Calling at the highway: The spatiotemporal constraint of road noise on Pacific chorus frog communication Calling at the highway: The spatiotemporal constraint of road noise on Pacific chorus frog communication Ecology and Evolution

Loss of acoustic habitat due to anthropogenic noise is a key environmental stressor for vocal amphibian species, a taxonomic group that is experiencing global population declines. The Pacific chorus frog (Pseudacris regilla) is the most common vocal species of the Pacific Northwest and can occupy human-dominated habitat types, including agricultural and urban wetlands. This species is exposed to anthropogenic noise, which can interfere with vocalizations during the breeding season. We hypothesized that Pacific chorus frogs would alter the spatial and temporal structure of their breeding vocalizations in response to road noise, a widespread anthropogenic stressor. We compared Pacific chorus frog call structure and ambient road noise levels along a gradient of road noise exposures in the Willamette Valley, Oregon, USA. We used both passive acoustic monitoring and directional recordings to determine source level (i.e., amplitude or volume), dominant frequency (i.e., pitch), call duration, and call rate of individual frogs and to quantify ambient road noise levels. Pacific chorus frogs were unable to change their vocalizations to compensate for road noise. A model of the active space and time ("spatiotemporal communication") over which a Pacific chorus frog vocalization could be heard revealed that in high-noise habitats, spatiotemporal communication was drastically reduced for an individual. This may have implications for the reproductive success of this species, which relies on specific call repertoires to portray relative fitness and attract mates. Using the acoustic call parameters defined by this study (frequency, source level, call rate, and call duration), we developed a simplified model of acoustic communication space-time for this species. This model can be used in combination with models that determine the insertion loss for various acoustic barriers to define the impact of anthropogenic noise on the radius of communication in threatened species. Additionally, this model can be applied to other vocal taxonomic groups provided the necessary acoustic parameters are determined, including the frequency parameters and perception thresholds. Reduction in acoustic habitat by anthropogenic noise may emerge as a compounding environmental stressor for an already sensitive taxonomic group.

人为噪声（anthropogenic noise）导致的声学栖息地（acoustic habitat）丧失，是发声两栖动物类群面临的关键环境胁迫因子（environmental stressor）——该类群正经历全球种群数量下滑。太平洋树蛙（Pseudacris regilla）是太平洋西北地区最常见的发声两栖物种，可栖息于受人类活动主导的生境类型，包括农业湿地与城市湿地。该物种会暴露于人为噪声中，这类噪声会干扰其繁殖季的鸣唱行为。我们假设，作为广泛分布的人为胁迫因子，道路噪声会诱导太平洋树蛙调整其繁殖鸣唱的空间与时间结构。我们在美国俄勒冈州威拉米特河谷，沿道路噪声暴露梯度，对比了太平洋树蛙的鸣唱结构与环境道路噪声水平。我们同时采用被动声学监测（passive acoustic monitoring）与定向录音（directional recordings）技术，测定单只个体的声源级（source level，即振幅或音量）、主频（dominant frequency，即音调）、鸣唱时长与鸣唱速率，并量化环境道路噪声水平。太平洋树蛙无法通过调整鸣唱行为来弥补道路噪声带来的干扰。通过构建可传播太平洋树蛙鸣唱的有效时空范围（"spatiotemporal communication"）模型，我们发现，在高噪声生境中，单只个体的时空通信范围大幅缩减。这可能会对该物种的繁殖成功率造成影响——该物种依赖特定的鸣唱曲目来展示相对适应度并吸引配偶。基于本研究界定的声学鸣唱参数（主频、声源级、鸣唱速率与鸣唱时长），我们构建了该物种的简化声学通信时空模型。该模型可与各类声学屏障的插入损耗（insertion loss）测定模型结合使用，以明确人为噪声对受胁物种（threatened species）通信半径的影响。此外，只要测定得到必要的声学参数（包括频率参数与感知阈值），该模型即可推广应用于其他发声分类类群。人为噪声导致的声学栖息地缩减，可能会对本已处于弱势的分类类群构成复合环境胁迫因子。