Species Assembly of Highland Anuran Communities in Equatorial Africa (Virunga Massif): Soundscape, Acoustic Niches, and Partitioning

The soundscape is a complex arrangement of sounds originating from animals and the environment. It is considered a reliable proxy for ecosystem niche structure at the community level. Acoustic communities of anuran species include advertising males, which compete in acoustic space for conspecific females. Stochastic niche theory predicts that all local niches are occupied, and the acoustic community is species-saturated. Acoustic niches, which include the spectral and temporal call structure and diel and seasonal patterns of call activity, are of similar breadth with small overlap. We tested these predictions in four communities inhabiting pristine wetlands at 2546–3188 m a.s.l. in the Volcanoes National Park, Rwanda. We sampled 74 days of hourly 5 min recordings of the local soundscape (September 2019–March 2020) using passive automated monitoring devices (Songmeter SM4). We identified species based on the advertisement call features and measured call activity as calls per minute. The communities included 4–6, species depending on wetland structure, with a shared stock of three species (Hyperolius castaneus, H. glandicolor, Leptopelis kivuensis). Independent of elevation, niche breadth for call features was similar among species and overlap reduced by partitioning the frequency range used. The diel and seasonal niche breadth of specific call activity varied according to the local temperature regime at different altitudes representing the variable part of the acoustic niche. We conclude that communities are indeed species-saturated and acoustic niches differ primarily by the fixed call features remaining locally adaptable by the modulation of the call activity pattern, corroborating the predictions of the stochastic niche theory.