Marine Soundscape Ecology: A New Mechanism for Biodiversity Measurement in Temperate Coastal Reefs

Abstract

In the face of accelerated global climate change, monitoring biodiversity has become a critical task for ecologists. Habitat loss is occurring at an alarming rate in both terrestrial and marine ecosystems, resulting in the endangerment and extinction of species up to 1,000 times faster than natural rates. However, traditional biodiversity measurement techniques present logistical and financial obstacles to conservation. Soundscape Ecology has recently emerged as a promising solution to these problems, providing a mechanism for measuring biodiversity using acoustic technology. Acoustic diversity indices have proven to be beneficial indicators of biodiversity in terrestrial landscapes. This is the first study to explore the relationship between biodiversity and acoustic diversity in temperate marine reefs. Three acoustic diversity indices originally developed for terrestrial use – the acoustic entropy index (H), acoustic richness index (AR) and acoustic complexity index (ACI) – were applied to recordings from six protected and three unprotected coastal reefs in northeastern New Zealand. Acoustic diversity in both the broadband (100 – 24,000 Hz) and low frequency (100 – 800 Hz) ranges was compared to traditional measures of species diversity, obtained from visual surveys of reef fish abundance at each site. Six widely used biodiversity indices were investigated based on fish species data: the observed number of species (S); Margalef’s species richness (d); Shannon’s index (H’); Pielou’s evenness (J’); Simpson’s index (1 – λ’) and the Chao2 estimated number of species. Criteria for successful acoustic diversity indices in temperate marine settings were outlined as follows: 1) positive correlations between biodiversity and acoustic diversity in relevant frequency ranges; 2) consistent diurnal behaviour as well as consistent behaviour over entire recording periods; 3) consistent behaviour within one season; 4) robustness to the inclusion of natural and anthropogenic noise interference in the dataset. While neither the acoustic entropy nor the acoustic richness indices met these criteria, the acoustic complexity index was found likely to be appropriate for use on temperate reefs. Specifically, results showed relatively strong (R2 = 0.640) positive correlations between the acoustic complexity index and Pielou’s evenness in the broadband frequency range, and stronger (R2 = 0.740) positive correlations between ACI and J’ in the low frequencies. It was therefore concluded that passive acoustic monitoring is likely appropriate as a non-invasive method of measuring biodiversity on temperate reefs.