Ecological assembly of natural soundscapes in the Garajonay National Park (Spain)

Abstract

Natural sound contains data about the ecology of animal populations, communities, and the full ecosystem, resulting from a complex evolution and varying according to the environment. Amongst the processes that are hypothesized to explain sound assemblages, or soundscapes, one is the acoustic niche hypothesis: sounds produced by species calling at the same time seek avoid overlapping, leading to an acoustic differentiation of signals. Soundscapes are more complex in the most pristine environments and show responses to habitat degradation and physical perturbations; hence here, we focus on La Gomera, in the Canary Islands (Spain). This island is the only location in Europe where primary cloud forests are well preserved and thrive on an island with varied orography, microclimates, disturbances, and vegetation types. In this article, we adapted a method to quantify the importance of acoustic niche partitioning and also the opposite process: acoustic aggregation. To do so, we explored soundscapes at different temporal scales in forests with variable degrees of perturbation and maturity. A secondary goal of this report is to compare how soundscapes could differ in an area affected by a wildfire, and undisturbed equivalents, in summer in winter, seasons with contrasting temperatures and wind regimes. We conclude that tracking faunal activity and behavior through soundscape monitoring could be a piece of useful complementary information to guide conservation decisions and future restoration efforts in the Garajonay National Park (La Gomera). HIGHLIGHTS: The mature forest and the transitional forest are similar in terms of sound levels, frequencies, and dynamics, although the transitional forest had higher sound levels of the lower frequencies (higher NDSI). The mature forest has stronger and more active assembly mechanisms (i.e., acoustic aggregation and acoustic niche partitioning) and a higher acoustic diversity index (ADI) than the transitional forest. Assembly organizes differently in both locations. In both the mature and the transitional forests, we found an inverse relationship between the ADI and acoustic niche partitioning. At the highest diversities (more frequency bands occupied), the weakest is the temporal avoidance of calls with similar frequencies. The vulnerability of the burned location is highest during the harsh summer, but in winter, it hosts a community very similar to the intact vegetation in terms of ADI and sound levels.