Urbanization Impacts on Bat Acoustic Diversity

Abstract

Urbanisation is one of the most rapid and pervasive drivers of habitat transformation globally, converting natural and agricultural landscapes into impervious surfaces, artificial light corridors, and noise-polluted environments that fundamentally alter the foraging, roosting, and communication ecology of echolocating bats. This study quantified bat acoustic diversity, call activity, and community composition along urbanisation gradients in Stockholm (Sweden) and Zurich (Switzerland) using passive acoustic monitoring with 48 Anabat Swift detectors deployed across 96 sites stratified by impervious surface cover (ISC: 0–15%, 15–40%, 40–70%, > 70%) over two full activity seasons (April–October 2020–2021). A total of 284,628 echolocation call sequences were recorded and classified to species level using BatClassify v2.1, identifying 18 bat species across 5 families. Species richness declined significantly with ISC (Spearman r s = -0.72, p < 0.001), from a mean of 12.4 ± 1.8 species per site at ISC < 15% to 4.8 ± 1.4 at ISC > 70%. Acoustic activity index (calls per detector-night) declined by 68% from low to high ISC sites. Functional diversity (FDis of acoustic trait space) declined significantly with ISC (r s = -0.68, p < 0.001), with high-ISC communities dominated by edge-adapted, high-frequency species (Pipistrellus pipistrellus, P. pygmaeus) while open-space, low-frequency foragers (Nyctalus noctula, Eptesicus serotinus) were disproportionately lost. Artificial light at night (ALAN) intensity was the single strongest predictor of bat species richness loss (R2 = 0.58, p < 0.001), outperforming ISC, noise pollution, and tree cover as predictors in multivariate models. Green corridor connectivity within 500 m significantly buffered species richness loss at intermediate ISC (40–70%), maintaining 78% of low-ISC richness at connected sites vs. 52% at disconnected sites. These findings identify ALAN management and urban green corridor design as the highest-priority interventions for bat conservation in European cities under the EU Biodiversity Strategy 2030