Long-duration recordings of the natural environment have many advantages in passive monitoring of animal diversity. Technological advances now enable the collection of far more audio than can be listened to, necessitating the development of scalable approaches for distinguishing signal from noise. Computational methods, using automated species recognisers, have improved in accuracy but require considerable coding expertise. The content of environmental recordings is unconstrained, and the creation of labelled datasets required for machine learning purposes is a time-consuming, expensive enterprise. Here, we describe a visual approach to the analysis of environmental recordings using long-duration false-colour (LDFC) spectrograms, prepared from combinations of spectral indices. The technique was originally developed to visualize 24-hour “soundscapes.” A soundscape is an ecoacoustics concept that encompasses the totality of sound in an ecosystem. We describe three case studies to demonstrate how LDFC spectrograms can be used, not only to study soundscapes, but also to monitor individual species within them. In the first case, LDFC spectrograms help to solve a “needle in the haystack” problem—to locate vocalisations of the furtive Lewin’s Rail (Tasmanian), Lewinia pectoralis brachipus. We extend the technique by using a machine learning method to scan multiple days of LDFC spectrograms. In the second case study, we demonstrate that frog choruses are easily identified in LDFC spectrograms because of their extended time-scale. Although calls of individual frogs are lost in the cacophony of sound, spectral indices can distinguish different chorus characteristics. Third, we demonstrate that the method can be extended to the detection of bat echolocation calls. By converting complex acoustic data into readily interpretable images, our practical approach bridges the gap between bioacoustics and ecoacoustics, encompassing temporal scales across three orders of magnitude. Using the one methodology, it is possible to monitor entire soundscapes and individual species within those soundscapes.

Fire is notably becoming more intense, frequent and widespread due to climate change. In northern Australia, inappropriate fire regimes have been implicated in mammal declines, yet nothing is known about how different aspects of fire regimes affect bats in this region. This study aimed to determine how fire intensity, associated with seasonality, affects insectivorous bats on a local scale. An experimental M BACI approach was used on five site replicates across Cape York Peninsula, where ultrasonic detectors were used to determine the activity of insectivorous bats in response to low intensity burns (LIBs) and high intensity burns (HIBs) on a local scale. Total bat activity increased due to LIBs, but showed no response to HIBs. Activity of edge-open guild bats also increased due to LIBs but decreased in response to HIBs. Activity of open guild bats was unaffected by LIBs, but exhibited a strong positive response to HIBs. Activity of closed guild bats showed no response to fire, or fire intensity. Responses were likely derived from changes in habitat structure and prey availability. Given that each bat guild responded differently to each fire intensity, this lends support to the ‘pyrodiversity begets biodiversity’ concept, which is currently the basis for many fire management practices for conservation in northern Australia.

Assessing the risk to threatened species of population decline from anthropogenic disturbances is challenging when there are issues with species identification, and little is known of their biology, distribution, population size, and habitat preference. The bare-rumped sheath-tailed bat (Saccolaimus saccolaimus) is one such species that has a poorly defined distribution over two broad areas of northern Australia. Environmental impact assessments are expected to consider the possibility of its presence in intervening areas outside the known distributions. Our study presents new empirical data that can assist with detection of S. saccolaimus across the entire expanse of northern Australia, provides a critical analysis of acoustics-based identification of the species, and assessed presence within the potentially high value habitat of tall Eucalyptus tetrodonta-dominated forest on the western side of Cape York Peninsula using a combination of trapping and acoustic recordings. Capture of other Saccolaimus species was the greatest of any survey conducted to date in Australia, demonstrating that the capture of these high-flying bat species in tall forest habitats can be relatively effective with mist net arrays hoisted into the tree canopy. In addition, reference echolocation call collections from the focal trapping area plus other locations across northern Australia allowed characterisation and comparison of the calls of most low-frequency-emitting (LFE) echolocating bat species of northern Australia. In addition to separation of species-specific search phase call types using multivariate statistics, a compilation of features from search phase, approach phase and feeding buzz echolocation calls will help distinguish S. saccolaimus from most other LFE species. However, the similarity of the echolocation calls of S. mixtus and S. saccolaimus prevented them from being distinguished from one another. A multi-method approach that emulates the present study and incorporates our recommendations and cautions will lead to robustness in ecological studies and greater clarity in environmental impact assessments.