Acoustic monitoring of invasive species: Continental patterns in calling activity of the invasive cane toads

Invasive populations are managed most effectively when surveillance aligns with periods and places of maximal activity. Understanding the conditions influencing breeding of invasive species may help provide information for studies of population dynamics and range expansion. Despite their continued successful advances, the patterns and drivers of invasive cane toad ( Rhinella marina ) calling activity, as a signal of potential breeding activities, remain poorly resolved at broad scales in Australia. Using continent-wide passive acoustic recording data paired with a machine-learning acoustic classifier, we describe patterns in cane toad choruses across most of their current range in Australia, spanning three States and climate zones (~ 15° latitude and 27° longitude). A total of 163,701 minutes of cane toad calls from 33,799 nights of recordings showed that they chorused nearly year-round, with seasonal peaks. Cane-toad calling probability and intensity increased on dark, calm, humid nights and near lentic systems or temporary water, with low- to medium- canopy cover and decreased with increasing wind and moonlight. Social cues appeared important: prior-week calling activity strongly elevated both probability and intensity of calling. Interestingly, temperature effects were context-dependent, without a prior chorus, calling probability and intensity peaked at moderate temperatures; following chorusing, both metrics remained high across a broader thermal range. However, variable importance differed amongst climate zones: atmospheric and lunar factors were more influential for initiating choruses in subtropical and semi-arid sites, whereas habitat and hydrology contributed more to sustaining choruses in the Tropics. Together, these results reveal a detailed picture of cane toad calling ecology in Australia, identifying when, where and under what conditions chorusing is favoured. More broadly, this study demonstrates the value of ecoacoustics for investigating vocal invasive species at large spatial scales and provides a useful basis for future monitoring and modelling. Graphical abstract