1. Species-area relationships (SARs) are one of the oldest, best documented, and most ubiquitous patterns in ecology, raising its status to ecological law. However, unlike species richness, little is known about the spatial scaling of alternate dimensions of diversity. Among these other diversity types, the quantification of soundscape diversity, or the spectro-temporal diversity of all sounds produced in the landscape, has seen rapid growth in recent years. Soundscape diversity metrics have successfully been used as descriptors of landscape configuration, habitat identity, ecosystem health, diel and seasonal dynamics, proxies for taxonomic diversity, and more. Still, the effects of island size on the soundscape richness remain unknown.
2. In this study, we examined the relationship between the soundscape richness and island size for a set of Amazonian land-bridge islands in the Balbina Hydroelectric Reservoir. We collected long-duration acoustic recordings for 40 plots on 21 islands ranging from 12 – 668 ha. For each island, we calculated the island-wide soundscape richness (gamma) and assessed its relationship with island size. Furthermore, to disentangle the ecological mechanisms underlying the observed pattern, we decomposed the soundscape richness into its alpha and beta components and evaluated their relationship with island size. Finally, to assess when and where sound is lost from the acoustic trait space, we quantified the soundscape richness-area relationship for various frequency subsets (above or below 11,025 Hz) and diel phases (day, night, dawn, dusk).
3. For the first time, we demonstrate strong Island SoundScape-Area Relationships (ISSARs), with slopes of comparable magnitude to those previously described in island SAR-studies. We observed a positive relationship between the plot-scale soundscape richness (alpha) and island size, indicative of an area per se effect on the soundscape richness. As the relationship between the beta soundscape richness and island size was negligible, habitat heterogeneity effects were likely not driving the observed ISSAR. Finally, we found that the ISSAR-slope was twice as steep for sounds above 11,025 Hz, indicating that organisms that produce sounds at higher frequencies, such as orthopterans and cicadas, are more vulnerable to the effects of island size. Subsetting the acoustic trait space into various diel phase subsets had a minimal impact on the observed slope values.
4. These findings suggest that soundscape richness metrics can effectively capture fundamental ecological patterns and provide novel insights into the mechanisms driving biodiversity change in complex systems such as rainforests, highlighting their use as a tool for ecological monitoring and conservation biogeography.
soundscape ecology, eco-acoustics, passive acoustic monitoring, species-area relationships