How Wildlife Respond to Natural Noise

Animal sensory systems have evolved in a natural din of noise since the evolution of sensory organs. Anthropogenic noise is a recent addition to the environment, which has had demonstrable, largely negative, effects on wildlife. Yet, we know relatively little about how animals respond to natural sources of noise, which can differ substantially in acoustic characteristics from human-caused noise. Here we review the noise literature and suggest an evolutionary approach for framing the study of novel, anthropogenic sources of noise. We also push for a more quantitative approach to acoustic ecology research. To build a better foundation around the effects of natural noise on wildlife, we experimentally and continuously broadcast whitewater river noise across a landscape for three summers. Additionally, we use spectrally-altered river noise to explicitly test the effects of masking as a mechanism driving patterns. We then monitored bird, bat, and arthropod abundance and activity and assessed predator-prey relationships with bird and bat foraging assays and by counting prey in spider webs. Birds and bats largely avoided high sound levels in noisy environments. Bats also avoided acoustic environments dominated by high frequency noise while birds avoided noise that overlapped with their song, the latter trend suggesting that communication is impaired. Yet, when sound levels were high overlapping noise was not any more disruptive than non-overlapping noise, which suggests that intense noise interferes with more than communication. Avoidance of noise that overlapped in frequency with song was stronger for low-frequency singers. Bats that employ higher frequency echolocation were more likely to avoid high sound level noise; we explore potential explanations for this pattern. Most arthropod Orders responded to noise, yet the directions of effects were not consistent across taxa. Some arthropods increased in abundance in high sound level areas - perhaps in response to the absence of bird and bat predators. Reinforcing this possibility, visually foraging birds and passively listening bats decreased foraging effort beyond what was expected based on declines in abundance and activity. Orb-weaving spiders increased dramatically in high sound level areas, which could be due to a release from predation, an increase in prey capture, or direct attraction to high sound level river noise. Overall, we demonstrated significant changes to many vertebrate and invertebrate taxa during playback of whitewater river noise. We were able to parse out the effects of sound pressure level and background frequency on these individual taxa and predator-prey behaviors. Our results reveal that animals have likely long been affected by particular characteristics of noise, which may help explain contemporary responses to anthropogenic noise. As the spatial and temporal footprint of anthropogenic noise is orders of magnitude greater than intense natural acoustic environments, the insights provided by our data increase the importance of mitigating noise pollution impacts on animals and their habitats. It is clear that natural noise has the power to alter animal abundances and behavior in a way that likely reverberates through entire communities and food webs. Future work should focus on strengthening the relationships between these potential predators and prey and highlight how the structure of the system changes under such noise treatments.