Biases of acoustic indices measuring biodiversity in urban areas

Urban green infrastructure, GI (e. g. , parks, gardens, green roofs) are potentially important habitats, however their full ecological capacity is poorly understood, in part due the difficulties of monitoring urban wildlife populations. Ecoacoustic surveying is a useful of monitoring habitats, where acoustic indices (AIs) are used to measure biodiversity by the activity or diversity of biotic sounds. However, the biases introduced to AIs acoustically complex urban habitats dominated by anthropogenic noise are not well. Here we measure the level of activity and diversity of the low (0-12 kHz, l) and (12-96 kHz, h) frequency biotic, anthropogenic, and geophonic components of 2452 of acoustic recordings from 15 sites across Greater London, UK from June to October 2013 based on acoustic and visual analysis of recordings. We used mixed-effects models to these measures to those from four commonly used AIs: Acoustic Complexity Index (ACI), Acoustic Diversity Index (ADI), Bioacoustic Index (BI), and Normalised Difference Index (NDSI). We found that three AIs (ACIl, BIl, NDSIl) were significantly correlated with our measures of bioticl activity and diversity. However, all three also correlated with anthropogenicl activity, and BIl and NDSIl were correlated with diversity. All low frequency AIs were correlated with the presence of sound. Regarding the high frequency recordings, only one AI (ACIh) was correlated with measured biotich activity, but was also positively correlated with activity, and no index was correlated with biotich diversity. The AIs tested are therefore not suitable for monitoring biodiversity acoustically in anthropogenically habitats without the prior removal of biasing sounds from recordings. However, further methodological research to overcome some of the limitations identified here, has enormous potential to facilitate urban biodiversity and ecosystem at the scales necessary to manage cities in the future.