. Furthermore, a singular example using a real-world recording also verified its effectiveness and highlighted the promising potential of FADI in applications of PAM.

Sounding out ecoacoustic metrics: Avian species richness is predicted by acoustic indices in temperate but not tropical habitats

Biodiversity loss is a global concern. Current technological advances allow the development of novel tools that can monitor biodiversity remotely with minimal disturbance. One example is passive acoustic monitoring (PAM), which involves recording the soundscape of an area using autonomous recording units, and processing these data using acoustic indices, for example, to estimate the diversity of various vocal animal groups. We explored the hypothesis that data obtained through PAM could also be used to study ecosystem functions. Specifically, we investigated the potential relationship between seven commonly used acoustic indices and insect leaf herbivory, measured as total leaf damage and as the damage from three major insect feeding guilds. Herbivory was quantified on seedlings in 13 plots in four subtropical forests in south China, and acoustic data, representing insect acoustic complexity, were obtained by recording the evening soundscapes in those same locations. Herbivory levels correlated positively with the acoustic entropy index, commonly reported as one of the best-performing indices, whose high values indicate higher acoustic complexity, likely due to greater insect diversity. Relationships for specific feeding guilds were moderately stronger for chewers, indicating that the acoustic indices capture some insect groups more than others (e.g., chewers include soniferous taxa such as crickets, whereas miners are mostly silent). Our findings suggest that the use of PAM to monitor ecosystem functions deserves to be explored further, as this is a research field with unexplored potential. Well-designed targeted studies could help us better understand how to best use novel technologies to monitor ecosystem functions.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00442-024-05536-9.

Frogs and toads (Amphibia: Anura) were very sensitive to environmental changes. Due to their sensitivity, usually they can be used as indicator for environmental changes. On the other hands, there has been only limited data on the diversity and distribution of frogs and toads in Java. This could lead to the reducing population of frogs and toads in Java. Based on those facts, this research was aimed to collect data on species diversity, species richness, species composition and species distribution of frogs and toad from Sawangan Ecotourism, Magelang, Central Java Province. This study was expected to complete the database of anurans in Central Java. The research was conducted from June to July 2009, mostly at night. The VES ( Visual Encounter Survey ) method combined with 200 m transects were used for sampling. Seven species of anurans were recorded during the survey. They were Duttaphrynus melanostictus, Limnonectes kuhlii, Limnonectes microdiscus, Microhyla achatina, Odorrana hosii, Hylarana chalconotus and Huia masonii . From those seven species, only three ( Huia masonii , Microhyla achatina and Limnonectes kuhlii ) were known as endemic of Java.

Passive acoustic monitoring is a potentially valuable tool in biodiversity hotspots, where surveying can occur at large scales across land conversion types. However, in order to extract meaningful biological information from resulting enormous acoustic datasets, rapid analytical techniques are required. Here we tested the ability of a suite of acoustic indices to predict avian bioacoustic activity in recordings collected from the Western Ghats, a biodiversity hotspot in southwestern India. Recordings were collected at 28 sites in a range of land-use types, from tea, coffee, and cardamom plantations to remnant forest stands. Using 36 acoustic indices we developed random forest models to predict the richness, diversity, and total number of avian vocalizations observed in recordings. We found limited evidence that acoustic indices predict the richness and total number of avian species vocalizations in recordings (R2 < 0.51). However, acoustic indices predicted the diversity of avian species vocalizations with high accuracy (R2 = 0.64, mean squared error = 0.17). Index models predicted low and high diversity best, with the highest residuals for medium diversity values and when continuous biological sounds were present (e.g., insect sounds >8 sec). The acoustic complexity index and roughness index were the most important for predicting avian vocal diversity. Avian species richness was generally higher among shade-grown crops than in the open tea plantation. Our results suggest that models incorporating acoustic indices can accurately predict low and high avian species diversity from acoustic recordings. Thus, ecoacoustics could be an important contributor to biodiversity monitoring across landscapes like the Western Ghats, which are a complex mosaic of different land-use types and face continued changes in the future.