Soundscapes offer unique opportunities for studies of fish communities.

Resource partitioning is a fundamental ecological concept in which cooccurring species reduce competition by using or specializing on different resources (1). It is widely accepted as a mechanism permitting similar species to cooccur, leading to increased levels of species diversity (2). Typically, resources are thought of as food, habitat, or behavioral features such as timing of foraging and pattern of prey capture. However, the concept applies equally well to any niche parameter that affects species success. This includes the “space” to effectively communicate with conspecifics (3). In PNAS, Ruppé et al. (4) document apparent resource partitioning in the acoustic communication behavior of a community of nocturnal marine fishes found in a cave environment on the rocky coastline of South Africa. They recorded and analyzed 2,793 instances of 17 distinctive sounds that differed in peak frequency and pulsing characteristics. They assumed those distinctive sounds represent separate species and found that sounds from sonic species recorded during the day were less acoustically distinct from one another than those recorded at night. The authors interpret this pattern as indicative of resource partitioning among nocturnal species that are largely limited to acoustic communication modalities. In contrast, the acoustic signals of diurnal species in the same community, for which visual displays undoubtedly play a larger role, are not so constrained and overlap considerably in frequency at the resolution used in their analysis (∼700 Hz). The Ruppé et al. study (4) tackles several emerging topics in the area of marine soundscape research. By illuminating differences in daytime and nighttime patterns of acoustic signaling, they are contributing to a developing body of knowledge on the impact of environmental constraints such as daily and lunar cycles on marine soundscapes (e.g., ref. 5). These patterns point to one form of resource partitioning that seems to be occurring in this habitat but may exist more widely in marine soundscapes. Their study also offers a robust statistical treatment of signal characteristics, to identify distinct components of the soundscape. In future work, however, researchers wanting to further separate fine-scale components of soundscapes may find that additional acoustic features and higher-resolution analyses are needed. Finally, by looking at the whole acoustic picture instead of focusing on individual signals, Ruppé et al. open a door to a new tool for assessing community-level interactions, potentially merging behavioral, ecological, and evolutionary responses into a quantifiable measure. Partitioning of the acoustic environment by temporal separation of calling or by parsing out frequency space has been demonstrated in a variety of other animal groups, including, most notably, insects, anurans, birds, and mammals (6). The prevalence of acoustic resource partitioning in the marine realm is, however, not well documented. Acoustic resource partitioning may be common in marine mammals whose vocalizations can travel great distances, but it may also be expected on much smaller geographic scales in a variety of environments and communities. This includes coral reefs where fishes produce calls differing in type as well as timing of occurrence (7). In such highly diverse communities, clear temporal separation of calling between diurnal and nocturnal communities may be expected, with more pronounced partitioning of the soundscape at night when alternative communication modes are restricted. Although both temporal and frequency partitioning occur in the cave environment in South Africa (4), in high-diversity areas where acoustic space is even more crowded, it may be important for researchers to consider additional scales of resource partitioning, both temporally and in frequency. That may include partitioning of dusk, dawn, daytime, and nighttime, which may account for precisely timed chorusing, as well as investigation of longer-scale temporal partitioning as seen in superannual separation of cicadas (8). Alternatively, it may include finer partitioning of the frequency spectrum, as seen in baleen whales (Fig. 1). Baleen whales rely extensively on acoustic signals for