Hoefer, Sebastian, Allen-Ankins, Slade, McKnight, Donald T., Nordberg, Eric J., and Schwarzkopf, Lin (2025) Sensors versus surveyors: Comparing passive acoustic monitoring, camera trapping and observer-based monitoring for terrestrial mammals. Methods in Ecology and Evolution, 16 (11). pp. 2603-2621.

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Abstract

Mammals play vital roles in ecological communities, but many are in rapid decline worldwide. Comprehensive monitoring of mammal populations is crucial for effective conservation, but large-scale monitoring presents significant challenges. Remote sensing techniques such as passive acoustic monitoring offer viable and effective solutions for surveying animal communities. While passive acoustic monitoring has shown promising results for birds, its application in mammal biodiversity assessments has received little testing. In this study, we compared passive acoustic monitoring (combined with BirdNET embeddings) to traditional observer-based monitoring and camera trapping for assessing terrestrial mammal biodiversity over multiple years across an extensive spatial scale in eastern Australia. Using embeddings from the BirdNET deep learning model, we efficiently analysed a cumulative total of 317,410 h of continuous audio data, recorded at a sampling rate of 22 kHz (effective up to 11 kHz), from six sites, successfully detecting all 17 target vocal mammal species. Given the inherent inability of passive acoustic monitoring to detect non-vocalising species or those vocalising outside this frequency range, we considered the mammal community in two ways: (i) entire mammal community and (ii) vocal mammals only. For detecting species in the entire mammal community, observer-based monitoring performed the best, followed by camera trapping and then passive acoustic monitoring. However, when focusing on vocal mammals only, all methods showed comparable performance for the same 28-day survey period, with passive acoustic monitoring demonstrating significantly better performance when leveraging long-term audio data. Additionally, we found a significant positive correlation between species richness of vocal mammals and that of the entire mammal community, suggesting that vocal mammal richness may serve as a proxy for overall mammal biodiversity. Furthermore, PAM's effectiveness was not influenced by common life-history traits, suggesting PAM may be a broadly applicable survey tool for vocal mammals. Despite the benefits, the inability of passive acoustic monitoring to detect non-vocal mammals necessitates a combined approach with other survey methods such as camera trapping or observer-based monitoring to achieve a comprehensive assessment of terrestrial mammal biodiversity. This combined approach is likely to enhance future biodiversity monitoring, offering a more detailed understanding of ecosystems and supporting effective conservation practices.

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