Passive acoustic monitoring of Ensiferan calling diversity in a sub-tropical forest of Northeast India.

Abstract

Ensiferans are nocturnal insects (Order Orthoptera) that produce mating advertisement calls using stridulatory organs on modified forewings. These calls, typically made by males, are species-specific and serve as indicators of forest health. In biodiverse ecosystems like the subtropical forests, caller density is high, and ecological constraints such as intra- and interspecific acoustic competition, masking interference, and predation pressure can influence calling behavior. These pressures lead to variation in call structures and differences in spatiotemporal acoustic space use, leading to variations in community call type composition across the seasons. Passive Acoustic Monitoring (PAM), a non-invasive and cost-effective technique, is widely used for long-term monitoring in vertebrate taxa, but is less commonly applied to terrestrial invertebrates. In this study, we employed PAM to quantify acoustic diversity, acoustic space use, separation of different call types, seasonal calling patterns, and seasonal variation in call type composition among nocturnal Ensiferan callers. Year-round recordings were conducted using AudioMoth devices in the Khasi Hills of Meghalaya, part of the Indo-Burma Biodiversity Hotspot, at a 48 kHz sampling rate. Acoustic samples were processed using Raven Pro software. We identified 33 distinct call types, mutually distinctly differing in spectral, temporal, or both parameters. Principal Component Analysis revealed fine-scale separation of call types. While there were seasonal shifts in call types, with the dry season having the least number of callers, overall call type composition remained stable across pre-monsoon and monsoon seasons. Acoustic Space Use (ASU) analysis indicated greater use of lower frequency bands consistent with ground cricket presence, as well as seasonal variation in spectral occupancy. This foundational study is the first of its kind in Northeast India and demonstrates the potential of PAM in studying invertebrate soundscapes.