The Hills Are Alive with the Sound of Music: The interaction between soundscape and vegetation structure in a successional forest

Abstract

Soundscape ecology is the study of the ecological impacts of a soundscape, or the sounds that are emitted from a landscape. The soundscape is an ecological information resource for species that use auditory signal communication. Species will partition their signals temporally or spectrally to avoid overlap with other species, which would result in different sound niches and the environment also modifies and alters acoustic signals, where the signal with the greatest broadcast area will be selected. This study investigates how the soundscape differs throughout a biodiverse forest that is undergoing different states of succession across vegetation structures. It predicts that older forest should show higher complexity and heterogeneity values than newer forest, regardless of vegetation structure, due to a longer evolutionary history. However, the sound structure should remain dependent on the vegetation structure. Data collection was performed in Santa Rosa National Park, Costa Rica, where recordings and vegetation measurements were obtained at different areas of the park that varied in their stand age and growth. Three locations were chosen: new growth, older growth, and new growth of an older forest. Acoustic data was analyzed with the acoustic indicators (dominant frequency, Acoustic Complexity Index, Total Entropy Index, and Acoustic Diversity Index) as a way to quantify the soundscape according to sound structure, complexity, heterogeneity, and diversity. There was a significant temporal effect on these indices, where high levels of activity were associated with dawn and dusk (dominant frequency) or day-time (ACI and ADI). Sound was shaped by the vegetation structure, however the complexity and heterogeneity was shaped more by the evolutionary history of the forest. This provides evidence that both the acoustic niche hypothesis and acoustic adaptation hypothesis are at play in a soundscape. Conservation efforts should investigate the complexity of soundscapes and use it in an assessment of what forest to preserve. Further studies should focus on more vegetation indicators and a larger acoustic dataset to provide a clearer interpretation of the patterns and ecological forces at play.